How to calculate how many solar panels you need

The reliable starting point is annual electricity consumption—not square footage or bedroom count. Then estimate how much energy one panel produces in your location after normal system losses.

Formula: Annual electricity target ÷ annual energy produced per panel = panels needed. Always round up to a whole panel.

Step 1: Find annual electricity use

Add twelve months of electricity usage in kWh. If you only have an average monthly number, multiply it by 12. To offset less or more than 100%, multiply annual use by your chosen offset percentage.

Step 2: Estimate annual output per panel

Multiply panel power in kilowatts by peak sun hours per day, 365 days and a performance factor. Watt My Roof uses 0.80 by default to allow 20% for temperature, inverter, wiring, soiling, mismatch and other practical losses.

400 W ÷ 1,000 × 5 peak sun hours × 365 × 0.80 = 584 kWh per panel per year.

Step 3: Divide and round up

A home using 9,924 kWh annually would require 9,924 ÷ 584 = 16.99 panels, rounded up to 17 panels. At 400 watts each, that is a 6.8 kW DC array.

What this simple formula leaves out

Professional production models use site-specific irradiance, roof orientation, tilt, shade, module specifications and monthly weather data. Utility limits, roof fit and economics may also change the final design. Use the formula to ask better questions and compare proposals—not to approve construction.