Solar Panel Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A rooftop solar system is usually sized backward from the electricity you want it to produce. This calculator starts with annual consumption and available sun, adjusts for the desired bill offset and a system-performance factor, then converts the required array size into a panel count and roof-area check.

What this calculator does

The Solar Panel Calculator estimates required array capacity in kW, number of panels, required panel area, and modeled annual output. It uses annual electricity consumption, equivalent solar hours per day, the percentage of consumption you want to offset, an environmental or system factor, panel wattage, panel area, and usable roof area.

How to use it

Enter annual electricity consumption and average daily solar hours for the site. Set the bill-offset target and system factor, then provide the wattage and area of the panel you are considering plus usable roof area. Use usable—not total—roof area because setbacks, shading, vents, orientation, and structural constraints can reduce the space available for modules.

How the calculation works

Raw array size is annual kWh ÷ (365 × solar hours). The calculator multiplies by the offset fraction and divides by the environmental factor to get required system kW. Panel count is rounded up from system watts ÷ panel watts. Required roof area = panel count × panel area, and modeled production reverses the same sun-hour and system-factor assumptions.

Example

With 7,500 kWh/year of consumption, 4 solar hours/day, 100% offset, and a 75% system factor, the calculator requires about 6.849 kW. Using 400 W panels means 18 panels after rounding up. At 1.9 m² each, they require about 34.2 m², which fits within a 50 m² usable roof input.

How to interpret the result

The system-size result is the modeled capacity needed to meet the selected energy-offset target under the entered solar resource and performance factor. The panel count is deliberately rounded upward, so expected production can slightly exceed the exact target. The roof-area status is a geometry check, not a structural or shading assessment.

Limitations and notes

Actual solar yield depends on location, tilt, azimuth, shading, temperature, inverter efficiency, soiling, snow, degradation, clipping, module mismatch, and local weather variability. Solar hours and the environmental factor compress many of those effects into simple inputs. The calculator does not design wiring, inverters, batteries, structural supports, or grid interconnection.