Calculator
Solar Array Size Calculator
Dividing annual electricity use by a panel's nameplate wattage and the hours the sun is up gives a number that looks like a system size and is not one, because a panel almost never produces its nameplate wattage and the sun is not equally strong across every hour it is above the horizon. What actually determines system size is peak sun hours — a location-specific average of full-strength-equivalent sunlight per day — combined with the real loss stack between a panel's rating and the power that actually reaches your meter: inverter conversion, wiring resistance, dust and soiling, and heat, which all take a bite before any of it counts.
This calculator works backward from a target production figure to an exact DC system size, rounds that up to a whole number of panels — because a fraction of a panel is not a purchasable thing — and then, rather than stopping there, recalculates what that whole-panel system will actually produce and what percentage of your usage it actually offsets. A naive kWh-divided-by-hours estimate stops at the theoretical size and never checks what the buildable system, panel by panel, comes out to.
Calculate your quantity
kWh — add up twelve bills
Daily average for your location.
watts
System size
8.4 kW DC
Panels
21 panels
- Target production
- 11,000 kWh/yr
- Exact size required
- 8.37 kW DC
- Estimated production
- 11,038 kWh/yr
- Average month
- 920 kWh
- Roof area needed
- 441 sq ft
- Actual offset
- 100.3 %
- The 0.8 derate is the whole system loss stack — inverter, wiring, soiling, temperature and mismatch. It is not optional padding; leaving it out undersizes an array by roughly 25%.
Shopping summary
- • 21 x 400 W panels (8.4 kW), needing about 441 sq ft of unshaded roof
This is an estimate — confirm structural work with a professional.
How this calculation works
Target production starts from your annual usage times the share of it you want the array to offset — 100% for a system meant to fully replace grid electricity, less for a partial system. Dividing that target by peak sun hours times 365 days times the derate gives the exact DC kilowatt size needed, in decimal kilowatts that no panel actually comes in.
Panel count rounds that exact size up to a whole number, because you cannot buy 20.9 panels — and once the panel count is fixed, the calculator works forward again from that real, whole-number system to figure out its actual production and actual offset percentage, which will usually land a little above the target rather than exactly on it. That final step is the one a back-of-envelope estimate skips: it stops at the theoretical size and never checks what the array you can actually build produces.
- The roof area this page reports is a first check, not a design — it says nothing about shading, roof orientation, structural bracing, or setbacks a real installer has to account for.
- Peak sun hours is a published average for your general location, not a guarantee for your specific roof's orientation and shading — a south-facing, unshaded roof will beat the average; a shaded or poorly oriented one will fall short of it.
- The derate is not a padding factor you can shave off on a good site. It is the aggregate of several real losses that exist on every grid-tied system, and skipping it undersizes the array by roughly the derate's own gap from 1.0.
- Utility net-metering rules — what exported power is actually worth compared to imported power — are outside this page entirely, and they can make a lower offset target the better economic choice even on a roof that could physically support 100%.
The formula
targetKwh = annualKwh x offsetPct/100; kwDc = targetKwh / (peakSunHours x 365 x derate); panels = ceil(kwDc x 1000 / panelWatts); actualProduction = (panels x panelWatts/1000) x peakSunHours x 365 x derate
- annualKwh x offsetPct/100
- Your total yearly electricity use, scaled to the share of it you actually want this array to offset — the target production the system size is solved backward from.
- peakSunHours
- The daily average of full-strength-equivalent sunlight for your location, a published resource figure that is not the same as hours of daylight and is usually meaningfully lower than it.
- derate
- The combined real-world loss fraction between a panel's DC nameplate rating and the AC power that actually reaches your meter — inverter conversion, wiring, soiling, temperature, and module mismatch, folded into one multiplier.
- panels = ceil(...)
- The exact kilowatt figure rounded UP to a whole panel count, because the exact decimal size the arithmetic produces is never itself a purchasable system.
- actualProduction
- Recalculated from the real, whole-panel system size rather than the theoretical target — this is the number that answers what the array you can actually order will produce, and it is what the offset percentage on this page is based on.
Where these numbers come from
- 4.5 peak sun hours default
- Published solar-resource data: a mid-range daily average drawn from the kind of location-specific figures NREL's PVWatts tool provides. Real values run from roughly 3 in cloudier northern regions to over 6 in the sunniest parts of the desert Southwest, which is why this field is meant to be replaced with your own location's figure rather than left at default.
- 0.8 system derate
- Trade convention: a commonly used starting point for the combined inverter, wiring, soiling, temperature and mismatch losses on a typical grid-tied residential system. Some tools default closer to 0.85; a shadier site or a hotter climate can push real losses higher than either.
- 400 W panel, 21 sq ft per panel
- Manufacturer-published: representative of a common modern residential panel's rated output and physical footprint. Panel wattage and size both vary meaningfully by product, which is why both are page inputs rather than fixed constants.
- 100% target offset default
- Trade convention: a common design goal for a net-metered system aiming to zero out grid electricity purchases over a year, though it is a starting point rather than the right number for every household or every utility's export rules.
Worked examples
A full-offset system on a sunny roof
| Annual electricity use | 11000 |
|---|---|
| Peak sun hours | 4.5 |
| System derate | 0.8 |
| Panel rating | 400 |
| Target offset | 100 % |
| Area per panel | 21 sq ft |
| System size | 8.4 kW DC |
|---|---|
| Panels | 21 panels |
| Target production | 11000 kWh/yr |
| Exact size required | 8.37 kW DC |
| Estimated production | 11038 kWh/yr |
| Average month | 920 kWh |
| Roof area needed | 441 sq ft |
| Actual offset | 100.3 % |
The exact size needed is 8.37 kW, which rounds up to 21 panels at 400 watts each — 8.4 kW of real, buildable system. Recalculating production from that whole number rather than the theoretical 8.37 kW gives 11,038 kWh a year against an 11,000 kWh target, a 100.3% actual offset: the rounding-up step naturally lands you slightly above the target, never below it.
441 square feet of roof is what 21 panels physically need, which is a first pass at whether the roof has room — the real answer also depends on which face of the roof that area sits on and how much of it is shaded at different times of day, neither of which this page can see.
A partial-offset system on a cloudier site with higher usage
| Annual electricity use | 16000 |
|---|---|
| Peak sun hours | 3.2 |
| System derate | 0.75 |
| Panel rating | 350 |
| Target offset | 60 % |
| Area per panel | 18 sq ft |
| System size | 11.2 kW DC |
|---|---|
| Panels | 32 panels |
| Target production | 9600 kWh/yr |
| Exact size required | 10.96 kW DC |
| Estimated production | 9812 kWh/yr |
| Average month | 818 kWh |
| Roof area needed | 576 sq ft |
| Actual offset | 61.3 % |
This household targets only 60% offset, yet ends up with more than half again as many panels as the fully-offset example above — 32 against 21 — because 3.2 peak sun hours and a 0.75 derate mean every panel here produces meaningfully less than one in the sunnier example, and 16,000 kWh of annual use is nearly half again as much to begin with. Offset percentage alone tells you almost nothing about panel count without knowing the site and the usage behind it.
The lower panel wattage (350 W) and smaller per-panel footprint (18 sq ft) here also mean the 576 square feet of roof this system needs is spread across more, smaller units than the first example's — worth knowing before assuming a bigger kW number always means a proportionally bigger physical footprint.
Common mistakes
- Treating peak sun hours as the same thing as hours of daylight. A location can have 14 hours of daylight and only 4.5 peak sun hours, because peak sun hours measure sunlight intensity averaged into full-strength-equivalent hours, not literal time the sun is above the horizon.
- Skipping the derate to make a system look smaller and cheaper on paper. It is not padding — omitting it typically undersizes a real array by roughly a quarter, which shows up months later as a system that never quite reaches the offset it was sold on.
- Assuming the roof-area figure this page reports means the array will fit as designed. It is the raw square footage the panel count needs with no allowance for shading, setbacks, structural bracing points, or which roof face the panels actually sit on.
- Sizing to 100% offset by default without checking what your specific utility actually pays for exported power under its net-metering rules — on some rate structures, a smaller system that offsets less but exports less surplus is the better financial outcome even on a roof that could carry more.
- Reading the actual-offset percentage as a guarantee rather than a modeled estimate tied to the peak-sun-hour figure entered. A below-average sun year, added shading from a tree that grows in over a decade, or panel degradation over the system's life will all pull real production below this page's number.
- Rounding panel count down instead of up to hit a target budget. The panel math here rounds up on purpose — a system built a fraction of a panel short of the target size does not average out to the goal, it simply falls short of it every single year.
Shopping summary
Take the panel count and system size to installers for quotes, and treat the roof-area figure as a starting conversation about layout, shading, and which roof face the array should actually go on — not as a finished design.
If you are adding an EV to the household, add its expected annual charging kWh into the usage figure here before sizing rather than sizing the array from pre-EV bills alone — the EV charger circuit calculator sizes the circuit itself, but the charging load belongs in this page's annual usage input.
A heat pump water heater draws differently than a standard tank and is worth adding to your usage total the same way; check its demand on the water heater size calculator. And if backup power during an outage matters as much as offsetting your bill, compare a battery-backed solar system's economics against the generator size calculator rather than assuming solar alone covers both goals.
FAQ
What's the actual difference between peak sun hours and daylight hours?
That gap is exactly why a weather app's 'hours of sunshine' figure is the wrong number to size a system from — those apps typically report something closer to daylight hours or cloud-cover duration, not the intensity-weighted figure this page needs. Pull your actual peak sun hour average from a resource like NREL's PVWatts tool for your specific location instead; the difference between a nearby city's published figure and your own can be enough to shift the panel count by two or three panels on an otherwise identical roof.
Why doesn't my actual offset land exactly on the percentage I targeted?
Because panel count has to round up to a whole number, and the real system built from that whole number almost always produces slightly more than the exact target. Aiming for 100% offset typically lands a system in the 100 to 105% range once the panel count is rounded up rather than exactly at 100.0%.
Is the derate figure just conservative padding I can skip on a really good site?
No — it represents real, physical losses that exist on every grid-tied system: the inverter converting DC to AC loses a few percent on its own, wiring has resistance, panels lose output as they heat up in the sun, and dust or pollen reduces output between cleanings. A very good site can push the derate toward the higher end of typical values, but it does not eliminate the loss stack.
Does the roof area figure mean my roof will actually fit the array?
It means the panel count needs at least that much unshaded, usable area — it does not check whether your roof has that much contiguous space on a workable face, whether that space is shaded by trees or vents at any point in the day, or whether the structure can carry the load without reinforcement. That is design work for an installer's site visit.
Should I always size to 100% offset if my roof can physically support it?
Not necessarily. Whether 100% offset is the right financial target depends on your utility's net-metering rules — some pay less for exported power than they charge for imported power, which can make a smaller system that exports less surplus the better economic choice even when a larger one would fit.
How should I account for adding an EV or a heat pump water heater later?
Add their expected annual kWh into this page's usage figure before sizing, rather than sizing for current usage and hoping there's headroom. An EV alone can add several thousand kWh a year depending on how much it's driven, which is often enough to change the panel count meaningfully.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.