Estimate solar panel system size based on monthly usage, sun hours, and system efficiency.
Usage Tip
Peak sun hours vary by region and season; size for the lower winter figure if you want year-round coverage.
system size (kW) = daily kWh ÷ sun hours × (100 ÷ efficiency)
panels = round up( system size × 1000 ÷ panel wattage )
The result rounds up to whole panels.
How many solar panels do I need?
It comes down to three numbers: how much electricity you use, how much sun your location gets, and how big each panel is. Take your monthly usage in kilowatt-hours, work out the daily average, and divide by the usable sun your roof sees each day — that gives the system size in kilowatts. Divide that by the panel wattage and you have the panel count. A typical home using about 900 kWh a month in a sunny region needs roughly a 7 to 9 kW system, which is around 18 to 22 modern panels. This calculator does that sizing, then estimates production, bill savings and payback.
Solar system sizing
The core formula is straightforward:
System kW = daily kWh ÷ (sun hours × system efficiency × shading)
System efficiency — sometimes called the derate factor — accounts for losses in the inverter, wiring, heat and dust, usually around 80 to 85 percent. Shading from trees or buildings cuts production further. Once you have the system size, the panel count is the size in watts divided by each panel’s wattage, rounded up.
Sun hours by region
Peak sun hours are the equivalent hours of full-strength sun per day, averaged over the year — not the same as daylight hours:
| Region | Peak sun hours/day |
|---|---|
| Pacific Northwest | 3.5 to 4 |
| Northeast | 4 to 4.5 |
| Midwest | 4.5 to 5 |
| Southeast | 5 to 5.5 |
| California | 5.5 to 6 |
| Southwest | 6 to 6.5 |
How much roof do you need?
A modern residential panel is about 18 square feet. Multiply by the panel count for the rough roof area — a 20-panel system wants around 360 square feet of unshaded, well-oriented roof. South-facing slopes produce the most in the northern hemisphere; east and west faces give up roughly 10 to 20 percent. If the array does not fit, higher-wattage panels pack more power into the same space.
Solar payback and savings
Payback is the install cost, minus incentives, divided by the yearly bill savings. Savings are the production times your electricity rate — the more you pay per kWh, the faster solar pays off. At a typical installed cost of around 3 dollars per watt and the 30 percent federal tax credit, many homes land in the 8 to 12 year range, after which the power is essentially free for the panels’ remaining 15-plus years. A higher electricity rate, more sun, or a smaller bill all shorten that.
Net metering
Net metering lets you push surplus daytime production back to the grid and pull it back at night, effectively using the grid as a battery. Where full one-to-one net metering exists, every kWh you generate offsets one you would have bought, which is the best case for savings. Many areas have moved to lower export rates, so check your utility’s policy — it strongly affects how much a system over your own usage is worth.
Estimate vs reality
These numbers are planning estimates, not a quote. Real production varies with weather, panel tilt and azimuth, temperature, soiling, roof obstructions and the specific equipment. A site assessment with shading analysis and a real utility-rate schedule will refine them. Treat this as the starting point that tells you whether solar is worth a serious look, not the final design.
Frequently asked questions
How many solar panels to power a house?
For an average home around 900 kWh a month, roughly 18 to 22 panels, or a 7 to 9 kW system, depending on sun and shading.
How much does a solar system cost?
Commonly around 3 dollars per watt installed before incentives, so an 8 kW system is about 24,000 dollars, or roughly 17,000 after the 30 percent federal credit.
What is the payback period for solar?
Often 8 to 12 years, shorter where electricity is expensive or sun is plentiful, longer where rates are low.
What are peak sun hours?
The equivalent hours of full 1,000 W per square meter sun per day, averaged yearly — a measure of usable solar energy, not daylight length.
Does shading really matter?
A lot. Even partial shade on a few panels can cut a string’s output sharply. Heavy shading can reduce production by 30 percent or more.
Will solar cover my whole bill?
It can, if the system is sized to your annual usage and net metering credits the surplus. Many homes offset most but not every kWh.
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Estimates use a standard sizing model with a typical derate and simplified savings and payback math, for planning and education only. Actual production, cost, incentives and net-metering terms vary widely by site, equipment, installer and utility. Get a professional site assessment and quotes before making a purchase decision. Incentive figures are illustrative and change over time.