How big a solar system do I need?

Quick answer: size usually starts with your electric bill

Most solar sizing starts with how much electricity you use each month (kWh), not with the roof’s square footage alone.

A typical goal is to make solar produce a big portion of your annual usage. But the best size can vary a lot because of shading, roof orientation, your local utility rates, and state incentives.

If you want a simple next step, look for your last 12 months of electric bills and find your total kWh. Then compare that number with what a system of different sizes can produce in your area—your matched providers can help estimate this.

For more basics, see our Solar basics: lease, PPA, and loan.

How system size is measured (and what numbers to expect)

Solar system size is usually measured in kilowatts (kW). A 5 kW system is smaller than a 10 kW system. Many US homes land somewhere around 5–10 kW, but your needs could be smaller or larger.

Production is measured in kilowatt-hours (kWh). A bigger system usually makes more electricity, but whether you benefit depends on your contract terms, your utility’s net metering (if available), and your electricity rates.

Because homes and utilities differ, don’t rely on one “standard size.” A provider should show you:
- Your estimated annual kWh production for the proposed system size
- How much of your usage that could offset
- Any assumptions about shading, roof area, and system performance

A simple sizing method you can do today

You can get a rough estimate before you request quotes.

1. Find your last 12 months of electricity use (kWh) from your bills.

2. Decide your “solar coverage” goal. Many homeowners aim for partial to near-full offset, such as 60% to 100% of annual usage. The best choice depends on budget, roof conditions, and contract details.

3. Ask providers to propose system sizes that match that goal, then compare them using the same assumptions.

Even if your estimate is rough, it helps you ask better questions. And it helps you spot quotes that look too small or too large for your usage.

Why bigger isn’t always better (leases/PPAs vs buying/owning)

The “right” size isn’t only about your roof and usage—it also depends on the way you finance solar.

• Lease or PPA (third-party ownership): You usually pay a monthly amount for the system’s electricity. The provider typically claims the federal tax credit. Monthly payments may include escalators, which can raise what you pay over time.
• Solar loan or buying/owning: You may pay more upfront, but you may get more long-term upside if the system produces as expected.

Because contract terms change the outcome, don’t choose a system size based only on the advertised monthly payment. Compare the full contract numbers, how payments change over time, and what happens if your roof needs repairs.

For comparisons and questions to ask, check solar solutions and how matching works.

Roof limits, shading, and real-world performance

A system can be “the right size on paper” but still underperform if the roof can’t support it or if sunlight is blocked.

Common real-world factors include:
- Shading: Trees, neighboring buildings, chimneys, and roof vents can reduce output.
- Roof angle and direction: Panels usually produce more when they face the sun more directly.
- Roof condition: Some providers may limit size if the roof needs repairs or replacement soon.
- Local weather and grid conditions: Cloud cover and temperature affect production.

That’s why you should ask your matched providers for an estimate of annual production (kWh) and the assumptions behind it. If they can’t explain those numbers clearly, that’s a warning sign.

What to ask when you request quotes (so you can compare apples to apples)

When you get matched with vetted local providers, ask for the same key details from each offer. This helps you compare system sizes fairly and understand trade-offs.

Use this checklist:
- Proposed system size (kW) and why that size was chosen
- Estimated annual production (kWh)
- Your estimated bill offset (or savings estimate) stated clearly, with assumptions
- For leases/PPAs: monthly payment, payment escalator, contract length, and cancellation/transfer terms
- For loans/buying: loan terms (rate, term length), total cost, and what happens at the end of the loan
- Warranty coverage (panels and inverter) and what it covers
- Roof and workmanship terms (including what happens if you need a roof replacement)

Also, never sign on the spot. If a salesperson pressures you—by phone, door-to-door, or “limited-time” offers—pause and review the full contract. Rules can vary by state, and it’s okay to take your time.