What solar panel sizing actually is
A house that uses 1,000 kWh a month, in a place with 5.0 peak sun hours, needs a 7.65 kW array. That is twenty 400-watt panels, about 38.1 m² (410.1 ft²) of roof, producing roughly 12,000 kWh a year. That is what "how many solar panels do I need" is really asking, and it is the whole job of the Solar Panel Calculator: you enter your monthly kWh (or your bill) and your local sunshine, and it returns the system size in kW, the panel count, the annual production and the roof area. Four numbers in, four numbers out, no salesperson.
Sizing an array means matching two quantities. The first is how much electricity your household pulls off the grid — that number is printed on your bill every month. The second is how much a given slab of silicon can push back, which depends far more on where you live than on which panel you buy. The same panel in the desert Southwest makes close to twice what it makes in western Washington. Location is the variable that moves the answer, which is why a panel count offered without a location is a guess dressed up as arithmetic.
Worth saying plainly: sizing is not the same question as savings. What an array costs, what it pays back, and what your utility will credit you for exported power are all local, political, and stale within a year. How many panels cover 1,000 kWh a month is physics and division. The Solar Panel Calculator answers the second question and leaves the first one to the three installers you should be getting quotes from.
How to use the Solar Panel Calculator
- Pick how you know your usage. kWh per month is the accurate route — it is the headline figure on your electricity bill. If you only have the dollar amount, switch to Bill mode and add your rate per kWh, printed on the same bill near the total.
- Set your peak sun hours. This is the annual daily average of full-intensity sunshine at your address, in kWh/m²/day, and it moves the answer more than anything else. The 4.5 default is a US average. Real figures run from about 3.2 in the Pacific Northwest to over 6 in Arizona. There is a link to NREL's PVWatts under the field — that is where the real number lives.
- Choose your panel wattage. 400 W is a fair default for a residential module sold today. Quick-pick chips cover 350 to 500 W, and you can type whatever figure a quote puts in front of you.
- Set your offset. 100% sizes the array to cover all of your electricity use. Drop it if your roof will not fit a full array, or if your utility caps what you are allowed to install.
- Read the four numbers. System size in kW DC is what installers quote against. Panel count is rounded up to a whole panel, with the installed capacity beside it. Annual production and roof area tell you whether this fits your roof and your bill.
- Copy the summary and paste all four numbers plus the assumptions into an email — so the quote you get back can be checked against the same arithmetic.
Nothing is sent anywhere. The math runs in your browser, there is no address field, and closing the tab clears it.
The formula behind the panel count
The Solar Panel Calculator runs four lines of arithmetic, in order:
daily kWh = monthly kWh ÷ 30.4 × (offset ÷ 100)
system kW = daily kWh ÷ (peak sun hours × 0.86)
panels = ⌈ system kW × 1,000 ÷ panel watts ⌉
roof m² = panels × panel watts ÷ 210
Walk the canonical case through it. 1,000 kWh a month divided by 30.4 days is 32.88 kWh a day. Divide that by 5.0 peak sun hours times the 0.86 derate — 4.3 effective hours — and you get 7.65 kW. Multiply by 1,000 and divide by 400 W and you need 19.11 panels, which rounds up to 20. Twenty panels at 400 W is 8.0 kW installed. Roof area is 20 × 400 ÷ 210 = 38.1 m², or 410.1 ft².
The 0.86 is the part people skip, and skipping it is the single most common way a back-of-envelope estimate comes out too small. A panel's nameplate rating is measured in a lab; your roof is not a lab. Inverter conversion, resistance in the wiring, heat (panels get less efficient as they get hotter, and a roof in July is very hot), soiling, shading and module mismatch eat roughly 14% of what the array generates before it reaches your meter. That 14% is the default system-loss figure in NREL's PVWatts model, which is what most of the industry sizes against. Leave it out and the same house comes back at 6.58 kW and 17 panels — three panels short, forever.
Why 30.4 and not 30. A month is 365 ÷ 12 = 30.4 days. Rounding it to 30 looks harmless and inflates the array by about 1.4%: the same house comes back at 7.75 kW producing 12,167 kWh a year against 12,000 kWh of actual usage. At 100% offset, annual production should equal annual usage exactly. The extra 0.4 of a day is what keeps that true.
How many panels your usage needs
Start with usage. Every row below is 400 W panels at 4.5 peak sun hours — the US average — sized to cover 100% of the bill.
| Monthly usage | System size | Panels (400 W) | Annual production | Roof area |
|---|---|---|---|---|
| 500 kWh | 4.25 kW | 11 | 6,000 kWh | 21.0 m² / 225.5 ft² |
| 750 kWh | 6.37 kW | 16 | 9,000 kWh | 30.5 m² / 328.0 ft² |
| 1,000 kWh | 8.50 kW | 22 | 12,000 kWh | 41.9 m² / 451.1 ft² |
| 1,250 kWh | 10.62 kW | 27 | 15,000 kWh | 51.4 m² / 553.6 ft² |
| 1,500 kWh | 12.74 kW | 32 | 18,000 kWh | 61.0 m² / 656.1 ft² |
| 2,000 kWh | 16.99 kW | 43 | 24,000 kWh | 81.9 m² / 881.6 ft² |
Now hold usage still at 1,000 kWh and move the sunshine. This is the table that explains why national averages are useless advice.
| Peak sun hours | Roughly where | System size | Panels (400 W) | Roof area |
|---|---|---|---|---|
| 3.5 | Pacific Northwest | 10.92 kW | 28 | 53.3 m² / 574.1 ft² |
| 4.0 | Upper Midwest, New England | 9.56 kW | 24 | 45.7 m² / 492.1 ft² |
| 4.5 | US average | 8.50 kW | 22 | 41.9 m² / 451.1 ft² |
| 5.0 | Mid-Atlantic, central plains | 7.65 kW | 20 | 38.1 m² / 410.1 ft² |
| 5.5 | Southern California, Texas | 6.95 kW | 18 | 34.3 m² / 369.0 ft² |
| 6.0 | Nevada, New Mexico | 6.37 kW | 16 | 30.5 m² / 328.0 ft² |
| 6.5 | Arizona desert | 5.88 kW | 15 | 28.6 m² / 307.5 ft² |
Same house, same bill, same panels: 28 modules in Seattle, 15 in Phoenix. Nearly double the hardware for identical electricity. Anyone answering "how many solar panels do I need" with a single national number is answering a different question than the one you asked. Look up your own figure and the answer stops being a range and becomes a number.
Panel wattage is the lever people expect to matter most, and it mostly does not — not for roof space, anyway.
| Panel wattage | Panels needed | Installed capacity | Roof area |
|---|---|---|---|
| 350 W | 22 | 7.7 kW | 36.7 m² / 394.7 ft² |
| 400 W | 20 | 8.0 kW | 38.1 m² / 410.1 ft² |
| 450 W | 17 | 7.7 kW | 36.4 m² / 392.1 ft² |
| 500 W | 16 | 8.0 kW | 38.1 m² / 410.1 ft² |
Going from 350 W to 500 W cuts the count from 22 panels to 16, and the roof area lands within two square metres of where it started. Higher-wattage modules are physically bigger in almost the same proportion. What you actually buy is fewer roof penetrations, rails and mounting points — a labour saving, not a space saving. The number that genuinely shrinks your footprint is module efficiency, watts per square metre. This assumes 21% modules, about 210 W/m², which is typical of what is sold today.
Where a sizing estimate goes wrong
The roof area the Solar Panel Calculator reports is panel area. It does not include walkways, fire-code setbacks from the roof edge, or clearance around vents, chimneys and skylights. Treat 410 ft² as a floor and plan on meaningfully more usable roof. Orientation is not modelled either: peak sun hours assume a sensibly tilted, unshaded, roughly south-facing array. A north slope or an afternoon of shade from one oak tree will undercut the production figure, and no calculator can see your trees.
The panel count always rounds up, because you cannot buy 19.11 panels. The system size is the honest answer to the arithmetic; the count is what gets installed. Twenty panels at 400 W is 8.0 kW rather than the 7.65 kW you strictly needed. That overshoot is usually welcome — headroom for an EV, a heat pump, or the slow degradation that accumulates over 25 years.
And if your usage is about to change, last year's bill is the wrong target. Adding an EV can push a household up by 250–400 kWh a month on its own. Size for the house you are about to have, not the one on the bill. If your roof or your utility will not allow a full array, drop the offset instead: at 70%, the 1,000 kWh house falls from 20 panels to 14 and produces 8,400 kWh a year.
Related calculations
If you do not know your monthly kWh and cannot find a bill, build it up appliance by appliance with the energy consumption calculator — it turns watts and hours-per-day into the kWh figure the Solar Panel Calculator expects.
On the electrical side, watts to amps and kW to amps answer the question that comes next: whether your existing service panel has room for the array you just sized. An 8 kW system is a real current draw on the AC side, and busbar capacity is a common reason a quote comes back smaller than the calculation.
For the roof itself, the roof pitch calculator converts a rise-over-run into the tilt angle installers work in, and the square footage calculator adds up your actual roof planes so you can check them against the 410 ft² this tool asks for.
Frequently asked questions
How many solar panels do I need for a 2,000 square foot house?
Square footage is a poor proxy — two 2,000 ft² houses can differ threefold in electricity use depending on heating, air conditioning and how many people live there. Use the bill instead. If you genuinely have no bill to read, roughly 900 kWh a month is a common US household figure, which at 4.5 peak sun hours works out to 7.65 kW and 20 panels of 400 W, producing about 10,800 kWh a year. Replace that placeholder with your real number the moment you have it.
How many panels does a $200 electricity bill need?
Depends entirely on your rate. At $0.17/kWh, a $200 bill is about 1,176 kWh a month — at 4.5 peak sun hours that is a 9.99 kW system, 25 panels, and 14,118 kWh a year. At $0.34/kWh the same $200 buys half the electricity and needs half the array. That is why Bill mode asks for your rate rather than assuming one: the dollar figure on its own tells you nothing about how much power you actually used.
Why does annual production stay at 12,000 kWh when I change the sun hours?
Because that is the point of the offset. At 100%, the array is sized by definition to make exactly what you consume — 1,000 kWh × 12 = 12,000 kWh a year. Moving the sun hours changes how much hardware it takes to hit that target, not the target itself. Seattle needs 28 panels and Phoenix needs 15, and both produce 12,000 kWh. If that number ever drifts away from your annual usage at 100% offset, the arithmetic is wrong — which is exactly why a month is counted as 30.4 days here.
Can I use this outside the United States?
Yes. Kilowatt-hours and peak sun hours are both international units, and PVWatts covers most of the world. The dollar sign in Bill mode is cosmetic — the calculation is a division, so any currency works as long as the bill and the rate are in the same one. The only thing to watch is that some utilities bill every two months; halve that figure before entering it.
What if I only have my annual kWh, not my monthly?
Divide by 12 first. Entering an annual figure in the monthly field returns an array twelve times too big, and the number looks plausible enough that people have been known to take it to an installer. If your usage swings hard between summer and winter, the annual total divided by 12 is the better input anyway — solar sizing is an annual balance, and a single August bill will oversize the array badly.
Is a 10 kW system big enough for my house?
Run it backwards: a 10 kW array at 4.5 peak sun hours produces about 14,125 kWh a year, which covers roughly 1,177 kWh a month. At 5.0 sun hours it is closer to 15,695 kWh, or 1,308 kWh a month. Compare that against your own bill. If your usage is under it, 10 kW covers you with room to spare; if it is over, either add panels or accept a partial offset and pay the utility for the difference.
Why is there no address field, no email, and no savings estimate?
Because that is the shape of the trap in this category. The standard solar panel calculator is a lead form wearing a calculator's clothes: it asks for your address, then your email, then your phone number, and the "custom report" arrives in the form of installers calling you. The calculation was never the product — your contact details were. This one asks for four numbers, runs entirely in your browser, sends nothing anywhere, and stores nothing. You look up your own sun hours, which is the honest trade for not handing over your address. Savings and payback need local install costs, this year's incentives and your utility's net-metering rules, all of which go stale fast and none of which belong in a sizing tool. Take the system size to two or three installers and let them compete on the money.