How big a solar system your house really needs

How do you work out the size you need?

Use four steps. Take your last 12 months of electricity use in kilowatt-hours. Decide what share you want solar to cover, such as 70% or 100%. Divide by the yearly yield per kilowatt of panels at your address, which an installer's production model supplies. The answer is the system size in kilowatts.

The yield varies with location, tilt, direction and shade, so use the model, not a guess. The ideas behind it are in peak sun hours in Canada and how to read a solar production estimate. The broader counting method is in how many solar panels to power a house, and this page adds household types to it.

What does sizing look like for different households?

The numbers below are placeholders to show the method, not typical Canadian figures. Assume a yield of 1,200 kWh per kW per year in every case.

• Gas-heated house. Uses 7,500 kWh a year and wants 80% covered. Target output 6,000 kWh, divided by 1,200, gives a 5 kW system.
• Heat pump added. The same house adds 3,000 kWh a year for a heat pump. Use is 10,500 kWh, and 80% is 8,400 kWh, or a 7 kW system.
• Heat pump and electric vehicle. Add 2,500 kWh for the car. Use is 13,000 kWh, and covering 100% needs about 10.8 kW.

Check your own situation against a heat pump load calculation, and see the cold-climate heat pump guide for how much electricity heating can add. If you are planning electric additions later, size for them now, since adding panels later costs more per watt.

Will the system fit on your roof?

Panels take space. A modern residential panel is a bit under two square metres and, depending on its rating, a kilowatt of panels needs roughly five to six square metres of usable roof with allowance for setbacks and obstructions. On that rough basis, a 5 kW system needs about 25 to 30 square metres of clear roof and a 10 kW system about 50 to 60. Fire-code setbacks, vents, chimneys and dormers cut into the usable area, and a good installer will draw a layout.

If the roof is too small, higher-efficiency panels, a second roof face or a ground mount may solve it. Roof faces towards the south, east or west all produce, and the best panel direction explains the trade-offs. Shade from trees and neighbouring buildings can matter more than orientation.

Is a house with low energy use easier to size?

Yes, and it is cheaper to serve. A well-insulated house with efficient appliances needs a smaller system for the same share of its power. The R-2000 homes at Drake Landing in Okotoks were built to a high efficiency standard, which is part of why a modest collector area could carry most of their space heating; see the homes page and R-2000 homes explained. For an existing house, a home energy audit can show cheaper savings to take before buying panels.

Should you size for 100% of your use?

Not necessarily. Exports may be credited at a different rate than you pay for power, and your utility may limit system size to your annual consumption or a fixed cap. Covering 100% of annual use with panels still leaves you buying power in winter nights, since output and demand do not match by the hour or the season. A smaller system that you use mostly yourself can give a better return than a large one that mostly exports. Ask your utility about size limits and export credit rules, and read Alberta net metering explained if you live in Alberta. Storage can shift timing, as set out in solar battery storage in Canada.

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