Previously on this blog, we published a step-by-step walkthrough on how to properly size a solar electric system, which is an excellent resource for anyone who wants to design and build their own solar system.
I want to follow up on that advice with some specific tips on designing a system around a SolarEdge inverter with power optimizers.
Systems with power optimizers excel at minimizing the impact of shade on solar panels. If your build space is blocked by trees, buildings, or other obstacles that will cast shade on your panels, a SolarEdge system will help you get as much production as possible out of your solar panels.
Systems with optimizers have some other perks as well, including improved monitoring, easier troubleshooting, and simplified installation. Optimizers give you more control over your system, and (as the name implies) optimize the output from your solar panels.
That said, let’s dive in.
SolarEdge Design Overview
- Pick your inverter based on your array size
- Pair panels to the appropriate optimizers
- Determine panel string size
Step 1: Match DC Array Size to Inverter Input Size
In our example, we’re going to design for an array with 30 Astronergy 345W solar panels. The array size is 10,350 watts or 10.35 kW:
30 x 345 = 10,350 watts
Once we know our desired array size, we need to pick an inverter that is large enough to handle the input from the array.
To determine which inverters are suitable, check the inverter spec sheet for the maximum DC power @ 240V for each model. We choose 240V because that is the standard voltage on most electrical service panels installed on residential properties.
So we have a decision to make. The SE7600 would be easier to install, because it can go on a standard 200 amp service panel without derating the main breaker or doing a line side connection. On the other hand, the SE10000H would allow some room for future expansion.
Inverter Input vs. Output Sizes
Note that inverters have different input and output ratings. The SolarEdge SE7600 outputs 7,600 watts, but can safely handle 11,800 watts of input from the array.
It’s fairly common for your solar array DC wattage to be greater than the inverter output. The array is usually not producing at max power due to real-world losses caused by cloud cover, high temperatures, and module soiling.
Oversizing your PV array will better utilize that inverter, resulting in lower system costs with similar production.
In this case, I would suggest the SE7600H unless you plan to expand your system down the road.
Now that we know which inverter we’ll be using, we can pair the appropriate optimizer.
Step 2: Pick an optimizer for your inverter
Picking the right optimizer is fairly easy when you look at the input section on the optimizer spec sheet. These optimizer models work with SolarEdge inverters:
- Standard inverters: P320 / P340 (available soon) / P370 / P400 / P405 / P505
- Commercial inverters: P700 / P730 / P800 / P850 / P860
Let’s start by looking at the spec sheet for the standard optimizers:
Compare the optimizer input requirements to the solar panel’s spec sheet to make sure the voltage and current are within spec:
The P370 and P400 both have a high enough DC input rating to cover the 345W output of the solar panel.
The P400’s 80V max input voltage covers the 46.37V open-circuit voltage of the panel, and its 10.1A short circuit current is enough to handle the 9.67A current from the panel. The P370 satisfies these benchmarks as well.
With those values covered, we know we can safely pair either the P370 or P400 with Astronergy’s 345W panels.
Next, we need to figure out our string size—the number of panels that can be wired into a single input on the inverter.
Step 3: Determine string size
Go back to the optimizer spec sheet and scroll down to the bottom where it covers system design:
Based on our single-phase HD-wave 7600 inverter and P400 optimizer, we can now determine minimum and maximum string size:
- Minimum string length: (8) P370 or P400 optimizers
- Max string size: 25 optimizers or 6,000 watts (whichever is smaller)
Divide the panel wattage by the maximum string input to ballpark your maximum string size:
345W / 6,000W = 17.39
We round down to a max string size of 17 panels. That means that for this system, a string size ranging anywhere from 8-17 panels would work.
Note that in most cases, the 6000W limit is going to dictate string size. But you could conceivably hit the 25-panel limit if you are using smaller solar panels (in this case 240W or below).
Once we have our string size, look back at the inverter specs to determine how many inputs it has for strings of PV:
The SE7600H inverter can handle 2 strings, so we would use 2 equal strings of (15) Astronergy 345W panels. That is 5,175 watts per string, which is below the 6,000-watt string size limit.
The same string sizing would work for the SE10000H inverter, but that model has a 3rd string input, giving us more flexibility with our array layout. On the SE10000H, you could just as easily do 3 strings of 10 if it worked out better with the layout.
Sizing Three-Phase SolarEdge Inverters
The minimum and maximum string sizes are going to change with the commercial three-phase inverters, because they operate at a higher nominal DC voltage. String size is increased on commercial inverters, and we can also use different optimizers.
The optimizers for three-phase systems (P700 / P730 / P800 / P850 / P860) are designed to pair with two panels, unlike in smaller systems, where each panel would have its own optimizer.
So three-phase systems have the option to use half as many optimizers. Otherwise, the process follows the same outline as above.
Here are a few examples of three-phase system sizing:
Three-phase 208v system
Recommended design: 3 strings of 16 panels
- Max power per string: 6000w
- Minimum string length: 8 optimizers / 16 panels
Strings of 16 are the only option with this configuration. If needed, you could also switch to P400 optimizers on every panel, which would allow for more flexible string sizing.
Three-phase 480v system
Recommended design: 3 strings of 32 and 6 strings of 34
- Max power per string: 12750w
- Minimum string length: 13 optimizers / 26 panels
Wherever possible, maintain even string lengths for dual optimizers to avoid needing an extra optimizer for odd-numbered strings.
Hopefully this shines a light on our thought process when we design SolarEdge systems. If you need help with your project, reach out to us for a free solar design consultation. And if you plan on building the system yourself, don’t miss our guide to installing your own solar system.