How to Size a Home Solar System and Estimate Payback
October 21, 2026
Sizing a solar system starts with your electricity use. Divide your annual or monthly consumption by the energy each kilowatt of panels can produce where you live. That production depends on peak sun hours, the equivalent number of hours per day at full-strength sunlight, and on losses from wiring, the inverter, temperature, and shading. A system rating of 8.3 kW, for example, can offset about 900 kWh a month in a location with 4.5 peak sun hours.
This guide walks through the sizing, the cost, and the simple payback calculation, with a warning that real quotes and incentives vary widely by place and time.
Skip the math and get your answer instantly:
Open the Savings Calculator →Step 1: Find Your Daily Use
Look at 12 months of bills to capture seasonal swings. If you use 10,800 kWh a year, that is 900 kWh a month or 30 kWh a day. If you plan to add an electric vehicle or heat pump, add the expected extra consumption: a typical EV driven 12,000 miles a year uses around 3,500 to 4,000 kWh.
Step 2: Convert to System Size
System size (kW) = daily kWh ÷ peak sun hours ÷ system efficiency. A derate factor of about 0.75 to 0.85 accounts for real-world losses. With 30 kWh a day, 4.5 peak sun hours, and 0.80 efficiency: 30 ÷ 4.5 ÷ 0.80 = 8.33 kW. With 400-watt panels, that is about 21 panels.
Peak sun hours vary: roughly 3 in cloudy northern climates, 4 to 5 in much of the US, and 6 or more in desert regions. Tools from national laboratories provide location-specific estimates.
| Daily use | Peak sun hours | Required system (80% efficiency) | Panels at 400 W |
|---|---|---|---|
| 20 kWh | 4.0 | 6.25 kW | 16 |
| 30 kWh | 4.5 | 8.33 kW | 21 |
| 30 kWh | 6.0 | 6.25 kW | 16 |
| 40 kWh | 3.5 | 14.3 kW | 36 |
Step 3: Estimate Cost and Savings
Installed residential solar has commonly cost around $2.50 to $3.50 per watt before incentives in the US, depending on market and equipment. At $3 per watt, an 8.33 kW system costs about $25,000. If it offsets all of 900 kWh a month at $0.16 per kWh, annual bill savings are 900 × 12 × 0.16 = $1,728. The simple payback is $25,000 ÷ $1,728 = 14.5 years.
Incentives such as tax credits, rebates, and net metering rules change often and differ by location, so check current programs. They can shorten payback substantially, while falling export rates for surplus energy can lengthen it.
Factors That Change the Result
Roof orientation (south-facing is best in the northern hemisphere), tilt, shading from trees and buildings, panel degradation (about 0.5% a year), electricity price increases over time, financing costs, and whether you get credit for exported power. A system that lasts 25 years or more at a payback of 14 years provides roughly a decade of near-free electricity, but you should also consider inverter replacement (often 10 to 15 years) and the time value of money.
Compare solar with other uses of the same money, such as efficiency upgrades or paying down debt.
Getting Quotes
Collect at least three quotes, ask for the price per watt, the production estimate, equipment warranties, and how the savings were calculated. Be skeptical of sales claims about guaranteed savings, and check that the installer is licensed and insured. Remember that shading analysis and roof condition matter. If your roof needs replacement in a few years, do it before installing panels.
Frequently Asked Questions
How do I calculate what size solar system I need?
Divide your daily electricity use by the peak sun hours and by an efficiency factor of about 0.8.
What are peak sun hours?
The equivalent number of hours per day with sunlight at 1,000 watts per square meter.
How many panels do I need?
System size in watts divided by the wattage of each panel. An 8.3 kW system needs about 21 panels of 400 W.
How long is the payback period?
Often 7 to 15 years depending on cost, incentives, and electricity prices.
Do panels still work on cloudy days?
Yes, but output is reduced, typically to 10% to 25% of full output.