Solar Panel Calculator: 1200 kWh / month
Estimate the solar system size and number of panels to cover your electricity use, from your monthly kWh.
// Bill of materials
Costs are rough, materials only — edit the price to match your quote.
Pre-filled for a 1200 kWh / month. Change any value to recalculate, or start from scratch.
How it's calculated
Daily use = monthly kWh ÷ 30. System size (kW) = daily use ÷ (peak sun hours × efficiency). Panels = system watts ÷ panel wattage, rounded up. Roof area assumes about 18 sq ft per panel.
Worked example — 1200 kWh / month
- Monthly electricity use
- 1200 kWh
- Peak sun hours/day
- 4.5 hrs
- Panel wattage
- 400 W
- System efficiency
- 0.8 ×
- Installed price
- 3 $/W
Nearby sizes
Tips for buying solar panel
- Start with a full year of power bills — usage swings a lot between summer and winter.
- Peak sun hours aren't daylight hours; they're the equivalent hours of full-strength sun for your region.
- Real systems lose 15–25% to heat, wiring and the inverter — that's the efficiency factor.
- Check your roof's orientation and shading; north-facing or shaded roofs need more panels.
- Cutting usage first (LEDs, efficient appliances) is cheaper per watt than adding panels.
- Get local quotes — installed price per watt varies widely by market and incentives.
Frequently asked questions
How many solar panels do I need?
For 900 kWh/month at 4.5 peak sun hours and 400 W panels, about 21 panels — roughly an 8.3 kW system. Sunnier regions need fewer.
What size solar system do I need?
System kW = (monthly kWh ÷ 30) ÷ (peak sun hours × efficiency). The calculator does this and converts it to panels and roof area.
What are peak sun hours?
The number of hours per day of full-strength (1,000 W/m²) sun your location effectively gets — about 4 in the north, 5.5 in the southwest.
How much roof space do I need?
About 18 sq ft per panel. A 21-panel system needs roughly 380 sq ft of unshaded roof.