Solar
Solar Panel Calculator: How Many Panels Do You Need?
A home using 900 kWh per month needs about 22 400 W panels under the visible example assumptions. Your result changes with local production, panel wattage, roof direction, shade, and system losses. Production inputs follow the dated NLR PVWatts, EIA, and Census-derived sources documented on the Solar Data Sources page.
Estimate how many solar panels you need, required system size, annual production, and nominal panel area from monthly energy use or a bill, ZIP, panel watts, roof 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. Calculator lookups use checked-in data rather than sending inputs to those source agencies; information submitted through quote or email forms is handled under the Privacy policy. Read the solar data provenance and limitations.
What this calculator returns
Use the whole-panel count as a planning answer and the required array kW as the sizing basis. The nominal panel area does not include setbacks, access paths, spacing, obstructions, or a roof-layout review.
What changes the number of solar panels you need
Monthly kWh is the strongest starting input. The same household usage can require a different panel count when local solar production, panel wattage, orientation, shade, or other losses change.
- Energy use
- Use the last 12 months of utility kWh when available; a single bill can miss seasonal heating or cooling.
- Location and roof
- ZIP selects a nearby production baseline, while orientation and shade adjust the usable output estimate.
- Panel model
- Enter the exact module wattage and use its datasheet dimensions for a real roof layout.
- Whole panels
- The result rounds up because a design cannot install a fraction of a module.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Annual use = monthly kWh × 12; required kW = annual use ÷ adjusted annual production 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.
Adjusted production uses the nearest static location baseline when available, otherwise the visible peak-sun-hours fallback, then applies orientation, shading, and other loss assumptions. Panel count always rounds up to a whole module.
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
| Monthly electricity use | Annual electricity use | Estimated 400 W panels | Nominal panel area at 21.5 ft² each |
|---|---|---|---|
| 500 kWh | 6,000 kWh | 12 | 258 ft² |
| 750 kWh | 9,000 kWh | 18 | 387 ft² |
| 900 kWh | 10,800 kWh | 22 | 473 ft² |
| 1,000 kWh | 12,000 kWh | 24 | 516 ft² |
| 1,200 kWh | 14,400 kWh | 29 | 623.5 ft² |
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.
How do I calculate how many solar panels I need?
Multiply average monthly electricity use by 12, divide annual use by the estimated annual production of one installed kilowatt, convert the required kilowatts to watts, divide by panel wattage, and round up to a whole panel.
How many solar panels do I need for 1,000 kWh per month?
With the page's example assumptions of 4.5 peak sun hours, south orientation, 10% shading, 14% other losses, and 400 W panels, 1,000 kWh per month requires about 24 panels. Change location, shade, orientation, losses, and module wattage for your site.
How much roof space will the panels need?
The result includes nominal module area from a planning area per panel. A real layout needs exact module dimensions plus setbacks, access paths, spacing, roof edges, vents, obstructions, and structural review.
Does the calculator send my ZIP to NREL or another source agency?
The calculator resolves ZIP centroids from checked-in Power Totals data rather than sending the lookup to NREL or another source agency. If you submit a quote or email form, that submitted information is collected and handled as described in the Privacy policy.
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 provenance. Content release .
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