Solar Panel Calculator
Enter your monthly electricity use, your region, and what you want to offset. You get the number of panels, system size, roof area, installed cost at today's price per watt, annual savings at your rate, and the payback period.
From your bill; US average is about 900.
US average about $2.75 before incentives; quotes range $2.25-3.50.
The federal residential credit ended for systems placed in service after 2025; enter any state or utility incentive.
Panels needed
20
400 W each · 400 sq ft of roof
System size
8.0 kW DC
produces ~11,242 kWh/yr
Installed cost
$22,000
before any incentives
Simple payback
12.0 yr
saves ~$1,836/yr at $0.17/kWh
kW = annual kWh ÷ (peak sun hours × 365 × 0.77). The 0.77 derate is PVWatts' default for inverter, wiring, soiling, shading and temperature losses. Payback ignores net-metering rules, rate escalation and degradation (~0.5%/yr); a quote with PVWatts run on your actual roof is the real number.
Panels needed by annual use and region
400 W panels, 100% offset, 0.77 derate. The US average home uses about 10,800 kWh a year.
| Annual kWh | Arizona6.5 h | California5.8 h | Texas5.3 h | Florida5 h | Missouri4.6 h | New York4 h | Washington3.7 h |
|---|---|---|---|---|---|---|---|
| 6,000 | 9 (3.6 kW) | 10 (4.0 kW) | 11 (4.4 kW) | 11 (4.4 kW) | 12 (4.8 kW) | 14 (5.6 kW) | 15 (6.0 kW) |
| 9,000 | 13 (5.2 kW) | 14 (5.6 kW) | 16 (6.4 kW) | 17 (6.8 kW) | 18 (7.2 kW) | 21 (8.4 kW) | 22 (8.8 kW) |
| 10,800 | 15 (6.0 kW) | 17 (6.8 kW) | 19 (7.6 kW) | 20 (8.0 kW) | 21 (8.4 kW) | 25 (10.0 kW) | 26 (10.4 kW) |
| 12,000 | 17 (6.8 kW) | 19 (7.6 kW) | 21 (8.4 kW) | 22 (8.8 kW) | 24 (9.6 kW) | 27 (10.8 kW) | 29 (11.6 kW) |
| 15,000 | 21 (8.4 kW) | 24 (9.6 kW) | 26 (10.4 kW) | 27 (10.8 kW) | 30 (12.0 kW) | 34 (13.6 kW) | 37 (14.8 kW) |
| 18,000 | 25 (10.0 kW) | 28 (11.2 kW) | 31 (12.4 kW) | 33 (13.2 kW) | 35 (14.0 kW) | 41 (16.4 kW) | 44 (17.6 kW) |
Cost and payback for the average home, by electricity rate
10,800 kWh a year, 5 peak sun hours, $2.75/W, no incentives.
| Rate | System | Cost | Saves / yr | Payback | 25-yr savings |
|---|---|---|---|---|---|
| $0.11/kWh | 8.0 kW | $22,000 | $1,188 | 18.5 yr | $7,700 |
| $0.14/kWh | 8.0 kW | $22,000 | $1,512 | 14.6 yr | $15,800 |
| $0.17/kWh | 8.0 kW | $22,000 | $1,836 | 12.0 yr | $23,900 |
| $0.22/kWh | 8.0 kW | $22,000 | $2,376 | 9.3 yr | $37,400 |
| $0.28/kWh | 8.0 kW | $22,000 | $3,024 | 7.3 yr | $53,600 |
| $0.40/kWh | 8.0 kW | $22,000 | $4,320 | 5.1 yr | $86,000 |
Simple payback: no rate escalation (which shortens it), no degradation or inverter replacement (which lengthen it), full net-metering credit assumed.
What changes the answer most
- Roof orientation and tilt. South-facing at roughly your latitude is the reference. East or west loses 10-20%; north-facing is usually not worth building.
- Shade. One shaded panel on a string inverter drags the string down; microinverters or optimisers isolate it. Ask for a shade analysis.
- Net metering. Full retail credit for exported power is disappearing (California's NEM 3.0 pays a fraction). Where credit is low, a battery or sizing to daytime self-consumption changes the economics.
- Your usage, not the average. Twelve months of bills, plus anything you plan to add: an EV adds 3,000-4,000 kWh a year, a heat pump replacing gas 4,000-8,000.
Frequently Asked Questions
How many solar panels do I need?
Annual kWh ÷ (peak sun hours × 365 × 0.77) gives the kW of panels; divide by the panel wattage. A typical US home using 10,800 kWh a year in a 5-sun-hour region needs about 7.7 kW, which is 20 panels of 400 W covering roughly 400 sq ft of roof.
What are peak sun hours?
The equivalent number of hours per day at full 1,000 W/m² sunlight, averaged over the year. Arizona gets about 6.5, Florida 5, New York 4, Seattle 3.7. It is not daylight hours; a 14-hour summer day in Seattle still averages under 4 peak sun hours across the year. NREL's PVWatts tool has the figure for any address.
Why the 0.77 derate?
Panels are rated at their DC output under lab conditions. Real systems lose output to the inverter, wiring, module mismatch, dirt, shading, heat and downtime. NREL's classic PVWatts overall derate for all of that is 0.77; the newer PVWatts default of 14% system losses with a 96% inverter works out to about 0.825. The calculator uses 0.77 so it sizes slightly large rather than slightly small; your installer's production estimate should land between the two.
How much does solar cost in 2026?
Around $2.75 per watt DC installed for a typical residential system before incentives, so $19,250 for 7 kW. The 30% federal residential credit ended for systems placed in service after 2025; some states and utilities still offer rebates, which the calculator takes as a percentage. Get three quotes: the spread between installers is routinely 30%.
What is the payback period?
Installed cost ÷ annual savings. At the US average rate, the example system saves about $1,836 a year against a $22,000 cost, a 12-year simple payback on 25-year-warranted panels. High-rate states (California, the Northeast, Hawaii) pay back in 6-9 years; low-rate states 12-16. Net-metering rules decide how much of your production you are actually credited for, and they vary by utility.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.