If you are asking how many solar panels do I need, the honest answer is that it depends on three numbers: how much electricity you use, how much sun your roof gets, and how powerful each panel is. For a typical home using 900 kWh a month where the sun gives 5 peak hours a day, the answer is about 19 panels of 400 W — a 7.6 kW system. This guide gives you the quick table by usage, the four steps behind it, and the roof-space maths, so you can work out your own figure on paper or check it in seconds with our solar panel calculator.
Quick answer: panels by monthly usage
Start here if you already know your monthly usage. The table assumes 400 W panels, 5 peak sun hours a day and a 0.8 real-world efficiency factor — the same basis as the calculator.
| Monthly usage | System size needed | Panels (400 W) |
|---|---|---|
| 300 kWh | 2.5 kW | 7 |
| 600 kWh | 5 kW | 13 |
| 900 kWh | 7.5 kW | 19 |
| 1,200 kWh | 10 kW | 25 |
| 1,500 kWh | 12.5 kW | 32 |
Panel counts are rounded up to whole panels, so the fitted system is always a little bigger than the exact requirement — which is why a 900 kWh home shows 19 panels for a 7.5 kW need. If your usage sits between rows, round up to the next row: the extra panel or two costs little and covers the months when you use more than average.
The four steps
Every installer’s sizing quote follows the same four steps. Here they are, so you can follow along or check a quote against them.
- Find your usage. Your electricity bill shows kilowatt-hours used each month; if it only gives a yearly total, divide by 12. Twelve months of bills averaged is better than any single month.
- Find your sun hours. Use the regional ranges: roughly 6–7 peak sun hours a day in the Southwest, 5.5–6 in California and Florida, 4–4.5 in the Northeast and 3.5–4 in the Pacific Northwest.
- Run the formula. Daily kWh ÷ (sun hours × 0.8) = the system size in kW you need. For 900 kWh a month: 30 kWh a day ÷ (5 × 0.8) = 7.5 kW.
- Divide by panel wattage. A 7,500 W requirement ÷ 400 W panels = 18.75, rounded up to 19 panels.
Each step exists for a reason. Usage sets the target, sun hours set how hard each panel can work, the 0.8 factor allows for inverter, wiring, temperature and soiling losses, and the final division turns kilowatts into something you can actually buy. Change any one input and the count moves — the solar panel calculator runs all four steps at once if you would rather not do them by hand.
Averages hide the seasons
One caution about monthly averages: most homes use more electricity in some months than others. Air conditioning pushes summer usage up, electric heating does the same in winter, and sun hours move the opposite way, peaking in summer and dipping in winter. Sizing to your annual average — which is what a monthly figure represents — balances those swings across the year when your utility lets credits carry between months. If it does not, or if you want one particular month covered in full, size to that month instead and accept exporting more in the others.
How panel wattage changes the count
Panel wattage is the quiet variable in most guides. Panels are sold by their rating under laboratory conditions, and most home panels today are rated between 370 and 430 W. The system size you need does not change with the panel you pick — only the count does. The same 7.5 kW requirement takes 21 panels at 370 W, 19 panels at 400 W or 18 panels at 430 W. Higher-wattage panels are usually a little larger and a little more expensive each, so the roof space and the total price end up closer than the counts suggest. What matters is the total watts on the roof, not the number of boxes.
Roof space: will they fit?
Panels need somewhere to go. At about 17.5 sq ft per panel, the popular 19-panel system needs roughly 333 sq ft of clear roof — 19 × 17.5 — before any extras. A real layout also needs setbacks from the ridge, eaves and edges, and room around chimneys, vents and skylights, so plan on more like 380 to 420 sq ft of usable slope. Direction matters as much as area: in the northern hemisphere a south-facing slope is best, east and west are workable, and a north-facing slope produces noticeably less. If your best slope is too small, a slightly smaller system that covers most of your usage is usually better value than panels squeezed onto a poor roof.
One honest note about 2026 costs
The panel counts in this guide are the same whatever year it is, but the costs are not. Figures for 2026 assume no federal tax credit — the 30% residential credit ended on 31 December 2025 — so a system costs its full installed price, around $3.00 per watt before state incentives. Our guide to the solar panel payback period shows what that change does to the payback maths.
What to calculate next
Ready to run your own numbers? The Solar Panel Calculator turns your usage or your bill into panels, system size, roof area, cost and payback in one go. And if you want to see which appliances drive your usage in the first place, the Electricity Cost Calculator breaks a bill down appliance by appliance.
Frequently asked questions
How many solar panels does a typical 3-bedroom house need?
Most need 19–25 panels of 400 W, covering typical usage of 900–1,200 kWh a month at average sun hours. Smaller or very efficient homes need fewer; homes with electric heating, a pool or an EV need more.
Do solar panels produce their rated wattage?
No. The rating is measured under laboratory conditions. Heat, wiring, the inverter and ordinary dirt all take a share, which is why the 0.8 factor exists — a 400 W panel behaves more like 320 W across a typical day.
Can I add more panels later?
Usually yes, if the inverter has spare capacity (or the system uses microinverters, where each panel works independently) and the roof has room. Tell your installer expansion is likely, so the original design leaves both.
How many panels for 2,000 kWh a month?
About 42 panels of 400 W — a 16.7 kW system — at 5 peak sun hours a day. That is a large home system, needing roughly 735 sq ft of clear roof.
Estimates only — shading, roof angle and your utility’s rules move the real answer.