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Solar Panel Calculator
Use this free solar panel calculator to estimate what size solar system your home needs. Enter your monthly electricity use or your monthly bill, plus your peak sun hours and panel wattage, and it estimates the system size, the number of panels, the roof area, the cost, the annual savings, and the payback time.
All results are estimates. Defaults are editable: peak sun hours 4.5, panel size 400 W, rate $0.17 per kWh, and cost $2.80 per watt. Check your electric bill for your real usage and rate.
| Item | Amount |
|---|---|
| Annual electricity use | 10,800 kWh |
| Daily electricity use | 29.59 kWh/day |
| System size needed | 8.22 kW |
| Panels needed (400 W each) | 21 |
| Installed panel capacity | 8.40 kW |
| Roof area needed (18 sq ft per panel) | 378 sq ft |
| Estimated system cost | $23,014 |
| Estimated annual savings | $1,836/year |
| Estimated payback | 12.5 years |
How it works
Sizing a solar system starts with how much electricity you actually use, not with the size of your roof. Your electric bill shows your monthly use in kilowatt-hours (kWh) — one kWh is the energy of running a 1,000 watt appliance for one hour. If you only know your bill in dollars, divide it by your electricity rate to get the kWh: a $153 bill at $0.17 per kWh is 900 kWh a month. Use a full year of bills if you can, because summer air conditioning and winter heating change the monthly figure a lot.
The next number is peak sun hours: how many hours a day your roof gets full-strength sun. It is not the same as daylight hours. A summer day may have 14 hours of daylight but only 5 peak sun hours, because the morning and evening sun is weaker. Most of the country averages 3.5 to 5.5 peak sun hours a day, with the Southwest highest and the Pacific Northwest and Northeast lowest. The default here is 4.5, and you should change it to the figure for your area — your installer or a local solar map can give you the exact number.
Daily use = annual use ÷ 365
System size (kW) = annual use ÷ (peak sun hours × 365 × 0.80)
Panels = system size (W) ÷ panel wattage, rounded up
Roof area = panels × 18 sq ft
Estimated cost = system size (W) × cost per watt
Annual savings = annual use × electricity rate × offset
Payback = estimated cost ÷ annual savings
The 0.80 in the formula is a derating factor: real panels do not produce their laboratory rating all day. Heat, dust, shade, wiring, and the inverter that turns the panels' direct current into household alternating current all lose a little energy, so a system is sized about 20 percent larger than the raw sunshine math suggests. The panel count is then rounded up to whole panels, which is why the installed capacity — 21 panels at 400 watts, or 8.40 kW — is slightly larger than the 8.22 kW the calculation asks for.
Roof area uses 18 square feet per panel, the footprint of a standard residential panel, plus the understanding that you also need gaps around the edges and between rows for access and fire code setbacks. Cost is the system size times your installed cost per watt, which covers panels, inverter, racking, wiring, and labor. The default of $2.80 per watt is a typical national estimate before incentives; the federal solar tax credit, currently 30 percent for homeowners who qualify, is not subtracted here, so your after-credit cost could be much lower and your payback much shorter.
Annual savings assume the system covers the share of your use you entered — 100 percent by default — and that every kWh it produces is worth your electricity rate. In practice, net metering rules, time-of-use rates, and how much power you use during the day all change the real savings. Treat every number on this page as an estimate for planning, and get a quote from a local installer who can measure your roof, check shading, and apply the incentives and utility rules in your area.
Worked example
Your home uses 900 kWh a month. Your roof gets 4.5 peak sun hours a day, you choose 400 watt panels, your electricity rate is $0.17 per kWh, installed cost is $2.80 per watt, and you want to cover 100 percent of your use.
Step 1: Annual use — 900 × 12 = 10,800 kWh a year, or 10,800 ÷ 365 = 29.59 kWh a day.
Step 2: System size — 10,800 ÷ (4.5 × 365 × 0.80) = 10,800 ÷ 1,314 = 8.22 kW.
Step 3: Panels — 8,220 watts ÷ 400 watts = 20.55, so buy 21 panels, giving 8.40 kW of installed capacity.
Step 4: Roof area — 21 × 18 = 378 square feet.
Step 5: Cost — 8,219 watts × $2.80 = $23,014 estimated.
Step 6: Savings and payback — 10,800 × $0.17 = $1,836 saved a year, and $23,014 ÷ $1,836 = 12.5 years to pay back.
The result is an 8.22 kW system with 21 panels — about $23,014 estimated cost, about $1,836 saved per year, and about a 12.5 year payback. That is exactly what the calculator shows for these inputs.
FAQ
How many solar panels do I need for my home?
It depends on how much electricity you use and how much sun your roof gets. A home using 900 kWh a month with 4.5 peak sun hours needs about an 8.2 kW system, which is about 21 panels at 400 watts each. Homes that use less, or roofs in sunnier areas, need fewer panels.
What are peak sun hours?
Peak sun hours are the number of hours per day when sunlight is strong enough to count as full sun, about 1,000 watts per square meter. Most of the United States gets 3.5 to 5.5 peak sun hours a day on average, with the Southwest at the high end and the Pacific Northwest and Northeast at the low end. Your installer can give you the exact figure for your roof.
How much does a home solar system cost?
A typical installed price is about $2.50 to $3.50 per watt before incentives, so an 8.2 kW system costs roughly $20,500 to $28,700. The federal solar tax credit can cut that by 30 percent if you qualify, and some states add their own rebates. This calculator uses $2.80 per watt as an editable estimate.
How long does it take for solar panels to pay for themselves?
Divide the system cost by the annual savings on your electric bill. At $2.80 per watt and $0.17 per kWh, an 8.2 kW system costing about $23,014 and saving about $1,836 a year pays back in about 12.5 years. Tax credits, higher electric rates, and rising rates all shorten the payback.
How much roof space do solar panels need?
A standard residential panel is about 18 square feet, so 21 panels need about 378 square feet of usable roof, plus space around the edges and between rows for access. The roof also needs to face roughly south, west, or east, be in good condition, and be mostly free of shade during the middle of the day.