Sizing
How Big a Solar System Do You Actually Need?
May 1, 2026
At a glance
The right system size is not the biggest one your roof can hold — it is the one matched to how much power you actually use. Oversizing wastes money; undersizing leaves savings on the table. Start with your annual kilowatt-hours, not your roof.
The most common mistake in a solar quote is sizing to the roof or to a round number instead of to the home's real energy use. Here is how to think about it properly.
Start with your annual usage
Pull a year of electricity bills and find your total kilowatt-hours. That number, not your roof area, is the anchor. A rough rule in much of Canada: each 1 kW of panels produces about 1,000 to 1,300 kWh a year (lower on the cloudy BC coast, higher on the sunny Prairies), so a home using 9,000 kWh might need roughly 7 to 8 kW — adjusted for shading and orientation.
How your incentive changes the target
Sizing also depends on your program. Under a net-metering approach you can size to cover most of your yearly usage and bank summer credits for winter. Under Ontario's rebate, which funds load-displacement systems, the system is sized to serve your own home rather than to export — a different target for the same house.
Not sure which path applies to you? See every solar rebate by province.
Why oversizing backfires
An oversized system costs more upfront and, in many provinces, you are not paid well (or at all) for large amounts of exported power. Past the point where you offset your own use, each extra panel earns less. Bigger is not automatically better.
Should I size for future EV or heat pump use?
If those are coming soon, yes — size for the higher usage so you are not adding panels later. Just be realistic about the timeline.
What if my roof can't fit the ideal size?
Then the roof sets a ceiling. Higher-efficiency panels help you fit more capacity into limited space.
Can I add panels later?
Sometimes, but it is usually cheaper to size correctly once than to expand a system afterward.
Rules of thumb only — real sizing depends on your usage, roof, and program. This is general information, not financial advice.
