Solar Panel Calculator
Your estimate
- Yearly production
- 11,314 kWh
- Panel area
- 441.06 ft²49.01 yd² · 40.98 m²
- Estimated cost
- $23,520
System size and number of panels to cover your electricity use.
Updated 2026-10-20 · Based on: Ohm’s law, NEC Chapter 9 Table 8, 210.20 and 310.16; DIN VDE 0100-520 for metric.
How to use the Solar Panel Calculator
- Enter your monthly electricity use in kWh (average it over a year of bills).
- Enter the peak sun hours for your area: about 3.5–4 in the Northeast and Northwest, 4.5–5 in the South and Midwest, 5.5–6.5 in the Southwest.
- Enter the panel wattage you plan to buy (most home panels are 400–450 W).
- Leave 18 % for losses (inverter, wiring, heat, dirt) and 100 % to cover all your use.
Formula
System size (kW) = monthly kWh × 12 ÷ (365 × peak sun hours × (1 − losses)).
Panels = system size ÷ panel wattage, rounded up. Yearly production = installed kW × 365 × sun hours × (1 − losses).
Panel area assumes 20.5 % efficient modules (a 400 W panel is about 1.95 m² / 21 ft²). Cost = installed watts × price per watt.
Worked example
With the values preset in the calculator above:
- Monthly electricity use
- 900 kWh per month
- Peak sun hours
- 4.5 hours per day
- Panel wattage
- 400 W
- System losses
- 18 %
- Share of your use to cover
- 100 %
- Solar system size
- 8.4 kW (8,400 W)
- Panels
- 21
- Yearly production
- 11,314 kWh
- Panel area
- 441.06 ft² (49.01 yd²)
- Estimated cost
- $23,520
How many solar panels do I need?
A home using 900 kWh a month with 4.5 peak sun hours needs about 8.4 kW — 21 panels of 400 W, which produce 11,314 kWh a year. In a sunnier place with 5.5 sun hours the same home needs 17 panels.
Peak sun hours are not daylight hours: they are the hours of full 1,000 W/m² sun the day adds up to. NREL’s PVWatts gives the exact figure for your address and roof angle.
How much does a solar system cost in 2026?
Installed home systems cost about $2.50–$3.50 per watt before incentives, so an 8 kW system is roughly $20,000–$28,000. The 30 % federal residential credit (25D) ended for systems installed after 2025; check your state and utility for remaining rebates and net-metering rules.
System size by monthly use (4.5 sun hours)
| Monthly electricity use | Solar system size | Panels |
|---|---|---|
| 500 | 4.8 kW | 12 |
| 750 | 6.8 kW | 17 |
| 1,000 | 9.2 kW | 23 |
| 1,250 | 11.2 kW | 28 |
| 1,500 | 13.6 kW | 34 |
| 2,000 | 18 kW | 45 |
Frequently asked questions
How many solar panels to power a house?
Most US homes need 15–25 panels of 400 W. At 900 kWh a month and 4.5 sun hours: 21 panels (8.4 kW).
How much does one solar panel produce?
A 400 W panel makes about 400 × sun hours × 0.82 Wh a day — about 1.5 kWh at 4.5 sun hours, or 540 kWh a year.
How much roof space do I need?
About 21 ft² per 400 W panel plus gaps and setbacks: 441.06 ft² of panels for the example system.
Should I cover 100 % of my use?
Only if your utility credits exported power at a good rate. With low export rates, many homeowners cover 70–90 % and add a battery later.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.