Solar Panel Calculator
From a power bill to a panel count.
Solar Panel Calculator
Array kW = annual kWh / (365 x peak sun hours x (1 - losses)); panels = ceiling(array W / panel W).
Calculated instantly
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What This Calculator Does
The Solar Panel Calculator sizes an array from annual electricity use, peak sun hours, panel wattage, and system losses. It returns the array size needed, the number of panels after rounding, the roof area, and the generation the rounded array should produce.
How It Works
Annual kWh is divided by 365 to get daily use, then by peak sun hours and by the efficiency remaining after losses. That gives the direct current array size in kilowatts. Multiplying by 1,000 and dividing by panel wattage gives a fractional panel count, which is rounded up. The installed array size and estimated generation are recalculated from the whole panel count so the numbers agree.
Example
A home using 10,000 kWh a year in a 4.5 peak sun hour location with 15 percent losses needs 10,000 / (365 x 4.5 x 0.85) = 7.16 kW. At 400 W per panel that is 18 panels, an installed array of 7.2 kW, roughly 10,052 kWh a year, and about 324 sq ft of roof area.
Formula
annual kWh / (365 x peak sun hours x (1 - losses))ceiling(array watts / panel watts)panels x panel watts x sun hours x 365 x (1 - losses) / 1000panels x area per panelReading the Results
Array size needed
The direct current size before rounding to whole panels. It is a planning target, not a hardware configuration.
Panels needed
Whole panels at your wattage, rounded up. Panel dimensions and wattage vary, so confirm the actual product before committing to a layout.
Estimated annual generation
What the rounded array should produce under the assumed sun hours and losses. Real output varies by weather, shading, tilt, and inverter clipping.
Sources and References
National Renewable Energy Laboratory - PVWatts Calculator
Hourly PV energy production model
A location-specific simulation that accounts for weather, tilt, and orientation better than a hand calculation.
US Department of Energy - Solar Energy Technologies Office
Solar system sizing and performance basics
Background on capacity, capacity factor, and why real output differs from nameplate rating.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How many solar panels do I need for a normal house?
It depends on consumption, location, panel wattage, and roof orientation. A low-use home in a sunny climate may need 10-14 panels; a high-use home in a cloudy region may need 25 or more. Use your own annual kWh rather than a national average.
What are peak sun hours?
A measure of solar energy received per day expressed as hours of full-strength sun. Four peak sun hours means the day delivered the equivalent of four hours at 1,000 W per square metre. It is not the length of the day, and it changes with season.
What system losses should I assume?
Ten to twenty percent is a reasonable range for a residential system, covering inverter conversion, wiring, soiling, temperature, and module mismatch. Shade, snow, and an undersized inverter can push losses higher. A site-specific simulation is more accurate.
Does the calculator account for shading and roof orientation?
No. It assumes the array faces the sun well and is unshaded at the peak. A north-facing roof, a chimney shadow, or a steep pitch can cut output substantially. Use a shade study or PVWatts with your actual tilt and azimuth before sizing.
Should I size to 100 percent of my use?
That is a common starting point, but the best size depends on net metering, export rates, battery economics, and budget. In some tariffs, offsetting evening load with a battery is worth more than adding panels that export at a low rate. Model the tariff, not just the kWh.