Solar Panel Calculator
Size a solar array from your daily energy use, peak sun hours, panel wattage and system losses.
How it works
system kW = daily kWh ÷ (peak sun hours × (1 − losses%)) · panels = ceil(system kW × 1000 ÷ panel watts)
A panel only makes its rated power during “peak sun hours,” so the array must be big enough that those hours cover a full day’s use even after losses from wiring, inverter conversion, heat and dust — typically around 20%. Dividing daily kWh by the productive sun hours gives the kilowatts of array needed; dividing that by each panel’s wattage and rounding up gives the panel count. Roof area is estimated at about 20 ft² per residential panel.
Worked example
For 30 kWh a day at 4.5 peak sun hours with 20% losses: 30 ÷ (4.5 × 0.8) = 8.33 kW. With 400 W panels that is 8.33 × 1000 ÷ 400 = 20.8, rounded up to 21 panels — roughly 420 ft² of roof.
Frequently asked questions
What are peak sun hours?
They are the equivalent hours per day the sun delivers 1,000 W/m² — not daylight hours. A location averaging 4.5 peak sun hours might have 10 hours of daylight but weaker morning and evening sun. Local solar maps and calculators publish this figure by region.
Why include system losses?
Panels are rated under ideal lab conditions. In the field, inverter conversion, wiring resistance, heat, dust, shading and panel aging shave off energy — commonly about 20% combined. Sizing without a loss factor leaves a system that falls short on real days.
Will this array zero out my bill?
It sizes the array to your average daily kWh, but production is seasonal and daily use varies. Net metering, battery storage and local sun all affect whether you truly reach net-zero. Treat the result as the starting design size for a detailed quote.
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Estimate for planning. Add a waste margin and confirm with your supplier and local codes.