Home Solar Panel Calculator
Calculate how many solar panels you need to cover your home's electricity usage based on your location's sun hours, panel wattage, and system efficiency. Use it when planning a residential solar installation or getting quotes.
About this calculator
The number of solar panels needed depends on annual energy demand, available sunlight, panel capacity, and system losses. The formula is: Panels = ceil((monthly_usage × 12) / (365 × sun_hours × (panel_watts / 1000) × system_efficiency)). Monthly usage multiplied by 12 gives annual kWh demand. Each panel produces panel_watts / 1000 kW of rated power. Multiplying by daily peak sun hours and 365 days gives the annual kWh output of a single panel before losses. System efficiency (typically 0.75–0.85) accounts for real-world losses including inverter inefficiency, wiring resistance, soiling, and temperature derating. Dividing demand by single-panel output and rounding up gives the minimum panel count. Peak sun hours — not daylight hours — represent hours per day when irradiance averages 1,000 W/m², and range from about 3.5 (Pacific Northwest) to 6.5 (Arizona desert).
How to use
A home uses 900 kWh/month, has 5 peak sun hours per day, plans to use 400-watt panels, and expects 80% system efficiency. Annual demand: 900 × 12 = 10,800 kWh. Single-panel annual output: 365 × 5 × (400/1000) × 0.80 = 365 × 5 × 0.4 × 0.8 = 584 kWh/panel/year. Panels needed: ceil(10,800 / 584) = ceil(18.49) = 19 panels. So this home would need at least 19 × 400 W = 7.6 kW of installed solar capacity.
Frequently asked questions
How do I find the peak sun hours for my location when using the solar panel calculator?
Peak sun hours are available from the National Renewable Energy Laboratory's (NREL) PVWatts tool and solar irradiance maps, which provide city-level and ZIP-code-level data across the U.S. As a rough guide, the Southwest averages 5.5–6.5 peak sun hours, the Midwest 4–5, and the Northeast 3.5–4.5. Your solar installer will use precise data from NASA's satellite irradiance database for your exact roof orientation and tilt angle, which can shift the effective sun hours by 10–20% compared to a flat, south-facing surface.
What system efficiency should I use in the solar panel calculator?
A realistic system efficiency for a grid-tied residential solar installation with a string inverter is 75–80% (enter 0.75–0.80). Systems using modern microinverters or DC optimizers typically achieve 80–85% due to reduced mismatch and shading losses. The main losses come from inverter conversion (3–5%), temperature derating on hot days (5–10%), soiling and dust (2–5%), and wiring resistance (1–2%). Using 0.80 is a conservative and commonly accepted default for preliminary sizing.
Why does rounding up the panel count matter in a solar system design?
Solar panels must be purchased and installed in whole units, so any fractional result must be rounded up to ensure your system meets or exceeds your energy demand. Installing one fewer panel than the calculation requires means your system will produce slightly less electricity than you consume annually, leaving a shortfall you will pay the utility to cover. Rounding up also provides a small buffer for days with below-average sun, panel degradation over time (typically 0.5% per year), and potential increases in your household's electricity usage.