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. If you only know the bill amount, use the electric-bill solar calculator.

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.

Step 4: Verify what the formula cannot see

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. Read the full Watt My Roof methodology and use the formula to ask better questions—not to approve construction.

Frequently asked questions

What is the formula for calculating solar panels needed?

Divide your annual electricity target by the estimated annual energy produced by one panel, then round up. Watt My Roof estimates panel output as panel kilowatts multiplied by peak sun hours, 365 days and a 0.80 performance factor.

Should I calculate from monthly kWh or my electric-bill amount?

Use monthly or annual kWh whenever possible. Dollar amounts include rates and fixed charges that do not directly measure energy. If kWh is unavailable, estimate it from the energy portion of the bill and the price per kWh.

Why does Watt My Roof use a 20% system-loss allowance?

The 0.80 performance factor is a transparent planning allowance for temperature, inverter, wiring, soiling, mismatch and other practical losses. A site-specific production model may use different losses.

Why must the result be rounded up?

Solar panels are whole units. Rounding down would leave the modeled system below the selected annual energy target under the same assumptions.