How Many Solar Panels Do I Need? A Simple Way to Estimate

You don't need an engineering degree to get a rough system size estimate — here's the same basic logic installers use, simplified.

Step 1: Find your annual electricity usage

Your utility bill or online account typically shows kWh used per month or per year. If you only have monthly numbers, multiply your average month by 12 (or better, add up 12 actual months to account for seasonal swings in heating/cooling).

Step 2: Divide by your state's average sun hours

"Peak sun hours" is a measure of usable solar production hours per day, which varies by region — roughly 3.5-4.5 in the cloudier Northeast and Pacific Northwest, up to 5.5-6.5+ in the Southwest. Divide your annual kWh usage by (365 × your state's average sun hours) to estimate the system size in kW needed to offset 100% of your usage.

Example

A home using 11,000 kWh/year in a state averaging 5 peak sun hours: 11,000 ÷ (365 × 5) ≈ 6.0 kW system, before accounting for panel losses and system inefficiencies (typically add 10-15% to this raw number for a more realistic target).

Why most installers round up slightly

Panels lose some output to inverter conversion loss, wiring resistance, dust, and imperfect angle — typically 10-20% total system loss versus the theoretical maximum. Most installers build this into their sizing recommendation automatically, which is why a professional quote often comes in slightly larger than a simple back-of-envelope estimate like this one.

Should you size for 100% offset?

Not necessarily. In states with strong retail-rate net metering, sizing to fully offset your annual usage (or slightly over, to bank summer credits for winter) maximizes savings. In states with weaker export rates, sizing closer to your daytime usage — especially if paired with a battery — can be the more financially efficient choice. This is exactly the kind of decision worth a real conversation with an installer rather than a DIY estimate alone.