This is a the third installment in a three-part series on residential solar PV design. The goal is to provide a solid foundation for new system designers and installers. This section is dedicated to the basics of inverter sizing, string sizing and conductor sizing.

Download the full PDF “Solar PV Design and Installation Guide”

Part 1: How to Design a Solar PV System: The Basic Terms

Part 2: How to Design Solar PV – A Walk-Through of Array Sizing and Estimating Power Production

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This post is specifically focused on basic technical understanding of solar PV projects. However, more and more we’re getting questions from contractors who need to understand how to finance commercial solar projects.

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Learn best practices for setting up commercial solar power purchase agreements (PPAs).

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The goal of the article is to convey the basic process for sizing an inverter, strings, and the conductors. You may not be an expert at the end of the post, but you’ll have a better understanding of how to do these things.

As always, having specific numbers is the most useful for examples, so we’ll continue with the example from part 2 on sizing an array and estimate power production. The house was located in Houston, TX and the roof, given local shading conditions, has enough room on the roof for 20, 205 watt modules. (Read Part 2 to see how we arrived at this number.)

Here is the specification sheet on the Sanyo HIT 205 module we’ll use for the example.

So, the largest possible size of the array we can fit on the roof at STC is 4,100 watts. We can go lower then this but not higher.

1. Inverter Sizing and Selection

Given that we know how many modules can fit on the roof, how do we use this data to size the inverter? The size of the inverter is driven by answering two questions:

1 – What is the capacity of the existing electrical service?

Per NEC 690.64B2 (2008) 705.12 D2 (2011), an existing electrical service is only allowed to backfed up to 120% of the rated capacity.

What does this mean with a typical home?

100 amp service X 20% = 20 amp backfed breaker allowed

20 amp X 80% (for continuous load, we’ll talk about this below) = 16 amp continuous inverter output current

16 amps X 240 volts (or 208 volts, depending on the homes location) = 3840 watts. This is the maximum allowed AC power output of the inverter.

There are a few ways of getting around this, by upgrading the service, performing a line-side tap, and it can sometimes be accomplished with subpanels. However, for this example, let’s keep it simple.

If the existing service only had room for a 20amp breaker, we would not be able to have an inverter that has a rated AC continuous output that would exceed the 16 amp (see example above) or 3840 watts AC.

Per NEC 690.8 A3 the maximum AC output current from an inverter is defined as the manufacturer’s continued rated output current.

Max Current (inverter AC circuits) = continuous current output.

For our example, we’ll assume that the existing electrical service can supply an additional 25 amp back-fed breaker, 20 amps continuous allowed. This limits our choice of inverter to either a PVI 3000 or PVI 4000 inverter based on the electrical service capacity, as the PVI 5000 has a continue output current at 208 VAC of 20.7 amps.

Figure 1 – A Sampling of Solectria Residential Inverter Specs

2 – How many modules can we fit on the roof?

It’s very critical that you perform proper site visits before design the system, so you know the roof that you’re dealing with.

Performing high quality and efficient site visits is absolutely critical to the success of profitable and well designed solar projects, especially residential projects! You need to be able to capture all of the information you need to 1) quote the system correctly 2) design the project and 3) inform the installation crew what to expect. An efficient site visit process will lead to smooth operations and profitable jobs while complex process can lead to unprofitable jobs and a lot of confusion.

Back to our example. From our example, we know that we can fit 20, 205 watt Sanyo modules on the roof.

Here is the specification sheet for the module:

Figure 2 – Spec sheet for Sanyo 205 Module

First, we need to guess the size of the inverter. It’s a good rule of thumb to size the inverter, based on the rated AC continuous output, to be 80% smaller then the rated STC output of the array. The reason for this is that there is a lot of inefficiency from the array to the inverter, so if we undersize the inverter, the array is more likely to hitting the upper limit of the input ranges of the inverter and will more likely be operating within the MPPT operating range of the inverter.

For example, for our array size at 4,100 watts DC STC, we’ve guessed that the inverter would have a AC continuous output range of 80% of 4.1kW, or 3,280 watts AC.

You’ll notice that the naming of Solectria inverters (PVI 3000, 4000, 5000) also seem to match this relationship between the DC rated power of an array (the name of the inverter) and the AC continuous output of the inverter (2700W, 3400W, 4300W, respectively)

We will choose the Solectria PVI 4000 for our example from our choices between the PVI 3000 and 4000

3. How do we size the strings?

Right now, we have concluded two things. First, the inverter we’d like to use the PVI 4000 based on the number of modules that can fit on the roof and how their capacity relates to the inverter. Second, we know the number of max modules we can fit on the roof. Now, we must begin string sizing.

String sizing is the number of modules that we will connect in series and parallel before connecting them to the inverter. The size of our strings will determine the voltage and amperage that is inputted into the inverter.

When string sizing, our goals are:

Make sure we NEVER supply the inverter with too much voltage, which will kill it –> Maximum string length

Make sure that we can ALWAYS supply the inverter with enough voltage to turn on, given the array is receiving full sun –> Minimum string length

What is the maximum voltage allowed for the system? How many modules we can connect in series?

NEC 690.7 specifies that our worst-case voltage, the highest voltages that the DC array can create, must fall within the limits of the inverter.

The exact definition states that: The Voc of each module times the number of modules in a string, correct for lowest expected ambient temperature in the array’s location.

For the PVI 4000, maximum acceptable voltage is 600 VDC.

To calculate the maximum number of modules allowed, we need a few pieces of data

Voc at STC for the module at 77F/25C = 50.3 volts

The temperature coefficient for the module. Typically given in volts per degree C or % voltage per degree C. You will find all this data on any module spec sheet = -.14V/C

The lowest and highest temperatures seen in the specific jurisdiction. Below is the data for Houston from weather.com = 9F or -13C

Here are the calculations for the max system string size. The goal in determining the maximum system voltage is to make sure that power production from the array will never kill the inverter.

Temperature coefficient. -13C lowest temperature – 25C STC = -38C change from STC

-38C X -.14V/C = 5.32 voltage increase. (negative times a negative is a positive)

50.3 volts + 5.32 = 55.62 is the highest voltage we will ever expect to see from each module, and this is the voltage we will use to determine the maximum number of modules in a string.

600VDC (highest acceptable inverter voltage) / 55.62 = 10.78 modules.

We round down to 10 modules, because we cannot go over 600 volts.

Maximum system voltage (MSV) = 10 modules X 55.62 = 556 volts

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