How many solar panels do I need for my home?

Enter your monthly electric bill or target kWh, roof orientation, available roof area, and panel wattage. The calculator divides your energy need by per-panel production based on local sun hours to give a panel count and total system size in kW.

How do I calculate solar panel system size?

System size in kW equals monthly kWh usage divided by 30 times peak sun hours per day. The calculator multiplies this by 1,000 and divides by panel wattage to get the number of panels you need, then factors in your available roof area.

How much money can solar panels save me?

The estimator calculates annual kWh production from your system size and local sun hours, then multiplies by your electricity rate to show estimated yearly savings. It also displays the total system size in kilowatts and the panel count that fits your roof.

Energy

Solar Panel Estimator

Estimate solar panel count, system size in kW, annual production, and savings based on roof area and sun hours.

A good solar estimate starts with your electric use, not a sales pitch. This solar panel calculator helps homeowners translate annual kWh usage, peak sun hours, panel wattage, and real-world losses into a practical array size. Use it to plan a rooftop system before you compare installer quotes.

How it works

Enter your annual electricity use in kWh, your local peak sun hours, a derate factor, and the wattage of the solar panels you're considering. The core sizing formula is: system size kW = annual kWh usage / (365 × peak sun hours × derate). A common planning derate is about 0.8, which allows for inverter losses, heat, wiring, soiling, and other real-world conditions. Convert kW to watts by multiplying by 1,000. Then estimate panel count with: panel count = system watts / panel watts. With 400W panels, a 6,800W system needs 17 panels before rounding. Annual production is: system kW × 365 × peak sun hours × derate. Roof space is: rounded panel count × the footprint of your chosen panel.

Worked examples

Example 1: Moderate usage, steady sun Illustrative inputs: 9,000 kWh per year, 4.5 peak sun hours, 0.8 derate, and 400W panels. System size = 9,000 / (365 × 4.5 × 0.8) = 6.85 kW. Panel count = 6,850W / 400W = 17.13, rounded up to 18 panels. The rounded 18-panel array is 7.2 kW. Estimated annual production = 7.2 × 365 × 4.5 × 0.8 = 9,461 kWh before site-specific shading and roof-angle adjustments. Roof space needed = 18 × your selected panel footprint.
Example 2: Higher usage, stronger sun Illustrative inputs: 12,000 kWh per year, 5.0 peak sun hours, 0.8 derate, and 400W panels. System size = 12,000 / (365 × 5.0 × 0.8) = 8.22 kW. Panel count = 8,220W / 400W = 20.55, rounded up to 21 panels. The rounded 21-panel array is 8.4 kW. Estimated annual production = 8.4 × 365 × 5.0 × 0.8 = 12,264 kWh before site-specific shading and roof-angle adjustments. Roof space needed = 21 × your selected panel footprint.
Example 3: Lower usage, fewer sun hours Illustrative inputs: 6,000 kWh per year, 3.8 peak sun hours, 0.8 derate, and 400W panels. System size = 6,000 / (365 × 3.8 × 0.8) = 5.41 kW. Panel count = 5,410W / 400W = 13.53, rounded up to 14 panels. The rounded 14-panel array is 5.6 kW. Estimated annual production = 5.6 × 365 × 3.8 × 0.8 = 6,214 kWh before site-specific shading and roof-angle adjustments. Roof space needed = 14 × your selected panel footprint.

FAQ

What number should I use for annual kWh usage?

Use the total kWh from the last full year of electric bills if you have it. Solar sizing works best from actual usage because it reflects your home, appliances, heating and cooling habits, and any electric vehicle charging.

Why does peak sun hours matter so much?

Peak sun hours estimate how much usable sunlight your panels receive in a day. The same home may need a different system size in a sunnier location than it would in a cloudier or more shaded location.

What is the derate factor in the calculator?

The derate factor adjusts the ideal solar math for real-world losses. A planning value around 0.8 is often used in this calculator method to account for heat, inverter conversion, wiring, dirt, and other system losses.

Does the calculator replace a professional solar quote?

No. It gives you a clear first estimate, which is useful before talking to installers. A final design should check roof orientation, tilt, shading, panel layout, local rules, utility requirements, and equipment choices.