In this fifth video in this series about key concepts for people looking to buy a solar system, Kerim Baran of SolarAcademy explains how solar companies calculate a system size for a typical residential rooftop solar array. He accounts for a number of factors including shading, placement, angles, and location. The entire series can be viewed here.

Below is the video transcript:

Kerim Baran of SolarAcademy: The next concept to understand when going solar is how to size a system. We’ve discussed in the previous two videos the concepts of understanding power and energy and also the concepts of solar resource and the production factor. This is essentially a continuation of that understanding – how to size the system. In the solar resource and production factor video we covered the fact that generally in most places, in an average place in the mainland of the US, the solar resource is about 1500 kilowatt hours per kilowatt in average sunny and average latitude location.

How to size a system is you basically do the math in reverse. So let’s say it’s an average home that consumes about 10,000 kilowatt hours of energy per year, you basically divide into that 1500 kWh production factor that we discussed in the previous video. Of course depending on where you are that production factor might be higher than 1500. In a place like San Diego it could be closer to 1600 or 1650 in certain locations. Actually if you go to Hawaii it gets even closer to 1800. But if you go to Maine or if you go to Seattle it’ll be closer to 1300. So you basically take the total annual consumption of a house in kilowatt hours and divide that into that production factor number – let’s call it 1500 for this example – so if it is a 10,000 kilowatt hour annual consumption home and you want to offset that entire consumption and get net metering credit for it, then you design a 6.6 kilowatt system to generate almost exactly 10,000 kilowatt hours.

That is generally the way they size the system. But this math assumes that the solar panel is placed perfectly in a sunny location and also, ideally, facing slightly south. You want to place these panels on your south facing part of your roof rather than north facing or east or west facing. North facing will have a major loss in production because that side of the roof does not get direct sunlight generally speaking. Another option might be to place the panels east-west and in those cases those panels will get really good production in the morning on the east side and really good production on the west side in the evening. But the rest of the day because the sun is not shining so directly onto that other surface depending on the angle – you usually want these angles to be around 20 maximum 30 degree angles – you’re going to have some production losses. Usually a difference between a pure south facing installation versus the east face is probably 10 to 20 percent loss.

But there could be also unique cases where if the angles of those east and west are low and on a flat roof, for example, that can create some space advantages. That might become advantageous on large commercial installations. But that’s outside of our topic for now. So generally you want the panels facing south, and if it is not south, east-west. Or if it is in a really shady place like under a tree or one corner of the system is going to get some extra shading in the afternoons because of a certain tree or another building, those things will obviously affect your production negatively. There are some amazing software tools in the market that are capable of calculating what the shading losses would be. Tools like Aurora Solar’s design tool, Helioscope and a slew of others essentially have capability to calculate more accurately what your exact solar production factor is. Your installer generally will be using these tools if they want to accurately predict that.

You got to also sometimes take into account the additional loads that you’re going to add to your home. So for example if you’re going to add a pool or a hot tub or if you’re going to add EV chargers for your future EV cars these should be some considerations. Generally for an average American home, adding an EV charger means up-sizing the solar system by another 10 to 15%, two cars might be 20 or even 30%. Again these are just assuming the average consumption, average size home and so forth.