How big should your solar system be?
System size is where solar gets sold by the square foot instead of by the bill. The right size comes from three things: how much electricity you actually use, what your utility pays for the surplus, and how much good roof you really have. Get those three right and the kilowatt number mostly chooses itself.
Start with the bill, not the roof
Annual kilowatt-hours from 12 months of bills is the denominator. Seasonal patterns matter too: a home that peaks with summer cooling matches solar production; a home that peaks with winter heating exports its summer surplus. The same annual usage can justify two different sizes.
Exports are worth what the tariff says
Under full retail net metering, an exported kilowatt-hour is worth roughly a consumed one, and sizing near full annual usage makes sense. Under net billing or low export rates, every oversized kilowatt produces cheap exports. In those territories, size toward what the home uses while the sun is up, and treat remaining roof as future capacity, not a sales target.
The roof votes last
Shade, orientation, obstructions, and roof age cap what is sensible. A smaller array on the good plane beats a bigger array spread across poor ones, and any array over a tired roof inherits the roof problem. Size to the roof you have after the roof conversation, not before it.
A worked sizing check you can repeat on your bills
Take twelve months of bills and add the annual kilowatt-hours. The example on this page uses a home consuming 10,800 kWh a year in a climate producing about 1,400 kWh per kW each year. Dividing usage by production gives roughly 7.7 kW for a full annual offset on paper. That arithmetic is the starting point every installer should show you, with your production factor named and its tool identified, before any roof measurement enters the conversation.
Then ask what the last kilowatts are for. Under full retail net metering, exported energy is worth roughly what consumed energy costs, so sizing near full annual usage can make sense. Under a low export rate, the top slice of a big system produces mostly cheap exports. In the example, a 6 kW system sized toward daytime load can pay back years sooner than the 7.7 kW full-offset design while leaving roof room for a future addition. Neither size is virtuous on its own; the tariff decides which one your money should buy.
Finish with the roof vote. Shade, orientation, obstructions, and roof age cap what is sensible, and a smaller array on the good plane beats a larger array spread across poor ones. Price one smaller system alongside every recommended system. The comparison is information an honest installer expects to provide, and it tells you whether the recommended size came from your bill or from your roof area.
- Size from twelve months of bills, not one summer month.
- Ask which kilowatts serve daytime load and which only export.
- Price a smaller system next to every recommended system.
Worked example
A home using 10,800 kWh a year in a 1,400 kWh-per-kW climate needs about 7.7 kW to offset its full annual usage on paper. Under retail net metering, that full offset is roughly rational. Under a low export rate, the last 2 kW produces mostly cheap exports, and a 6 kW system sized to daytime load can pay back years sooner while leaving roof room for the future.
The trap most people miss
Sizing to 100% of annual usage because the phrase sounds complete, in a territory where the exported half of that production is paid at a fraction of retail.
Checklist
- Get 12 months of bills, not one summer month, and size against annual usage.
- Ask for the production estimate by month, not just per year.
- Price one smaller system alongside the recommended one. The comparison is information.
- If exports pay poorly, size toward self-consumption and ask for the math, not maximum roof coverage.
Common questions
Can I start small and add panels later?
Usually, within inverter and panel compatibility limits, and installers often design for it. It costs less to leave conduit, roof layout, and electrical capacity ready than to retrofit them. Ask for expansion readiness in writing if growth is likely.
What does 100% offset actually mean?
It means the system generated more than the home used over the year. Whether that is valuable depends completely on what the utility pays for the extra. At retail credit it is fine; at wholesale export prices it is expensive generosity.
Why does my installer want to cover the whole roof?
Because system size is easy to sell by area, while the right size comes from annual usage, the export rate, and the good roof you actually have. Under a low export rate, extra panels mostly produce cheap exports. Ask for the sizing arithmetic from your bills, then compare a smaller design priced the same day. The roof should vote last, not first.
What if my usage grows later, with an EV or a heat pump?
Known future load is a legitimate reason to size up or to build expansion-ready. Ask for conduit, roof layout, and electrical capacity that leave room to add panels, and get that readiness in writing. Guessing at panels today for load you may never add ties up money now; designing the roof so additions stay cheap keeps both options honest.
Sources and verification
- NREL PVWatts Calculator: Site-specific production modelling by location, tilt, azimuth, and system size. Checked October 4, 2026.
- U.S. Energy Information Administration: Residential electricity price and usage context. Checked October 4, 2026.
Production factors here are planning examples. Your roof's estimate should come from a site-specific model such as NREL PVWatts with your tilt, azimuth, and shade, and your usage from a full year of bills.