Solar System Size Calculator (kW Needed)
Enter how much electricity your home uses in a year, your location's average peak sun hours, and the wattage of the panels you are considering, and this calculator estimates the system size in kilowatts you would need to offset that usage — plus how many panels that works out to and roughly how much roof area they occupy. It is a planning estimate to size a system before you get quotes.
Usage & site
System size needed
8.22 kW
21 × 400 W panels · ≈ 40 m² roof
Panels
21
Target production
12,000 kWh
Size by peak sun hours
Required kW vs peak sun hours
How it works
The required system size is your target annual production divided by how much a single kilowatt of panels produces per year at your location. That per-kW figure is peak sun hours per day × 365 days × a derate factor (default 0.8) that accounts for inverter, wiring, soiling, and temperature losses. So required kW = (annual usage × offset fraction) / (peak sun hours × 365 × derate). Locations with more peak sun hours need fewer kilowatts to produce the same energy, which is why the sensitivity table shows how the answer shifts across the US range of roughly 3.5 to 6.5 sun hours.
Panel count is the system size converted to watts and divided by the wattage of one panel, rounded up: ceil(kW × 1000 / panel watts). A 400-watt panel is a common residential module today; higher-wattage panels mean fewer panels for the same system size. The roof-area estimate assumes about 1.9 square meters per typical 400-watt panel and scales with wattage, giving a rough sense of whether your available roof can physically hold the array — though usable area also depends on obstructions, setbacks, and orientation.
The offset fraction lets you size for less than 100% of your usage, which some homeowners do to fit a budget or a roof, or slightly more than 100% to cover future load growth such as an electric vehicle or heat pump. The calculator sizes purely on annual energy; it does not model monthly seasonality, net-metering rules, or battery storage, all of which a professional design would layer on top of this baseline number.
Frequently asked questions
How many peak sun hours does my location get?+
Peak sun hours are not the same as daylight hours — they represent the equivalent hours per day of full 1,000 W/m² sunshine, which folds in cloud cover, latitude, and season. Across the continental US the annual daily average ranges from roughly 3.5 in the cloudy Pacific Northwest and Northeast to over 6 in the desert Southwest. NREL's PVWatts tool gives a location-specific figure from your address, and it also lets you set roof tilt and azimuth, which meaningfully change production. When in doubt, run the calculator across a couple of sun-hour values using the sensitivity table to see how much your answer moves.
Why round the panel count up?+
You cannot install a fraction of a panel, so the calculator rounds the panel count up to the next whole number to ensure the array meets or slightly exceeds your target production. This means the installed system is usually a little larger than the exact kilowatt figure — for example, needing 8.22 kW with 400 W panels rounds to 21 panels, or 8.4 kW installed. That small over-sizing is normal and gives a modest production buffer. Real installations are also constrained by inverter sizing and string configuration, which an installer optimizes.
Does this account for battery storage or net metering?+
No. This tool sizes the panel array to your annual energy usage only. Whether you can actually use all that production to offset your bill depends on your utility's net-metering or net-billing policy, and whether you add battery storage to shift daytime production into evening use. Under full retail net metering, annual sizing like this works well. Under time-of-use rates or export limits, the economics change and you may want a battery or a different system size. Use this as the starting point and refine with an installer familiar with your utility's tariff.