Category: Systems

How many solar panels do I need to power my home?

How many solar panels do I need to power my home?

How many solar panels do I need to power my home?

Solar systems are sized based on your energy usage in kilowatt-hours (kWh). But if you don’t have those numbers handy, this article offers ballpark system sizes based on familiar reference points, like square footage or number of bedrooms in your home.

When people first consider the idea of going solar, one of the very first questions that comes to mind is “how many solar panels do I need to power my home?”

Though the only accurate and reliable way to size a solar system is based on your personal energy usage, we understand not everyone has that information readily accessible. For those of you just beginning your solar research, we wanted to provide benchmark system sizes based on familiar reference points, like square footage and number of bedrooms in your home, to give you a starting point for your research.

Before we go further, we should be clear: these tables are estimates projected based on US national averages. Your energy needs may differ from what is typical of the average American home.

These tables give a benchmark to help you answer preliminary questions like how many panels can fit on your roof and whether going solar fits within your budget. If you decide to move forward, you will need to gather energy usage data and size a system around your individual needs.

Jump to the data:

How many solar panels do I need…


We referenced US Census data on average household energy usage, cost of electricity, and occupancy per square foot in the US to work out these projections. In all cases, the tables shown reflects how many solar panels you would need to fully power an “average” American home based on the data available to us.

Because panels come in a wide range of wattages, we’ve run the numbers for two different panel sizes: 315W and 375W. If you opt for lower-efficiency panels, you’ll need more panels in your array to hit the target production number.

Keep this in mind if space is a concern. With limited roof space, you may need to invest in more efficient panels to be able to fit the array on your roof. We’ve provided a range in our projections (from 315W to 375W) to give you a sense of how much it impacts the physical layout of the array.

How many solar panels do I need based on the square footage of my house?

Know the square footage of your home? This table cross-references on US Census data on the average household square footage against the average monthly electric usage of an American household to estimate how many panels you may need based on the size of your home.

Square FootageTypical System Size# of panels (375W)# of panels (315W)Sample System
2500.66 kW23Shop
5001.33 kW45Shop
7501.99 kW67Shop
1,0002.66 kW89Shop
1,2503.32 kW911Shop
1,5003.99 kW1113Shop
1,7504.65 kW1315Shop
2,0005.32 kW1517Shop
2,2505.98 kW1619Shop
2,5006.65 kW1822Shop
2,7507.31 kW2024Shop
3,0007.98 kW2226Shop
3,2508.64 kW2428Shop
3,5009.31 kW2530Shop
3,7509.97 kW2732Shop
4,00010.64 kW2934Shop
4,25011.30 kW3136Shop
4,50011.97 kW3238Shop
4,75012.63 kW3441Shop
5,00013.30 kW3643Shop

How many solar panels do I need based on the number of bedrooms in my home?

If you don’t know the square footage of your house off the top of your head, we’ve also estimated average system size based on the number of bedrooms in your home. 

These estimates are based on research from the NAHB (National Association of Homebuilders) which reports the average American home has 3.38 bedrooms. We’ve referenced that figure against the average monthly electric usage of an American household to produce the table below.

BedroomsTypical System Size# of panels (375W)# of panels (315W)Sample System
11.94 kW67Shop
23.88 kW1113Shop
35.82 kW1619Shop
47.75 kW2125Shop
59.69 kW2631Shop
611.63 kW3237Shop

How many solar panels do I need to eliminate my electric bill?

Lastly, let’s assume you pay the national average rate for electricity, which is 13.3 cents/kWh.  Here’s how many solar panels you would need based on your average monthly electric bill.

Electric BillTypical System Size# of panels (375W)# of panels (315W)Sample System
$201.08 kW34Shop
$402.16 kW67Shop
$603.23 kW911Shop
$804.31 kW1214Shop
$1005.39 kW1518Shop
$1206.47 kW1821Shop
$1407.55 kW2124Shop
$1608.62 kW2328Shop
$1809.70 kW2631Shop
$20010.78 kW2935Shop
$22512.13 kW3339Shop
$25013.47 kW3643Shop
$27514.82 kW4048Shop
$30016.17 kW4452Shop
$32517.52 kW4756Shop
$35018.86 kW5160Shop
$37520.21 kW5465Shop
$40021.56 kW5869Shop

How many solar panels do you need based on your kWh usage?

The tables above simply give a starting point to get you in the ballpark. If you decide to move forward with your project, you’ll need to go through a more accurate sizing process based on your personal energy usage.

Energy usage is measured in kilowatt-hours (kWh), which can be found on your monthly electric bill. 

Ideally, you want to base your system design off the past 12 months of electric bills, to account for peaks and valleys in usage. Bills tend to be higher during summer and winter due to the need to run A/C and heat.

The table below shows benchmark system sizes based on your average monthly energy usage. This is the most accurate way to size out your system, so if you have your energy usage data available, we’d recommend starting here.

Monthly kWh UsageTypical System Size# of panels (375W)# of panels (315W)Sample System
1000.72 kW23Shop
2001.43 kW45Shop
3002.15 kW67Shop
4002.87 kW810Shop
5003.58 kW1012Shop
6004.30 kW1214Shop
7005.02 kW1416Shop
8005.73 kW1619Shop
9006.45 kW1821Shop
1,0007.17 kW2023Shop
1,2008.60 kW2328Shop
1,40010.04 kW2732Shop
1,60011.47 kW3137Shop
1,80012.90 kW3541Shop
2,00014.34 kW3946Shop
2,25016.13 kW4452Shop
2,50017.92 kW4857Shop
2,75019.71 kW5363Shop
3,00021.51 kW5869Shop

To hone in on a more accurate figure, head over to our solar cost calculator and enter your ZIP code and energy usage data. We’ll provide an accurate cost and system size estimate that takes your usage and local climate into account.

Download our free solar panel buying guide!
What is Voltage Drop? Wiring Your Solar System to Maximize Efficiency

What is Voltage Drop? Wiring Your Solar System to Maximize Efficiency

What is Voltage Drop?

When current moves through an electrical circuit, a small amount of voltage is lost due to resistance in the wires. This concept, known as voltage drop, leads to a slight production loss from your solar array.

Voltage drop is more pronounced over longer distances. A longer wiring run introduces more resistance to the circuit, which leads to greater voltage drop.

When you go solar, one of the goals is to design a system with minimal voltage drop so that your array can perform close to its peak rated output.

It is generally considered best practice to keep voltage drop at 3% or less, though many systems come in well under that mark. These recommendations are outlined in the National Electric Code (2017 NEC 210.19).

solar inverter guide

Free Solar Inverter Guide

Learn More »

Why Voltage Drop Matters

It’s pretty simple. Voltage drop has a direct impact on system production. If your wiring run is too long, your panels may not provide enough voltage to the inverter. The efficiency of the entire system will suffer and your real-world production won’t live up to the nameplate rating of the components.

With that in mind, let’s look at ways you can reduce voltage drop as you design your system.

How to Prevent Voltage Drop

There are four main approaches to counteracting voltage drop:

  1. Minimize the length of the wiring run.
  2. Consider your inverter placement carefully.
  3. Use a bigger wire size. Larger wire = less resistance.
  4. Design your system with higher voltage to overcome resistance.

This is designed to be a beginner-friendly article, so we’ll just run through a quick overview of the concept of voltage drop. If you work with a solar designer, they should take this into account as part of the design process. For example, our tech team checks every system for voltage drop concerns when we provide our electrical wiring diagrams as part of the permitting process.

How to Reduce Voltage Drop

1. Minimize the length of the wiring run.

Since longer wiring runs lead to more voltage drop, the simplest solution is to make the wiring run as short as possible.

As you design your system, plan for a layout that keeps system components close to each other.

If your wiring run is less than 100’, your system may already have less than the allowable voltage drop of 3% without any further design changes. 

2. Consider your inverter placement carefully.

AC wiring (from your inverter to your service panel) can be more prone to voltage drop than high voltage DC wiring (the wires running from the panels to the inverter or controller), though sometimes the reverse is true. It all depends on the voltage of the circuit: different equipment operates at different voltage ratings.

The side of the circuit that is operating at a higher voltage is going to push a stronger current through the wires, which reduces the impact of voltage drop. 

As a result, the inverter should be placed close to the lower-voltage end of the circuit, to minimize the effects of voltage drop in that wiring run.

If the DC voltage from the solar array is higher than the utility service panel, install the inverter closer to your utility service panel.

If the DC voltage from the solar array is lower than the utility service panel, install the inverter closer to your solar array.

Please note that this is just a general rule of thumb, and that the guidelines change depending on what products you use. For example, off-grid systems typically have a lower DC voltage, but there are high-voltage charge controllers to overcome that.

To evaluate your own project, use our voltage drop calculator to input the specs for the products you are considering and calculate voltage drop over the length of your run. You can tweak the wire length, size and other variables to find the sweet spot for your system.

(Of course, if you work with us to design your system, we take care of these calculations for you.)

3. Use a bigger wire size.

Some people need to go with a longer wiring run purely for logistical reasons. For example, you might need to run wires from your home to a barn, which may be several hundred feet apart.

In these cases, upgrade to a larger size wire. This is just like using a bigger hose. The wires have greater capacity, which means less resistance, ultimately making the system more efficient.

Large wires cost more, but they make your system more efficient. The extra output retained over the life of the system more than makes up for slightly higher wiring cost up front.

4. Design a system with higher voltage to overcome resistance.

Instead of (or in addition to) using a larger wire to reduce resistance, you can overcome that resistance by using higher-voltage products.

In some cases, you might prefer specific brands and products that are designed to operate at higher voltages.

For example, SolarEdge-based systems operate at 380V / 400V depending on the inverter model. The power optimizers regulate the panel strings to a fixed voltage, which allows you to design a system that consistently pushes the maximum power voltage through the circuit.

In contrast, string inverters like the SMA Sunny Boy don’t have power optimizers, so the voltage changes based on the number of panels in the string, as shown below. 

To overcome voltage drop, consider designing your system to operate at a higher overall voltage.

The Sunny Boy’s ideal operating range is 195V-480V, so you can end up on either side of the 240V service panel depending on how many solar panels are in a single string. In these situations, favoring larger strings can help overcome voltage drop.

Learn more about why string sizing matters in our string sizing guide.

And off-grid systems have entirely different considerations. If you are off the grid, it’s mandatory to install the inverter inside so it’s protected from the elements. That means you rarely have the luxury of placing the inverter next to the solar array. 

To counteract this limitation, off-grid systems use high-voltage charge controllers (up to 600V) to minimize voltage drop over long wiring runs. Of course, these changes need to be accounted for during the design process.

Voltage Rise: The Opposite of Voltage Drop

For grid-tied systems, voltage rise matters as well. Voltage rise is an equal-but-opposite effect that happens at the start of the circuit (the inverter). The calculations are the same, but the effects happen on opposite ends of the circuit.

Voltage drop is a loss of voltage (and subsequent loss of production) as the current is pushed from the inverter to the service panel. Voltage drop is measured at the end of the circuit, where voltage rise is measured at the start. If a grid-tied inverter is sending power into the grid, you would see voltage rise at the inverter terminals and voltage drop at the end of the wiring run, at the service panel.

Because the voltage is lower at the end of the circuit (the service panel), it follows that voltage should be higher at the start of the circuit (the inverter). That’s voltage rise – an increase in voltage at the start of the circuit.

To continue the hose analogy, picture what happens when you turn a hose on. Right at the spout, the pressure is highest because all of the current is being forced through a small tube. By the time it comes out, the pressure is lower because it had to push its way through the hose. 

So a hose that operates at a pressure of 50 PSI might be 55 PSI at the spout and 45 PSI when it comes out of the end of the hose.

The same goes for solar design. Due to voltage rise, voltage is at its highest where the current originates from the inverter. If that voltage exceeds the upper limit of the inverter’s AC voltage limit, it will cause a high voltage fault, causing your system to shut down.

As a result, systems need to be designed to account for voltage rise as well to ensure the extra voltage doesn’t push the inverter past its max AC voltage. Some grid-tie inverter manufacturers, like SolarEdge, recommend maintaining <1% voltage drop/rise to prevent issues.

Wrapping Up

This is a lot to take in, so let’s boil it down to what really matters.

Voltage drop matters because it causes you to lose wattage from your panels. More voltage drop = less production = less value from your investment into solar.

When designing a system, it helps to take a holistic approach. You should figure out where you plan to place your components, then pick equipment with those considerations in mind. 

If you have a long wiring run that can’t be avoided, it may be smarter to invest an extra $500 in high-voltage equipment to save yourself $2000 on a larger wire. Take a high-level view of the project and consider the most efficient design options given the constraints.

If you’d like guidance on the design process, reach out to us to request a free design consultation. We’re happy to help you design a system that is tailored to your needs.

Download your free solar inverter guide
Why You Should Oversize Your PV Array By 10-20%

Why You Should Oversize Your PV Array By 10-20%

Why You Should Oversize Your PV Array For Your Inverter

When designing a solar system, it is often smart to size components so that the panels supply 10-20% more wattage than the rating of the inverter. In this article, we’ll explain how oversizing your PV array can maximize your system’s overall efficiency.

Welcome to our solar tech tips series! In this article, we’re going to take a look at a concept that can create a lot of confusion during the system design process: oversizing your PV array for your inverter.

First, the basics: solar panels and inverters both have a wattage rating. For example, a 315W solar panel produces 315 watts, and a 290W micro-inverter can put out a max of 290 watts of power, if it’s available.

When the panel produces more power than the inverter can handle, the excess wattage is “clipped.” Anything above the 290W rating can’t be processed by the inverter, so if the 315W panel is producing at its rated output, 25 watts are wasted by the input limitations of the inverter.

Knowing all this, you might make the reasonable assumption that the panel wattage shouldn’t exceed the inverter wattage, because wasting power is generally a bad idea…right?

But that’s not quite true.

When designing a solar electric system, you’ll get the most bang for your buck if you oversize your panels by 10-20% in relation to your inverter. 

It’s counter-intuitive, but it’s true. Here’s why.

Why You Should Oversize Your PV Array

Note: this advice applies to grid-tie systems only. Off-grid systems have different sizing considerations – the inverter is sized to match your maximum load to run your appliances, rather than an average daily usage. Therefore, sizing considerations are different and this advice should not be applied to off-grid systems.

For help going off the grid, check out our off-grid system sizing guide.

In real-world conditions, the panel rarely produces at its rated output. There are two main reasons for that: 

  1. Efficiency Loss: real world factors like temperatures, shading and pollution affect the amount of light hitting your panel. This can cause the panel to produce below its rating.
  2. Production Curve: The array doesn’t produce a consistent amount of power throughout the day. Production is a curve, with less output during the morning and evening, and peak production at “solar noon.” During off-peak periods, the panel doesn’t produce as much as its wattage rating.

Let’s look at each of these points in more detail.

Standard Test Conditions vs. Real-World Conditions

When manufacturers test panels to give them a rating, they do so in ideal conditions:

  • Indoors, at a controlled temperature (about 77°F)
  • With a given amount of solar irradiance (1000 watts per square meter) 
  • At an ideal 90° angle of incidence (light shining directly on the panel)

These standard test conditions measure what the panel is capable in a perfect environment, but the real world rarely delivers these ideal conditions.

In reality, there are a number of factors that can reduce panel efficiency:

  • Hot temperatures
  • Tilt angle of the array
  • Time of day / sun’s position in the sky
  • Cloud cover and pollution

Under real-world conditions, a 315W panel rarely produces 315 watts of instantaneous power.

Our general rule of thumb is to ballpark 10% efficiency loss to account for real-world operating conditions. The true number changes based on your local environment, but 10% gives us a good baseline estimate.

Sun Hours & Peak Sunlight Window

For a solar panel to reach peak output, the sun has to be angled so it is directly perpendicular to the array, allowing it to absorb the maximum amount of light possible.

That only happens during a narrow window in the middle of the day. As the sun moves across the sky, the angle changes so that less light strikes the panels. This causes the panels to produce less power.

A map of average sun exposure across the United States.

We use the term sun hours to describe this concept. “Sun hours” refers to the amount of time the sun is in the right position in the sky so that the array can generate power. Sun hours are measured based on an irradiance of 1,000 watts per square meter—the same rating used to test panels under standard test conditions.

Most places in the US get between 4-6 sun hours per day on average, and all of the meaningful production from the panels comes within this short window. The production graph is a curve where production ramps up as the sun comes out, hits a peak when it is straight overhead, then falls off again into the evening.

The horizontal line represents the input limit of the inverter. Any production above the line (the light orange area) is clipped, or wasted.

The blue area below the line represents untapped potential production. In these periods, the inverter is capable of more throughput, but the panel is not supplying enough wattage to make full use of the inverter’s potential.

Our goal with system design is to balance out clipped production with “lost” potential production. On the graph, notice how the area of clipped production and lost production are roughly equal. 

By oversizing the array, you will make better use of your inverter’s capacity, producing more power overall. You want to find the “sweet spot” where you get the most overall production possible per dollar spent on your system – even if that means clipping a bit more power.

There’s another benefit to oversizing that takes advantage of this principle. Most electrical service panels can handle up to 7.6 kW input from solar. Anything larger makes the install more complicated, as you have to either derate the main breaker, or tap into the utility line side.

If you don’t want to tackle a complex install, it might be smarter to oversize your array on a 7.6kW inverter to extend the production window, so that you generate more power in the mornings and evenings, squeezing out that extra bit of production to bring you up to 100% energy offset for your property.

Guidelines for Oversizing Inverters

When designing your system, a good rule of thumb is that your solar panels should be 10-20% larger than your inverter. In hot climates, that can be extended up to 30%, due to greater efficiency losses from heat.

Two real-world examples:

For micro-inverters, we usually pair the 290W Enphase IQ7+ with a solar panel in the 320W-350W range.

For string inverters, the SolarEdge HDWave 7.6kW inverter can be paired with a 8360W-9120W solar array. For example, you could use 3 strings of 335W panels, with 9 panels in each string, for a total of 9,045 watts on a 7,600-watt inverter.

Note that the manufacturers recommend a much broader range in both cases—Enphase suggests 235W-440W panels for the IQ7+, and SolarEdge specs a max of 11,800 watts on their 7.6kW HD-Wave. These guidelines spec what the inverters can safely handle, but we recommend the narrower ranges above to help people maximize their production per dollar spent on their system. Keep in mind that this will vary depending on your climate and other factors that affect production.

Need help with the design process? Request a free consultation with our team. We’ve designed more than 10,000 systems since we came online in 2002, and we can help you navigate challenges like array oversizing to design the best possible system for your needs.

Download our free solar panel buying guide!
How to Size a Solar System: A Step-By-Step Walkthrough

How to Size a Solar System: A Step-By-Step Walkthrough

Once you have determined that a grid-tie solar system is the best option for your home, we want to help you size the system correctly. This article will teach you how to size a solar system that covers your energy use patterns, without over-sizing your PV array.

The process for sizing off-grid solar systems is different, due to the need to account for battery bank sizing. Click here for advice on how to size your off-grid solar system.

As a system designer, I follow a step-by-step process to size grid-tied systems that work with my client’s project constraints.

The first step is to figure out the main constraints on the project and using those restrictions as the starting point for the design. We can approach the project from one of three angles:

  • Budget constraints: Build a system within your target budget.
  • Space constraints: Build a system that is as space-efficient as possible.
  • Energy offset: Build a system that offsets a certain percentage of your energy usage.

I want to make sure I deliver a system that satisfies my client’s specs, but I also need to account for sizing factors that might not be immediately obvious to them.

Some common stumbling blocks that come up over and over again:

  • Local levels of sun exposure
  • Orientation of the array (facing and tilt angle)
  • Plans for future expansion
  • Product efficiency ratings
  • Natural degradation of performance over the life of the warranty

This article is intended to provide a step-by-step overview of the sizing process for grid-tied solar systems, taking the above restrictions into account.

Getting Started With Solar guide

Free Guide: Getting Started With Solar

Learn More »

Sizing Grid-Tied PV Systems: Getting a Quick Estimate

First, gather the Kilowatt Hours (kWh) usage from your electric bill. We want to have all 12 months available so we can look at peaks and valleys in usage. Energy consumption spikes in the summer and winter with heavy use of your A/C and heating units. A full year of energy consumption data gives us the big-picture overview we need.

We also want to average the data from 12 months of bills to know your average monthly kWh usage. Grid-tied systems tend to overproduce in the summer with peak sun exposure.

If your utility provides a favorable net metering policy, the energy your system generates can be banked with the utility as credit to be used later. Not all utility companies do this; check with your local provider.

Next, we want to look up your sunlight hours per day, through a sun hours chart or the PV Watts Calculator (I will get to this in the next steps).

For a general estimate we can use this simple formula, then fine-tune as we get further into the design:

(Yearly kWh Usage ÷ 365 days ÷ average sun hours) x 1.15 efficiency factor = DC solar array size required.

If the solar array cannot face south, on the preferred angle, we need to adjust the system by adding more solar.

Here is an example. I live in New Mexico where the PV Watts Calculator says I have an average of 6.10 sunlight hours per day. That is a LOT, I know, but that is why I live here. I use 1000 kWh per month, or 12,000 kWh in a year. According to the formula:

(12,000 kWh ÷ 365 days ÷ 6.1 sun hours) x 1.15 = 6.2 kW DC solar system required

Fine-Tuning the Estimated System Design

When I am ready to make a solar system estimate as accurate as possible, I pull up the address on Google Maps. I check to see if I have any viable south facing options for a roof mount.

(Your solar system should point toward the equator, so if you live in the Southern Hemisphere, look for north-facing options instead.)

A roof mount is the simplest and most cost-effective solution. It costs less than other racks. Many times the slope of the roof is already set up for solar gain, and it keeps the solar panels close to the inverter and service panel. This is great for the efficiency and costs less in conduit and wire.

To learn more about the pros and cons of each mount type, read this article: Ground Mount vs. Roof-Mount Racking: What’s the Best Way To Mount My Solar Panels?

Customer Tom M. with his roof mounted system in Albuquerque, NM.

If a roof mount is not an option, I will look into the possibility of a ground mount or pole-mounted solution.

Once we know how much area we have for solar panels, and what angles and directions we will be working with, I get out the PV Watts Calculator and follow these steps.

How to Use the PVWatts Calculator

  1. Enter the address and hit the orange arrow to the right.
  2. Once you are on the System Info page, enter the DC system size from the previous section.
  3. Choose standard module.
  4. For array type, select “fixed” for roof mounts, or “open” for ground mounts.
  5. Leave the system losses at around 15%.
  6. Enter the slope of your roof in degrees, and the azimuth. Azimuth is the degrees relating to north and south, with north being zero and south being 180. (Click here to learn how to fine-tune your angle and azimuth values.)

Once all the info has been entered, click the arrow to the right and it will tell you how much power your system will put out on a monthly basis.

This is our step-by-step process for honing in on an accurately sized system. We provide this info because our audience is heavily inclined to DIY, and most people prefer to research at their own pace.

Once you’re ready, we do encourage you to schedule a free design consultation with us so that we can double check your sizing, find compatible products, and ensure the system works within your constraints (budget, build space and energy offset). You can also give us a call at 1-800-472-1142 for an immediate consultation.

Choosing Grid-Tie Solar Equipment

Once we know how big the solar system needs to be, we will cross-reference that with the amount of space available. If you are doing a ground mount, that is usually not a problem.

From my example above, I know I need a 6.2 kW DC system. I can multiply this number by 1,000 to confirm that I need 6,200 watts of solar panels.

My fastest resource is to go to our grid-tied solar packages and scroll down until I see something in this range. If the client expresses a desire to buy American-made panels, or needs certain features like individual panel monitoring, I take those choices into account.

Here are a few viable options I’d consider. Note that the imported panels are more cost-effective, so you get roughly 10% more production for the same price.

Grid-tie systems with American-made panels:

  • 6.2 kW system with 310W Mission Solar panels and SolarEdge Inverter / optimizers
  • 6.2 kW system with 310W Mission Solar panels and Enphase IQ7+ micro-inverters
  • 6.2 kW system with 310W Mission Solar panels and SMA central inverter

Grid-tie systems with imported panels:

  • 6.7 kW system with 335W Astronergy solar panels and SolarEdge inverter / optimizers
  • 6.7 kW system with 335W Astronergy solar panels and Enphase IQ7+ micro-inverters
  • 6.7 kW system with 335W Astronergy solar panels and SMA central inverter

If you’re having trouble deciding which products to buy, we’ve written articles covering that ground as well:

Of course, sometimes it’s easier to talk to someone with experience and have them walk you through the design process. The fastest way to get a thorough evaluation of your solar needs is to call us at 1-800-472-1142 and connect with one of our designers. We’d love to help you design the perfect grid-tied system for your needs.

New call-to-action
How to Size an Off-Grid Solar System

How to Size an Off-Grid Solar System

Renewable energy sources like solar and wind are ideal for powering equipment in remote locations. In this article, we’ll outline a step-by-step process for sizing an off-grid solar system so you can generate electricity even when you’re miles from the nearest power line.

These types of systems power off-grid homes, as well as a variety of industrial applications where equipment needs to be powered at remote work sites. Common applications include lighting, telecommunication equipment, sensors, environmental monitors, security cameras, traffic signals, water pumps, cathodic protection systems, and anything else that requires reliable power in a remote location.

The following guidelines are intended to help size an off-grid solar system based on a given location, energy requirements, and desired days of autonomy (how long the battery bank can supply power before it needs to be recharged).

solar battery guide

Free Solar Battery Guide

Learn More »

Step 1: Determine Energy Requirements

First, you need to know how much energy the equipment uses on a daily basis. This is measured in watt-hours or kilowatt-hours per day. For example, let’s assume the equipment consumes 10 watts of power and operates 24 hours a day:

10 Watts x 24 hours = 240 watt hours per day or .24 kWh per day

How do you find this information? Check the data sheet or manual for your equipment to find out how much power it consumes (in Watts), and then multiply that by the number of operating hours per day. If possible, use a meter to measure the power consumption for an accurate real-world measurement.

If you are using an inverter to produce AC power for a load, remember to account for the inverter’s self-consumption and efficiency losses. Inverters consume a small amount of power while they are operating. Reference the inverter spec sheet, and add the self-consumption to your daily total. Inverter self-consumption typically ranges from under 1 watt, to around 30 watts depending on the inverter.

Efficiency losses can be from 5% to 15% depending on the inverter and how much it’s loaded. This will be accounted for when sizing the batteries. It’s important to invest in a quality, high-efficiency inverter.

Step 2: Evaluate Site Location

Next, determine where the system will be installed to estimate available solar energy.

Use a solar insolation map (also called a ‘sun hours map’) to estimate available PV resources. The system should be sized based on the month with the highest power consumption and/or lowest solar resource, typically December or January.

The National Renewable Energy Laboratory (NREL) has an online resource for mapping available solar radiation. Here is a map of the minimum daily sun hours in January in the United States with a south-facing array:

Minimum Solar Insolation in January

Most of the US has fairly low solar insolation in January. Generally, 2.5 sun hours is a good estimate, but it could be lower or higher depending on your location. We will use 2.5 minimum sun hours for our example.

Solar panels are designed to be installed in full sun. Shade is going to impact performance. Even partial shade on one panel will have a large impact. Inspect the site to make sure your solar array will be exposed to full sun during daily peak sun hours. Keep in mind that the sun’s angle will change throughout the year.

Other Considerations

There are a few other things to think about at this point:

System voltage: Determine what power requirements your equipment has. Off-grid PV systems typically output these common voltages: 12Vdc, 24Vdc, 48Vdc, or 120Vac.

Solar panels and batteries use DC power, and some equipment can be wired directly to the batteries provided it can handle real-world battery voltages. These can range from 10-15 volts for a 12-volt system, 20-30 volts for a 24-volt system, and 40-60 volts for a 48-volt system.

Days of autonomy: The number of days the equipment must operate on battery power with limited solar power. Between 5-20 days is typical, depending on the area and expectations for operating performance. You need enough autonomy to keep the equipment operating through extended periods of overcast weather.

Step 3: Calculate Battery Bank Size

Now we should have enough information to size the battery bank. After the battery bank is sized, we can determine how much solar power is required to keep it charged.

Here is how to calculate battery bank size in our example of 240Wh/day based on lead-acid batteries:

First, we need to account for the inefficiency of the inverter (if you are using an inverter). Depending on the equipment, 5-15% is usually reasonable. Check the spec sheet for the inverter to determine the efficiency. We’ll use a 10% inefficiency for this example:

240 Wh x 1.1 efficiency compensation = 264 watt hours

This is the amount of energy drawn from the battery to run the load through the inverter.

Next, we need to account for the effects of temperature on a battery’s capacity to deliver energy. Lead-acid batteries lose capacity as temps go down and we can use the following chart to increase battery capacity, based on the expected battery temperature:

For our example, we’ll add a 1.59 multiplier to our battery bank size to compensate for a battery temperature of 20°F in the winter:

240 Wh x 1.1 x 1.59 = 419.76 watt hours

Next, account for the efficiency loss that occurs when charging and discharging batteries. Typically we use 20% inefficiency for lead-acid batteries, and 5% for Lithium-ion.

240 Wh x 1.1 x 1.59 x 1.2 = 503.71 watt-hours minimum energy storage requirement

This is for a single day of autonomy, so we need to then multiply it by the number of days of required autonomy. For 5 days of autonomy, it would be:

504 wh x 5 days = 2,520 watt hours of energy storage

As you can see, the battery bank size is quickly increasing due to factors including temperature and required days of autonomy. All of these things affect your battery bank size significantly and need to be carefully considered.

Lead-acid batteries are commonly rated in amp hours (Ah) rather than watt-hours (Wh). To convert watt-hours to amp hours, divide by the system’s battery voltage. In our example:

2,625 Wh ÷ 12v = 220 Ah 12V battery bank

2,625 Wh ÷ 24v = 110 Ah 24V battery bank

2,625 Wh ÷ 48v = 55 Ah 48V battery bank

When sizing a battery bank, always consider the discharge depth, or how much capacity is discharged from the battery. Sizing a lead acid battery for a maximum 50% depth of discharge will extend the battery’s life. Lithium batteries are not as affected by deep discharges, and can typically handle deeper discharges without substantially affecting battery life.

Total required minimum battery capacity: 2.52 kilowatt hours

Note that this is the minimum amount of battery capacity needed, and increasing the battery size can make the system more reliable, especially in areas prone to extended overcast weather.

Step 4: Figure Out How Many Solar Panels You Need

Now that we’ve determined battery capacity, we can size the charging system. Normally we use solar panels, but a combination of wind and solar might make sense for areas with good wind resource, or for systems requiring more autonomy. The charging system needs to produce enough to fully replace the energy drawn out of the battery while accounting for all efficiency losses.

In our example, based on 2.5 peak sun hours and 240 Wh per day energy requirement:

240 Wh / 2.5 hours = 96 Watts PV array size

However, we need to account for real-world losses caused by inefficiencies, module soiling, aging, and voltage drop, which are generally estimated to be around 15%:

96 array watts / .85 = 112.94 W minimum size for the PV array

Note that this is the minimum size for the PV array. A larger array will make the system more reliable, especially if no other backup source of energy, such as a generator, is available.

These calculations also assume that the solar array will receive unobstructed direct sunlight from 8 AM to 4 PM during all seasons. If all or part of the solar array is shaded during the day, an adjustment to the PV array size needs to be made.

One other consideration needs to be addressed: lead-acid batteries need to be fully charged on a regular basis. They require a minimum of around 10 amps of charge current per 100 amp hours of battery capacity for optimal battery life. If lead-acid batteries aren’t recharged regularly, they will likely fail, usually within the first year of operation.

The maximum charge current for lead acid batteries is typically around 20 amps per 100 Ah (C/5 charge rate, or battery capacity in amp hours divided by 5) and somewhere between this range is ideal (10-20 amps of charge current per 100ah).

Refer to the battery specs and user manual to confirm the minimum and maximum charging guidelines. Failure to meet these guidelines will typically void your battery warranty and risk premature battery failure.

Here are standard configurations of PV arrays with battery banks. The battery capacity calculated in the previous step can be compared against this table to find a suitably sized system:

Array Size: PV Watts (STC)Battery Bank Size:
Watt Hours (@ C20 rate)
Battery Bank Ah Capacity
100-17560050Ah @ 12Vdc
200-3501,200100Ah @ 12Vdc
50Ah @ 24Vdc
400-7002,400200Ah @12Vdc
100Ah @ 24Vdc
800-1,4004,800400Ah @ 12Vdc
200Ah @ 24Vdc
100Ah @ 48Vdc
2,000-3,0009,600800Ah @ 12Vdc
400Ah @ 24Vdc
200Ah @ 48Vdc
4,000-6,00019,200800Ah @ 24Vdc
400Ah @ 48Vdc
8,000-12,00038,400800Ah @ 48Vdc

This information is intended to be a general guide and there are a lot of factors that can influence system size. There are also alternative options such as incorporating a backup gas generator or wind generator(s) to reduce the minimum battery requirement.

If the equipment is critical and in a remote location, it pays to oversize it because the cost of maintenance can quickly exceed the price of a few extra solar panels. On the other hand, for certain applications, you may be able to start small and expand later depending on how it performs. System size will ultimately be determined by your energy consumption, the site location and also the expectations for performance based on days of autonomy.

If you need help with this process, feel free to schedule a free consultation with us and we can design a system for your needs based on the location and energy requirements.

New call-to-action
SolarEdge, the New Kit on the Block

SolarEdge, the New Kit on the Block

Wholesale Solar is now offering solar power systems using the SolarEdge Systema new approach to gridtie systems. In these new systems, SolarEdge PowerBox optimizers are paired up with Astronergy solar panels and strung to an ETL listed SolarEdge inverter.

The Solar Edge Gridtie System
In a SolarEdge System, SolarEdge Power Boxes are paired with solar panels to provide DC to DC Power optimization. Click For a Larger View.

SolarEdge Systems share some similarities to Enphase Microinverter Systems, and offer many of the same benefits. Like SolarEdge PowerBox optimizers, microinverters control the output of each solar panel with Maximum Power Tracking. While microinverters optimize DC power and convert it to AC power, PowerBoxes offer DC to DC power optimization. Compared to a traditional gridtie system, both SolarEdge and Microinverter systems offer much more flexibility when it comes to system sizing and design:

  • There is no more need for string sizing.
  • Temperature is not a factor because fixed string voltage ensures the inverter always operates at its peak efficiency voltage and prevents under-voltage power losses even on hot days.
  • Panels don’t all have to be facing the same way
  • Different string lengths can now be accommodated.
  • Solar power ratings and technologies can vary.

A Traditional Gridtie System
In a traditional gridtie system, a centralized inverter is responsible for the DC to AC conversion.

A traditional gridtie system is similar to a SolarEdge system in that it has one or more inverters responsible for DC to AC conversion for all of the solar panels in an array. Some prefer traditional gridtie systems,because the inverters are more accessible than microinverters. Others prefer to rely on a system with proven technology that has been around awhile.

Read more about SolarEdge…

New call-to-action