How many solar panels do I need?
Divide your annual electricity use in kWh by your location’s peak sun hours times 365 times 0.80, and you get the system size in kilowatts; multiply that by 1,000 and divide by your panel wattage to get the panel count — so a home using 12,000 kWh a year in 5 peak sun hours needs about 8.2 kW, or roughly 21 panels at 400 W each.
There is no correct national panel count, because the two inputs that decide it — your usage and your sun — are both local. Anyone giving you a single number without asking for either is guessing.
The formula, and why each term is there
system kW = annual kWh ÷ (peak sun hours × 365 × 0.80)
panels = (system kW × 1,000) ÷ panel watts
Annual kWh is yours and only yours. Take twelve months of bills and add the kWh column — not the dollars, the kilowatt-hours. If you only have one bill, you can back into it: subtract the fixed monthly service charge (call it $15, it appears on your bill whether you use any power or not), divide what is left by your rate per kWh, and multiply by twelve. That is rougher, and it ignores seasonality, which in a house with air conditioning is not a small thing.
Peak sun hours is not daylight hours. It is the number of hours per day at which the sun would have to deliver a full 1,000 watts per square metre to match the day’s total energy. It is a way of collapsing a whole year of weather into one number. It runs from roughly 3 in Alaska to roughly 6 in Arizona.
0.80 is the derate, and it is where honest models separate from optimistic ones. NREL’s PVWatts default is 0.86 for a clean, ideally oriented array. Real roofs are not ideally oriented. Once you allow for actual azimuth and tilt, inverter losses, wiring, soiling, heat and some clipping, 0.80 is the number that stops the model lying to you. If a proposal implies something closer to 0.90, ask what they assumed about your roof.
Panel watts is the least interesting variable, and the one salespeople talk about most. Modules around 400 W are common now. Higher-wattage panels mean fewer panels for the same kW, which matters if roof space is tight and matters very little otherwise. You are buying kilowatts. The panel count is just how they got packaged.
Sizing is in kW. Panel count is a consequence. You will see pages quoting a panel count to one decimal place. There is no such thing as 0.3 of a panel, and a count carried to a decimal is a range dressed up as precision.
Worked example: the same 12,000 kWh house in six states
Same house, same usage, six different answers. This is why a static national number is useless: the sun does the dividing.
| State | Sun hours | kWh per kW/yr | System size | Panels @ 400 W |
|---|---|---|---|---|
| Arizona | 6.0 | 1,752 | 6.8 kW | 17 |
| California | 5.5 | 1,606 | 7.5 kW | 19 |
| Texas | 5.1 | 1,489 | 8.1 kW | 20 |
| Florida | 5.2 | 1,518 | 7.9 kW | 20 |
| Ohio | 4.1 | 1,197 | 10.0 kW | 25 |
| New York | 4.0 | 1,168 | 10.3 kW | 26 |
Worked example: by bill size
Most people know their bill better than their kWh, so here is the same arithmetic run backwards from a monthly bill. This uses the Texas rate of 15¢ and 5.1 sun hours, and subtracts a $15 fixed service charge first, because that part of your bill does not go away when you install solar.
| Monthly bill | Implied annual kWh | System size | Panels @ 400 W |
|---|---|---|---|
| $120 | 8,400 | 5.6 kW | 14 |
| $200 | 14,800 | 9.9 kW | 25 |
| $300 | 22,800 | 14.0 kW | 35 |
| $450 | 34,800 | 14.0 kW | 35 |
Note what the clamp does at the top end. A very large bill does not produce a very large array, because at some point you run out of south-facing roof and your utility starts treating you as a different class of customer. The honest answer for a $600 bill is often “solar covers most of it, not all of it, and you should look at why the bill is that size first.”
What a static answer gets wrong
Every widely-ranking answer to this question is a range: “most homes need 15 to 25 panels.” That is not wrong, exactly. It is just not an answer. The range spans a factor of nearly two, which on a $25,000 purchase is a five-figure difference, and it is presented as if the reader should pick a middle.
Four things the range cannot know:
| Input | Why it moves the answer | How much |
|---|---|---|
| Your annual kWh | It is the numerator. Two identical houses can differ twofold on habits, occupancy and heating fuel. | Everything |
| Peak sun hours | The denominator. Arizona to Alaska is roughly double. | Up to 2× |
| Shade | Partial shade takes a real bite out of annual output even with optimisers, so the array has to be larger to hit the same production. | Meaningful |
| Roof plane | Orientation, pitch and how many usable faces you have. Splitting an array across two planes costs production and money. | Meaningful |
All four are things you know or can find out in about ten minutes. There is no reason to accept an average when the arithmetic is this cheap to run — which is the entire argument for computing it instead of looking it up.
Cost per watt: how to compare two quotes
Once you know the size, price it per watt. Gross installed cost divided by system watts. It is the only number that lets you compare a 7 kW quote against a 10 kW quote, and it is the number a monthly payment is designed to obscure.
| State | Modelled $/W | 8 kW gross | Rate | Sun hours |
|---|---|---|---|---|
| Arizona | $2.50 | $20,000 | 14.5¢ | 6.0 |
| California | $3.30 | $26,400 | 31.8¢ | 5.5 |
| Texas | $2.55 | $20,400 | 15¢ | 5.1 |
| Florida | $2.45 | $19,600 | 15.3¢ | 5.2 |
| Ohio | $2.80 | $22,400 | 15.5¢ | 4.1 |
| New York | $3.40 | $27,200 | 24.5¢ | 4.0 |
Two rules when you have quotes in hand. First, compare gross cost per watt, before any incentive is subtracted — otherwise you are comparing two different guesses about your tax situation. Second, if one quote is far cheaper per watt, find out what is different: equipment tier, inverter type, whether the roof work is included, and what the workmanship warranty covers. Cheap per watt is good. Cheap per watt because the array is going on a roof that needs replacing in four years is not.
Questions about sizing
How many solar panels do I need for a house?
Take your annual electricity use in kWh, divide it by your state's peak sun hours times 365 times 0.80, and you get the system size in kilowatts. Multiply by 1,000 and divide by your panel wattage — 400 W is a common modern module — and that is your panel count. A home using 12,000 kWh a year in 5 peak sun hours needs about 8.2 kW, or roughly 21 panels. The same house in 4 sun hours needs about 26.
What size solar system do I need?
Size to your own annual kWh, not to your roof. Annual kWh divided by (peak sun hours × 365 × 0.80) gives kilowatts. Most single-family homes land between 6 and 12 kW. Below about 3 kW nobody will sell you an install; beyond about 14 kW you tend to run out of usable roof plane and your utility may push you onto a different tariff.
How many kWh does one solar panel produce per year?
A 400 W panel in a location averaging 5 peak sun hours produces roughly 584 kWh a year after a 0.80 derate for inverter losses, wiring, soiling, heat and imperfect roof orientation. In 4 sun hours the same panel makes about 467 kWh.
What is a good cost per watt for solar?
Cost per watt is gross installed price divided by system watts, and it is the only honest way to compare two quotes of different sizes. A 9 kW system at $27,000 is $3.00 per watt. Modelled installed costs across the states on this site run roughly $2.40 to $3.60 per watt before incentives, with labour, permitting and roof complexity driving most of the spread. Compare cost per watt before you compare monthly payments — the payment hides the price.
Should I size solar to cover 100% of my usage?
Usually about that, rarely more. Production beyond what you use gets exported, and exports are credited at whatever your utility pays — which in most states is now below the retail rate. Building past your own consumption means selling power cheaply to buy it back expensively. If you are adding an EV or a heat pump soon, size for that usage, not today's.
Do I need more panels if my roof faces east or west?
Yes, somewhat. A south-facing plane at a sensible pitch is the reference case. East or west costs you production, split arrays across two planes cost a little more, and north-facing planes are usually not worth mounting on. Shade costs more than orientation does: even partial shade on part of an array takes a meaningful bite out of annual output, and the fix is a site survey, not a bigger number in a spreadsheet.