How Many Solar Panels Do I Need?

How to use this calculator

Enter five values. The tool returns installed DC array size after whole-panel rounding, panel count, a rough module-area allowance, and estimated annual production.

1. Monthly electricity use (kWh) — average 12 consecutive bills. EIA’s latest complete annual customer data report 865 kWh/month of grid purchases in 2024, but a national mean is not a substitute for the home’s bills (EIA). Grid purchases can understate total use at a home with behind-the-meter solar (EIA methodology note).
2. Peak sun hours per day — use the site’s average daily plane-of-array solar radiation under the intended tilt and azimuth. PVWatts V8 uses NSRDB typical-meteorological-year data where available. The prefilled 4.8 is only a scenario input.
3. Panel wattage (W) — enter the STC nameplate wattage on the quoted module datasheet. Do not infer panel dimensions from wattage.
4. System efficiency (%) — an aggregate multiplier for effects not already represented in the peak-sun-hours input. The prefilled 78% is editable and is not a PVWatts default.
5. Offset target (%) — desired annual generated energy divided by annual use. Values above 100% require a separate check of future load, export economics, and utility limits.

Formula used by the calculator

target_kWh        = monthly_kWh × 12 × (offset_percent / 100)
required_DC_kW    = target_kWh / (peak_sun_hours × 365 × efficiency_decimal)
panel_count       = ceil(required_DC_kW × 1000 / panel_wattage_W)
installed_DC_kW   = panel_count × panel_wattage_W / 1000
annual_output_kWh = installed_DC_kW × peak_sun_hours × 365 × efficiency_decimal
area              = panel_count × 1.95 m² × 1.08

The displayed system size is the installed size after rounding up to whole panels, not the unrounded required size. The area equation is only the component’s fixed allowance; it does not use the selected module’s dimensions.

Reproducible verification example

Use the prefilled US values: 900 kWh/month, 4.8 peak sun hours, 400 W panels, 78% efficiency, and 100% offset.

• Target energy: 900 × 12 × 1.00 = 10,800 kWh/year
• Required DC size: 10,800 ÷ (4.8 × 365 × 0.78) = 7.903 kW
• Panel count: ceil(7.903 × 1,000 ÷ 400) = 20
• Installed array: 20 × 400 ÷ 1,000 = 8.0 kW DC
• Annual output: 8.0 × 4.8 × 365 × 0.78 = 10,932 kWh after display rounding
• Area output: 20 × 1.95 × 1.08 = 42.12 m² = 453 ft² after display rounding

The installed array produces slightly more than the target in this deterministic scenario because panel count rounds up.

Reference table for the default scenario

These values use 4.8 peak sun hours, 78%, 400 W panels, and 100% offset. “Installed array” includes whole-panel rounding; area uses 1.95 m² per panel plus the fixed 8% multiplier.

Inputs that need site-specific evidence

Solar resource, tilt, azimuth, and shade

The formula has no separate tilt, azimuth, or shading fields. Represent the intended array plane in the peak-sun-hours value, ideally from a site run. Use the solar panel tilt calculator only as a preliminary angle heuristic. NREL warns that PVWatts predictions contain assumptions and do not capture site characteristics absent from the inputs (NREL PVWatts caution).

Panel wattage and physical layout

Higher nameplate wattage can reduce whole-panel count for the same energy target, but module area is not proportional to wattage. Compare count and layout using each quoted module’s electrical and mechanical datasheets. The calculator omits roof shape, obstructions, access paths, setbacks, drainage, and structural capacity.

Future loads and utility rules

Add a future EV, heat pump, or water heater from its own documented annual-kWh estimate, not a generic allowance. Export credits, time-of-use periods, fixed charges, and interconnection limits are tariff-specific and can change. This calculator does not determine an economically optimal offset.

For federal tax planning, the IRS states that the Section 25D residential clean-energy credit is not available for property installed after December 31, 2025 (IRS); obtain advice for payment, installation and eligibility facts.

Limitations and common errors

• Using one bill: use 12 consecutive months and separately document known future load.
• Calling 4.8 or 78% local data: both are editable example defaults.
• Double-counting losses: if an input already expresses delivered AC kWh per installed kW, applying another 78% factor counts losses again.
• Treating area output as a roof plan: DOE notes that rooftop projects generally require local permitting and inspection (DOE).
• Equating annual energy offset with bill offset: exports, fixed charges, and tariff periods can materially change dollar savings.

This is a constant-average energy balance. It does not run hourly weather, temperature, inverter clipping, shading, snow, curtailment, outages, battery dispatch, or tariff intervals. It does not verify structural capacity, electrical design, fire access, permits, or interconnection. Use it for an initial scenario, then compare a site-specific model and a qualified installer’s design.

Sources

• NREL PVWatts V8 API — current model inputs, ranges, and NSRDB dataset description
• NREL PVWatts calculator — web loss starting point and model limitations
• EIA 2024 residential bills and grid purchases — 2024 US monthly consumption and price statistics
• IRS Residential Clean Energy Credit — current Section 25D end-date guidance
• DOE rooftop permitting and inspection — scope of local safety review