Solar Panel Count Calculator
How many panels, how big a system, how much roof — from your electric bill’s kWh, a panel wattage, and honest sun hours. Nothing else, on purpose.
Count and size only — deliberately. Production at your roof needs a site-specific PVWatts run; savings and payback need your current tariff and the NEM 3.0 export rules, which change. This page prints neither, on purpose.
How the math works
Daily use = monthly kWh ÷ 30. System size needed = daily kWh ÷ (peak sun hours × (1 − losses)). Panels = that size × 1,000 ÷ panel wattage, rounded up. The sun-hours preset is 5.4/day for Long Beach — the NREL PVWatts class figure for the coastal LA basin (checked November 2026) — and it is an input, because your roof’s orientation and shade are yours, not a preset’s.
This page deliberately stops at count and size. A production guarantee needs a site-specific PVWatts run; a savings or payback figure needs your current tariff and the NEM 3.0 export rules, which change. Any site printing your payback from three inputs is guessing with your money.
A worked example
A 900 kWh/month home with 400 W panels at 5.4 peak sun hours and 20% system losses: 30 kWh/day ÷ 4.32 effective hours = 6.94 kW needed → 18 panels, a 7.2 kW system, about 933 kWh/month of modeled production, and 324 sq ft of roof at 18 sq ft per panel.
Questions from the field
How many solar panels do I need for 900 kWh a month? At Long Beach sun hours with typical losses, about 18 modern 400 W panels (a 7.2 kW system). Shade, a north roof, or an older panel wattage moves the count — every one of those is an input here.
What are peak sun hours? The equivalent hours per day of full-strength sun (1,000 W/m²). Long Beach averages about 5.4 on the NREL PVWatts basis. Winter months run lower; this is an annual average.
Why no savings or payback number? Because payback depends on your tariff, NEM 3.0 export rates, and tax rules that change — none of it is pure math. Count and size are. Get the dollar figures from a site-specific proposal.
How much roof does a panel take? About 17–18 sq ft for a residential panel. The input defaults to 18; your panel’s spec sheet wins.
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