How many solar panels should I install?

Solar panels can supplement other sources of heat to reduce your overall heating costs and lower your contribution to greenhouse gas. While it is theoretically possible to design a system that could provide all your heating needs, it may not be economically practical because of the need to install large amounts of thermal storage for those times when the sun does not shine.

In northern climates we receive the least hours of sun when we need it the most. If we design for these conditions we will end up with heat we don’t know what to do with in the spring and fall. The following will assist you in estimating how many panels you should start with. This is based on our experience in testing our systems.

Let’s assume you want to heat your workshop. How many panels should you use?

The quantity of heat loss is a function of the building area, the temperature difference between indoor and outdoor and the resistance value R of the insulation and wall material. Other factors might include leakage and windows. We will try to keep it simple.

The WSE heat loss calculator below will help you determine how many panels you should install. This is an approximate value only.

Insert R value. If you are not sure of this, assume 12 for 4 inch walls or 20 for 6 inch walls if it is well insulated.

Insert the dimensions of your building. Choose feet or metres.

Insert desired temperature difference. This is the number of degrees above ambient that you would like to achieve in the building. Choose C or F.

Insert number of hours of daily sunshine for your area. You can obtain average numbers from the weather office or you can just take a guess. We suggest using a number that reflects operation in October-November or February-March instead of December-January.

For those of you engineer types out there who are busy checking my calculations, please note that I have included an arbitrary correction factor for air leakage.

To arrive at some design heat loss, we must correct for the hours of sun. This will mean that the temperature will overshoot the design conditions when the sun is shining and fall below design conditions when it is not. This fluctuation can be reduced by installing thermal storage if desired. If you happen to have in-floor heating and a concrete floor, the thermal storage is already there.

It is recommended that you install solar panels that will provide about 50% of design loss to start with. Remember that it is very simple to add more panels in the future.