Sizing a solar array starts with two numbers: how much electricity you use and how much sunlight your roof receives. This calculator turns your monthly or yearly kWh into the system size in kilowatts, the number of panels at your chosen wattage, the energy that array should produce, the roof area it needs and, if you enter prices, a simple payback period. A table shows how the answer changes with sun hours, since location matters as much as anything else.
How to use the solar panel calculator
- Enter your electricity use in kWh per month, year or day. Average twelve months of bills to smooth out seasons.
- Enter the peak sun hours for your location — the annual average daily solar energy in kWh/m².
- Set the share of use to cover. 100% aims to offset your whole bill; less fits a smaller roof or budget.
- Enter the system losses (15–20% is typical) and the panel rating in watts.
- Optionally add the area per panel, the installed cost per watt and your electricity rate for roof area, cost and payback.
Solar sizing formula
A 1 kW array under 1,000 W/m² of sunlight produces 1 kW, so one peak sun hour yields 1 kWh per kW of panels before losses. The rest of the formula simply scales that up to your usage.
Worked example
900 kWh per month, 4.5 sun hours, 15% losses, 400 W panels (the default)
Annual use = 900 × 12 = 10,800 kWh, or 29.59 kWh per day.
System size = 29.59 ÷ (4.5 × 0.85) = 7.736 kW; 7,736 ÷ 400 = 19.34, so 20 panels (8.0 kW).
Expected production = 8 × 4.5 × 0.85 × 365 = 11,169 kWh a year — 103% of use — on about 420 ft² of roof at 21 ft² per panel.
Adding costs. At an installed price of $3.00 per watt before incentives, the 8 kW system costs $24,000. If electricity costs $0.17 per kWh and every solar kWh offsets a retail kWh, the first-year savings are 10,800 × $0.17 = $1,836, for a simple payback of about 13 years. Incentives shorten that; lower export rates for surplus power lengthen it.
How location changes the answer
| Peak sun hours | Typical regions | System for 10,800 kWh/yr | 400 W panels |
|---|---|---|---|
| 3.0 | Pacific Northwest, northern New England | 11.6 kW | 30 |
| 4.0 | Great Lakes, mid-Atlantic | 8.7 kW | 22 |
| 5.0 | Southeast, Texas, Colorado | 6.96 kW | 18 |
| 6.0 | Arizona, Nevada, southern California deserts | 5.8 kW | 15 |
Sun-hour values are rough annual averages for a tilted, south-facing array; use a site-specific tool such as NREL’s PVWatts for your address before buying.
What the estimate leaves out
Orientation and tilt matter: an east- or west-facing roof may produce 10–20% less than a south-facing one at the same latitude, and partial shading from trees or chimneys can cut output far more than its area suggests. Panels lose roughly 0.5% of output per year, and production is seasonal, so a system sized for the annual total will over-produce in summer and under-produce in winter. Whether that matters depends on your utility’s net-metering or net-billing rules.
If you plan to store energy, the battery life calculator estimates how long a battery bank will run your loads. The electricity cost calculator shows what individual appliances add to your bill, which is often the cheapest place to start.
A planning estimate only. Solar installations connect to your home's electrical system and the grid; they require permits, must follow your local electrical code and utility rules, and should be installed by a licensed electrician or certified solar installer.
Frequently asked questions
How many solar panels do I need?
Divide your daily use by the energy one panel makes per day. Using 900 kWh a month (29.6 kWh a day) with 4.5 peak sun hours and 15% losses, each 400 W panel yields about 0.4 × 4.5 × 0.85 = 1.53 kWh a day, so you need about 19.3 panels — round up to 20, an 8 kW system.
What are peak sun hours?
The number of hours per day the sunlight would last if it always shone at the standard test intensity of 1,000 W/m². A location receiving 4.5 kWh/m² of sunlight per day has 4.5 peak sun hours. Annual averages range from about 3 in cloudy northern regions to 6 or more in the desert Southwest.
What losses should I assume?
Real systems lose energy to heat, wiring, dust, mismatch, shading and the inverter. NREL's PVWatts model uses 14% for system losses plus about 4% for the inverter, so 15–20% overall is a reasonable planning range. Heavily shaded roofs can lose much more.
How much roof space does a solar system need?
A typical 400 W residential panel measures about 19–22 ft² (1.8–2.0 m²). Twenty panels need roughly 400–440 ft² of usable roof, plus setbacks that fire codes often require along ridges and edges.
Does the calculator include tax credits?
No. Incentive programs, tax credits and net-metering rules vary by place and change often, so the cost and payback fields use your installed price before incentives. Check current federal, state and utility programs and your utility's export rate for an accurate payback.