Required DC capacity and rounded panel count
Estimated array size meets an average energy target under the entered solar assumptions. Panel count rounds capacity upward. It does not guarantee hourly supply, winter output, roof suitability, utility approval, or elimination of the electricity bill.
For forward generation estimates from a chosen array capacity, the Solar Energy Calculator models the corresponding solar energy task.
The formula
Peak sun hours express solar resource as equivalent hours at reference irradiance, not the number of daylight hours. The entered loss fraction reduces the simplified array output. Panel rounding can produce more installed DC capacity than the required estimate.
Worked example
For 30 kWh daily use, a 100% offset target, 4 peak sun hours, and 20% losses, estimated capacity is 30/(4 × 0.8) = 9.375 kW DC. With 400 W panels, 24 panels provide 9.6 kW DC.
How to use this calculator
- Enter average daily electricity use, average daily peak sun hours using the units shown.
- Set system output losses, electricity offset target, individual panel rating.
- Calculate and review estimated required array alongside the supporting quantities.
Annual averages hide seasonal variation
Daily use and peak sun hours should represent compatible averaging periods. An annual energy offset can still include imports at night and exports during sunny hours. Shade, orientation, roof tilt, weather, and inverter clipping require a location-specific production model. A high annual offset is not the same as operating independently from the grid.
From estimated capacity to a design
Use this calculation for an initial size range, then check an hourly model and actual roof or site constraints. Panel dimensions, setbacks, inverter selection, string voltage, interconnection, and structural capacity are separate design decisions. Batteries store energy but do not create the solar production assumed here; storage sizing needs a load and autonomy plan.
Assumptions & limitations
What this calculation assumes
- Average generation follows peak sun hours and one combined output-loss factor.
What to keep in mind
- Excludes location weather data, shading analysis, hourly matching, storage, and installation design.
Common questions
Are peak sun hours just daylight hours?
No. They represent equivalent full-reference irradiance hours. Local solar-resource estimates should provide the value.
Does 100% offset remove my electricity bill?
Not necessarily. Fixed charges, import/export prices, timing, and utility rules affect the bill even when annual energy balances.
Why is rounded capacity larger than required capacity?
Panels are counted as complete units. Rounding the panel count upward adds some DC capacity above the arithmetic estimate.
Sources & further reading
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