Solar
Solar System Size Calculator
Required solar system size equals annual energy use divided by adjusted annual AC production per installed kilowatt. ZIP, orientation, shading, and losses change the production assumption.
Estimate required solar array kW and annual production from monthly energy use, ZIP, orientation, shading, and visible loss assumptions.
Calculated result
Required solar array size
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
Layer B · economics
Economics estimate
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State averages are editable prefills, not utility tariffs. This section is not tax, financial, or investment advice.
Data provenance: NLR PVWatts v8 grid awaiting its first private-key refresh; the visible peak-sun-hours fallback is active. EIA residential state averages updated 2026-08-13. ZIP centroids use the 2025 Census Gazetteer. No ZIP or calculator input is sent to those sources. Read the solar data provenance and limitations.
What this calculator returns
This page targets array nameplate size. Use How Many Solar Panels Do I Need when module count and roof area are the primary question.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Required array kW = annual energy use / adjusted annual AC production per installed kW.
- Eannual
- Annual electrical energy use or production, in kilowatt-hours.
- Parray
- Required or installed solar array nameplate power, in kilowatts DC.
- Y
- Estimated annual AC energy produced per installed kilowatt DC.
- PSH
- Peak sun hours per day from dated static data or the visible fallback input.
- f
- Visible orientation, shading, loss, reserve, or efficiency factor used by the selected tool.
Adjusted production uses the nearest static baseline when available or the visible peak-sun fallback, plus orientation, shading, and loss assumptions.
Worked example
| Input | Value |
|---|---|
| Monthly energy use | 900 kWh |
| ZIP Code | 80202 |
| Array orientation | South |
| Annual shading estimate | 10 % |
At 900 kWh/month and the default visible production assumptions, the required nameplate size is about 8.5 kW DC before site-specific modeling.
Reference table
| Panel rating | Panels per 6 kW DC | Nominal area assumption | Planning note |
|---|---|---|---|
| 350 W | 18 | 21.5 ft² each | More modules for the same array rating |
| 400 W | 15 | 21.5 ft² each | Default calculator example |
| 450 W | 14 | 21.5 ft² each | Verify actual dimensions and mounting clearances |
Frequently asked questions
How does monthly kWh determine array size?
The calculator annualizes monthly use and divides it by adjusted annual AC production per installed kilowatt.
Why does location change the required system size?
Available solar production per installed kilowatt varies by location, so the same energy target can require a different DC array size.
Why is this separate from the panel-count calculator?
This page focuses on required array kW; the panel-count page converts that requirement into whole modules and a nominal roof-area estimate.
Assumptions and limitations
- Planning estimate only; not a binding proposal or code-compliance determination.
- ZIP interpolation is not a per-address simulation.
- Actual production varies with weather, soiling, shading, degradation, and equipment.
- Obtain a licensed installer site assessment.
Method and sources
Review the solar data sources, calculation methodology, and electrical formulas for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .