How many solar panels do I need? Sizing from your electric bill
Updated October 12, 2026 · By RoofPayback editorial · Reviewed by Automated fact and quality checks
Short answer
Start from your annual electricity use, divide it by what one kW of panels produces where you live, and you get the system size that would cover 100% of your use on paper. For a home using 10500 kWh/yr, that is about 7.4 kW in Texas, 6.3 kW in California, 8.1 kW in New York and 9.7 kW in Washington. Then check whether a slightly smaller system pays back faster — it often does, because credits for surplus power are capped.
Step 1: find your annual kWh
Add up the kWh from your last 12 electric bills, or download the usage history from your utility's online account. Use a full year: summer air conditioning and winter heating can make single months misleading. If you plan to add an electric car, a heat pump or a pool, add their expected use before sizing.
Step 2: look up output per kW where you live
One kW of panels produces very different amounts depending on location. In our PVWatts data for south-facing panels at a 20° pitch, the reference cities range from 1755 kWh per kW per year in Phoenix to 1088 in Seattle. Your state page lists the figure for your area, plus how much a roof facing east, west or southeast loses: see the state pages.
Step 3: divide
Annual use ÷ output per kW = size needed to cover 100% of your use.
For a 10500 kWh/yr home, our data gives:
| State | kW to cover 10500 kWh/yr |
|---|---|
| Arizona | 6 kW |
| California | 6.3 kW |
| Florida | 6.8 kW |
| Texas | 7.4 kW |
| Illinois | 8 kW |
| New York | 8.1 kW |
| Massachusetts | 8.1 kW |
| Washington | 9.7 kW |
Panel counts depend on the wattage of the panels in your quote; divide the system size in watts by the panel rating to get the number of panels.
Step 4: check payback at different sizes
Covering 100% of your use is not automatically the best financial choice. We modeled systems from 3 kW to 12 kW at $3000/kW for the same 10500 kWh/yr home (40% of use during daylight, full net metering, credits capped at a year of grid purchases, prices rising 2.5%/yr):
| Size | Texas payback | California payback | Florida payback | Arizona payback |
|---|---|---|---|---|
| 3 kW | 13.5 years | 5.4 years | 13.1 years | 11.3 years |
| 5 kW | 12.8 years | 5.3 years | 12.5 years | 10.8 years |
| 7 kW | 12.5 years | 5.8 years | 12.3 years | 12 years |
| 9 kW | 14.4 years | 7.3 years | 15.1 years | 14.8 years |
| 12 kW | 18.2 years | 9.4 years | 19 years | 18.7 years |
Two patterns stand out:
- Past roughly 100% of your use, extra panels stop paying. In Texas, a 9 kW and a 12 kW system both save $1667 in the first year: once credits cover a full year of grid purchases, additional exports earn nothing at true-up.
- The fastest payback and the biggest lifetime gain are different sizes. In Arizona the fastest payback is 5 kW (10.8 years), while 7 kW gives the largest 25-year gain ($31661 vs. $25659). Decide which matters more to you.
Step 5: adjust for your export credit and roof
If your utility pays less than retail for exported power, smaller systems look even better, because a larger share of their output is used directly. Read net metering vs. net billing for how much this moves payback. Roof space, shading and orientation set the upper limit: an east- or west-facing roof needs more kW for the same output.
Common sizing mistakes
- Using one month's bill times twelve. Seasonal swings distort the result.
- Sizing for future use you are not sure about. Oversizing today costs money now; adding panels later is usually possible.
- Comparing quotes by panel count. Compare kW, estimated annual kWh, and price per watt instead.
- Assuming incentives that no longer exist. The 30% federal credit ended for systems placed in service after 2025; see is solar still worth it without the tax credit.
Bottom line
Size from a full year of kWh and your local output, then test a few sizes around that number. The solar calculator does all of this with your own bill, roof and quote, and its "Which size pays off best?" table shows where extra panels stop adding value.