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

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

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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.

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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.

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