Solar
How Many Solar Panels Do I Need Calculator
Solar panel count equals the required array watts divided by the selected panel rating, rounded up. Required array size comes from annual energy use divided by estimated annual production per installed kilowatt.
Estimate solar system size, panel count, annual production, and roof area from energy use, ZIP, panel watts, orientation, and shading.
Calculated result
Estimated solar panel requirement
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
Layer B · economics
Economics estimate
Loading dated rate configuration…
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
The physics layer returns a complete production and roof-area estimate without incentives or policy data. Treat the economics layer separately and replace the state rate with your actual tariff.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Required kW = annual kWh use / annual kWh produced per installed kW; panel count = ceiling(required kW × 1,000 / panel watts).
- 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.
Production per kW uses the nearest static location baseline when available, otherwise the visible peak-sun-hours fallback, then applies orientation, shading, and loss assumptions.
Worked example
| Input | Value |
|---|---|
| Start from | Monthly energy use |
| Monthly energy use | 900 kWh |
| Monthly bill | 165 $ |
| ZIP Code | 80202 |
At 900 kWh per month, 4.5 peak sun hours, 14% other losses, south orientation, 10% shading, and 400 W panels, the estimate is about 8.5 kW or 22 panels before site-specific review.
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 much roof space will the panels require?
The result uses the entered module dimensions and whole-panel count to report nominal module area before setbacks, access paths, gaps, and roof obstructions.
How does shading affect panel count?
More entered shading reduces modeled retained production, so more nameplate capacity may be needed for the same annual energy target.
Does Power Totals send my ZIP to NREL or another service?
No. The browser resolves ZIP centroids and reads checked-in data files from Power Totals; no ZIP or calculator input is sent to a third party.
Should I use my electricity bill or kWh use?
Monthly kWh is the stronger physics input. If you start from a bill, the calculator divides it by the visible editable rate to estimate energy use.
Assumptions and limitations
- Planning estimate only; not a binding proposal or code-compliance determination.
- The ZIP selects a nearby static climate point, not a per-address simulation.
- Actual production varies with weather, soiling, shading, degradation, roof geometry, and equipment.
- Obtain a site assessment from a licensed installer.
- Economics are not tax, financial, or investment advice; state average rates differ from utility tariffs.
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 .