"How many panels do I need" is the first question almost everyone asks, and it's also where installer quotes vary the most — not because the math is hard, but because the assumptions underneath it rarely get explained. Here's the actual logic.
Start with usage, not roof space
The right starting point is your annual electricity consumption in kWh, not your roof size. Your utility bill either shows this directly or shows a dollar amount you can convert using your rate ($/kWh). A system is sized to produce roughly that much energy over a year — not more, not less, unless you have a specific reason to target a different offset.
The 0.8 is a derate factor — it accounts for inverter conversion loss, wiring resistance, dust, and the fact that panels lose a little efficiency as they heat up in direct sun. Skipping this factor is the single most common reason DIY size estimates come in optimistic.
Sun hours matter more than most people expect
"Sun hours" doesn't mean daylight hours — it means peak sun hours, the equivalent number of hours per day at maximum (1,000 W/m²) intensity. A cloudy northern city might average 3 peak sun hours a day; a desert city might average 6. That's a 2x swing in system size for the exact same electricity usage, which is why national "average install size" numbers you see quoted online aren't that useful for your specific home. Check your own number with the Solar Irradiance Calculator before trusting any generic panel-count answer.
Converting system size into panel count
Once you have a target system size in kW, panel count is simple division:
Panel wattage varies by product — common residential panels today run 400-550W. A higher-wattage panel means fewer panels for the same total capacity, which matters if your roof space is tight. Run your own numbers, including panel wattage, sun hours and usage together, in the Solar Panel Calculator.
Where installer quotes commonly overestimate
Three patterns show up repeatedly in oversized quotes:
- Skipping the derate factor — quoting production as if the system runs at 100% of nameplate rating year-round, which no real system does.
- Using a generic national sun-hours average instead of your actual location, especially common in quotes generated by phone without a real site assessment.
- Targeting well over 100% offset without checking whether your utility's export rate makes that extra capacity worth it — see our net metering guide for why this matters.
Don't forget future load
If you're planning to add an EV, heat pump, or pool in the next few years, it's worth sizing (or at least pre-wiring) for that now rather than adding a second undersized array later. The EV Charging Load Calculator and Appliance Solar Load Calculator both show exactly how much extra capacity a specific future addition would need.
Frequently asked questions
Is more solar panels always better?
No. Beyond the point where production matches your usage, extra panels only pay off if your utility credits exported power at a rate close to what you pay for imported power — otherwise you're paying for capacity you won't fully monetize.
How accurate is a panel count from an online calculator?
Accurate enough for budgeting and comparing installer quotes — generally within 10-15% of a professional design — but a real design needs a site survey for shading, roof condition, and exact orientation.