What size solar system do I need for my home?

Solar system size = monthly kWh usage ÷ (peak sun hours × 30 days × 0.85 efficiency). A home using 900 kWh/month in a 5-peak-sun-hour location needs: 900 ÷ (5 × 30 × 0.85) = about 7 kW. Most US homes need 6–12 kW systems. Always base sizing on your actual 12-month utility bills, not averages.

Is bigger always better for solar system size?

Not necessarily. Oversizing your system means you'll export significant electricity to the grid — sometimes at lower-than-retail rates. Most experts recommend sizing for 90–100% of your annual usage. If you're adding an EV, heat pump, or planning family growth, sizing up makes sense. Your installer can model different scenarios.

How do I calculate what size solar system I need?

Divide your annual kWh consumption by your location's peak sun hours per day, then divide by 365 to get the required daily kWh, and adjust for system efficiency losses (typically 80%). Example: 10,000 kWh/year ÷ 5 peak sun hours ÷ 365 × (1/0.80) = 6.85 kW system. This calculator automates this calculation based on your inputs.

Is it better to oversize or undersize a solar system?

Slight oversizing (10–15%) is generally better — it accounts for future energy usage increases (EV adoption, heat pump upgrades) and panel degradation over time. Under traditional net metering, excess production earns credits. Under newer net metering programs like California NEM 3.0, significant oversizing is less financially beneficial without battery storage.

Solar System Size Calculator

Find the ideal solar system size in kilowatts for your home. Starting from your electric bill and local conditions, this calculator works backward to recommend the right system capacity.

Monthly Bill ($)

Electricity Rate (¢/kWh)

Peak Sun (hrs/day)

About This Calculator

The Solar System Size Calculator determines how many kilowatts of solar capacity your home needs to meet a given percentage of your electricity consumption. It works backward from your electric bill — converting dollars to kilowatt-hours, then dividing by daily sun hours and accounting for system efficiency losses — to arrive at a recommended system size in kW DC. This is the number you will see on installer proposals and permits, and it determines everything from panel count to inverter size to final system cost.

The most impactful inputs are your monthly bill (or kWh usage), your local peak sun hours, and your target solar offset. Peak sun hours are not the same as hours of daylight — they represent the equivalent number of hours per day when sunlight intensity is at 1,000 W/m², the standard test condition for panels. A location with 5 peak sun hours generates significantly more power than one with 3.5, so using your actual local value (available via NREL PVWatts) rather than a national average is essential for accurate sizing.

System efficiency of 80% is a reasonable default for a well-installed rooftop system, reflecting inverter losses (3–5%), wiring losses (2–3%), temperature derating (5–10%), and minor soiling. Shaded roofs or older equipment may see efficiency closer to 70–75%. If your installer provides a specific production estimate, you can back-calculate efficiency by dividing projected annual kWh by (system kW × 365 × peak sun hours). That gives you a more accurate efficiency figure to plug in here.

System size directly affects permit requirements, HOA approvals, utility interconnection applications, and roof structural load calculations. Getting the size right before talking to installers helps you evaluate whether their proposals match your actual needs — and flags immediately if someone is proposing an under- or over-sized system relative to your usage.

Calculations based on NREL solar modeling data and industry-standard assumptions, built and maintained by the independent SolarToolsOnline research team.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.