How to Size a Solar Array

Solar Insolation Map of the World.

Insolation

Insolation is a funny word for the number of hours in a day that a solar panel will produce its rated voltage. While all the day's sunshine counts toward this total, not just the brightest hours, not every daytime hour counts equally. When the sun is low in the sky, a solar panel facing it doesn't produce as much energy as it would at noon. Another way of putting it would be to say that if you crammed all the day's sunlight--weak and strong--into equivalent hours of "peak" sunlight, you'd have the "sun hours", or insolation, number. In your area, while the sun may be up for 10 hours during a February day, not all that light is strong enough to be counted at full value, so the insolation value in your area may be closer to 2 sun hours. In our calculations, we use average insolation values taken from years of data collection. Insolation varies by location as well as by month. If you're planning a year-round solar electric system, the annual average insolation value will give you a good starting point for your estimate. If you only plan to use the array seasonally, then use the insolation values for those months only. Insolation tables can be found on the internet, in books, and on our website, here.

Calculations

We are trying to obtain the number of Watts, in solar panels, we need to produce a given amount of Watt-hours (or kWh) for our project in our location.

Now you know you can look for a 2.6 kW (or 2,600 Watt) system in order to produce, on average, 8 kWh per day (240 kWh per month) in our example location with its average of 4 hours of insolation. Expect an installed cost of approximately $8-$15* per Watt; this system would cost about $21,000 - $39,000 installed.

* Costs vary by region and site.