Sizing a Solar System for Your House

How do you work out how many panels you need?

Start with your yearly electricity use, found on your bills in kilowatt-hours. Divide it by the expected annual yield per installed kilowatt, then divide the resulting system size by the wattage of one panel.

Worked example with assumed figures: a home uses 9,000 kWh a year. In a sunny area, take 1,250 kWh per kW per year. System size is 9,000 divided by 1,250, so 7.2 kW. With 400-watt modules that is 18 panels. In a cloudier region with 1,000 kWh per kW, the same load calls for 9 kW, or about 23 panels. Local yield figures are explained in peak sun hours in Canada.

Do you need to offset 100 percent of your use?

Not necessarily. Many homeowners aim for most of their annual use and no more, because export credits vary and oversizing may not be rewarded. Others plan for a future load, such as a heat pump or an electric vehicle, and size up. If your household will add either soon, size to the new load. The comparison between heating options is in heat pump vs furnace in Canada.

Rules on how large a system you may connect, and how surplus is credited, depend on your province and utility. For one example, read Alberta micro-generation regulation, then confirm the current terms with your utility.

Some households size to a budget instead of to the load. A smaller array of, say, six to eight panels can still trim a meaningful slice of the bill, and it avoids electrical upgrades that a larger system might trigger. The saving per dollar is often similar to a bigger system, because the first panels use the energy you consume directly at the full retail value. Ask the installer to quote two sizes and show the production and cost per kilowatt-hour for each, so the larger option has to earn its place.

How much roof area do panels need?

A typical residential module is roughly 1.7 to 1.9 metres by 1.1 metres, about 2 square metres each. Eighteen panels therefore need around 35 to 40 square metres of usable roof, plus setbacks from edges and ridges that building and fire codes may require. Chimneys, vents, dormers and skylights remove space.

If the best roof face is small, higher-efficiency modules pack more watts into the same area. If the roof cannot fit the required size, options include a second roof face (east or west), a garage or outbuilding roof, or a ground mount. The effect of orientation is covered in the best direction for solar panels.

What can make the estimate wrong?

• Shading from trees, chimneys or neighbouring buildings, which lowers yield per kilowatt.
• Roof pitch and orientation that differ from the assumption behind the yield figure.
• Your own use changing, up or down, after efficiency work or new loads.
• Inverter limits, since the panel array and the inverter are sized as a pair (see inverter types).

A professional design uses site-specific modelling instead of a rule of thumb, and the installer should show its assumptions in writing. The process is outlined in the solar panel installation process.

Does reducing use change the number of panels?

Yes, and efficiency is cheaper than generation. Cutting the load first lets you buy a smaller system. Low-energy houses show how far that can go: the 52 homes at Okotoks were built to the R-2000 standard, which meant a small heating load for the solar heat system to meet. The homes page describes how they were built. For an ordinary house, an audit and some air sealing can cut the size, and cost, of the array you need.

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