Determine the right solar system size in kW based on your annual electricity usage, peak sun hours, and system efficiency. Free solar sizing tool.
Choosing the right solar system size is critical to maximizing savings and avoiding overspending. A system that's too small won't offset enough of your electricity bill, while one that's too large wastes money on panels you don't need — especially if your utility caps net metering credits.
The sizing formula divides your annual electricity consumption by the number of productive hours your panels see in a year. This accounts for your local sunlight (peak sun hours) and the system's real-world derate factor, which covers losses from inverter conversion, wiring, temperature, and soiling.
This calculator takes your yearly kWh usage and local conditions to recommend a system size in kilowatts (kW). You can find your annual consumption on your utility bill or account summary. Peak sun hours vary by region — check NREL PVWatts or your solar installer for local values.
Quantifying this parameter enables systematic comparison across facilities, time periods, and equipment configurations, revealing optimization opportunities that reduce both costs and emissions.
Getting the system size right means your solar investment pays back as quickly as possible. This calculator uses the same methodology professional installers use for initial estimates, giving you an informed starting point before you request quotes. This quantitative approach replaces rough estimates with precise figures, enabling facility managers to identify the most cost-effective opportunities for reducing energy consumption.
System Size (kW) = Annual kWh / (365 × Peak Sun Hours × Derate Factor)
Result: 7.19 kW system
With 10,500 kWh annual usage, 5 peak sun hours, and a 0.80 derate factor: 10,500 / (365 × 5 × 0.80) = 10,500 / 1,460 = 7.19 kW. A 7.2 kW system would offset approximately 100% of this household's electricity consumption.
The kW rating of your solar system determines both the upfront cost and the long-term savings. Each additional kW costs $2,500–$3,500 installed (before incentives), so oversizing by even 2 kW can add $5,000–$7,000. Proper sizing ensures every panel pays for itself in energy savings.
A 10,000 kWh home in Arizona needs roughly a 5.3 kW system, while the same home in Michigan needs about 8.5 kW. The difference is entirely due to peak sun hours. Always use location-specific data rather than national averages when sizing your system.
If you plan to add an EV, heat pump, pool pump, or home addition in the next few years, factor that additional load into your sizing calculation now. Most installers recommend sizing 10–15% above current usage to account for natural consumption growth.
Check your utility account online for a 12-month usage summary, or add up the kWh from your monthly bills for the past year. Most utilities provide this data in a chart or table. The average U.S. household uses about 10,500 kWh per year.
Most U.S. homes install between 5 kW and 10 kW systems. A 7 kW system is average and covers about 10,000–11,000 kWh/year in areas with 5 peak sun hours. Larger homes or those with EVs may need 10–15 kW.
It depends on your utility's net metering policy. If they offer full retail credits, sizing to 100% makes sense. If credits are below retail or capped, you might size to 80–90% and use the grid for the rest.
An EV can add 2,500–5,000 kWh/year to your consumption. Add this to your current usage before sizing your solar system. It's cheaper to install a larger system upfront than to add panels later.
Not always. If your utility doesn't offer net metering, excess production is wasted or credited at wholesale rates. Size your system to match your consumption pattern for maximum ROI.
Sunnier climates need fewer kW to produce the same energy. A home in Phoenix (6.5 PSH) needs a 5.5 kW system for 10,500 kWh, while Seattle (3.5 PSH) needs about 10.3 kW. Climate is the single biggest factor in sizing.