How to use the solar panel calculator
- Find your electricity use on your bill. A yearly total is best because use changes with the seasons.
- Enter your location’s average peak sun hours (see the table below or NREL PVWatts).
- Keep system losses at 14% unless you have shading or a better estimate.
- Choose how much of your use to cover and the panel wattage you plan to buy.
- Optionally add the installed cost per watt and your electricity price to see the payback.
The formula
Daily target (kWh) = Yearly use × Offset% ÷ 365 System size (kW) = Daily target ÷ (Peak sun hours × (1 − Losses)) Panels = System size × 1,000 ÷ Panel watts → round up Production (kWh/yr)= Panels × Panel kW × Peak sun hours × 365 × (1 − Losses)
Worked example
A home using 900 kWh a month in a 5 peak-sun-hour location:
- Daily use = 900 × 12 ÷ 365 = 29.6 kWh
- System size = 29.6 ÷ (5 × 0.86) = 6.88 kW
- Panels = 6,881 ÷ 400 = 17.2 → 18 panels (7.2 kW), producing about 11,300 kWh a year
- At $2.80 per watt the system costs about $20,160; saving 10,800 kWh × $0.17 = $1,836 a year gives a simple payback of about 11 years
Average peak sun hours (approximate)
| Location | Peak sun hours/day |
|---|---|
| Phoenix, Las Vegas, southern California | 6–7 |
| Texas, Florida, Georgia, Colorado | 5–5.5 |
| Midwest, Mid-Atlantic | 4–4.5 |
| New England, Pacific Northwest | 3.5–4 |
| Australia (Sydney, Brisbane, Perth) | 4.5–6 |
| Southern Europe (Spain, Italy) | 4.5–5.5 |
| UK, Germany, northern Europe | 2.5–3 |
These are yearly averages for a well-angled, south-facing (north-facing in the southern hemisphere) roof. Summer values are higher and winter values lower.
Things that change the result
- Roof direction and tilt: east- or west-facing roofs produce about 10–20% less than south-facing ones.
- Shade from trees, chimneys or neighbouring buildings can cut output sharply — microinverters or optimizers help.
- Net metering rules: if your utility credits exported power at less than the retail rate, covering 100% of your use may not pay back as fast. Many owners size for 70–90% offset.
- Future loads: an electric car adds roughly 3,000–4,000 kWh a year and a heat pump can add several thousand more, so plan ahead.
- Batteries: home batteries add backup power and help with time-of-use rates but lengthen the payback.
US incentives in 2026
The federal 30% residential clean energy credit (Section 25D) ended for systems paid for after December 31, 2025, so homeowners who buy a system now generally cannot claim it. Some states, cities and utilities still offer rebates, property-tax exemptions or performance payments — enter any rebate as a percentage in the incentives field. Leased systems and power purchase agreements are owned by the installer and are handled under different business credits, which can affect the price you are offered.