What size solar system do I need?

Divide your annual kilowatt-hours by peak sun hours, 365 and a derate factor. A house using 14,000 kWh a year at 4.5 peak sun hours and a 0.80 derate needs about 10.65 kW DC, which is roughly 27 panels at 400 W. The answer is driven far more by where you live than by anything about the equipment.

What are peak sun hours and how do they differ from daylight hours?

Peak sun hours is the equivalent number of hours per day at full test-condition irradiance, typically 3.5 to 6 across the continental US. Daylight hours are how long the sun is up, which is 12 or more. Using daylight hours in this calculator will undersize your array by half or worse, and it is the most common mistake made with this formula.

What is a derate factor?

A single multiplier covering every loss between the module label and your meter: inverter conversion, wiring resistance, soiling, high cell temperature, mismatch between modules and downtime. The 0.80 default is a conventional planning value. PVWatts models about 14% system losses, corresponding to roughly 0.86.

How many solar panels is 10 kW?

It depends entirely on module wattage. Ten kilowatts is 25 modules at 400 W, 23 at 440 W or 20 at 500 W. System size in kW is what determines production; panel count determines how much roof it covers. Our panel count calculator handles the conversion with a specific module.

How much roof area do I need?

The area figure here is modules only, about 511 square feet for a 10.65 kW system at 440 W. Real installations need fire access pathways and setbacks from ridges and eaves, and every roof has vents, hips and chimneys. Plan on needing substantially more usable plane than the module area suggests.

Should I size for 100% of my usage?

Not necessarily. This model assumes full annual offset, but under net billing or a buyback tariff the kilowatt-hours you export are credited below retail, so generating a large surplus can be worth much less than it costs. Sizing closer to your daytime load is a legitimate strategy and sometimes the better one.

Should I size for an electric car I do not own yet?

If it is genuinely coming, yes. Adding capacity later means a second mobilization, possibly a second interconnection application and sometimes a panel upgrade that a single co-ordinated design would have avoided. Add the vehicle’s expected annual kilowatt-hours to your usage figure before sizing.

Does this calculator account for shade?

No, and that is its most important limitation. There is no shading input. A federal methodology excludes roof area that cannot produce 80% of an unshaded system at the same orientation, so a shaded plane may not be usable at all regardless of what this size says. Read our page on shade before trusting a number for a shaded roof.

Why is my inverter smaller than my system size?

Because DC nameplate is measured under standard test conditions that a real roof rarely reaches. Deliberately pairing a smaller inverter with a larger array is normal practice and costs very little annual production, since the array only exceeds the inverter’s rating for a few hours in the year.

My roof cannot fit the size this gives me. What now?

Three options, in order of how often they work. Use higher-wattage modules, which cuts the count and the area for the same capacity. Accept a smaller system and a partial offset, which is usually fine economically. Or move the array off the roof entirely, our ground mount page covers what the code requires when you do.

HyreSolar tools

Solar system size calculator

Turns a year of electricity use into kilowatts of panels, a panel count and a roof area. A planning figure, not a shade-aware design.

Checked September 3, 2026. Data as of August 16, 2026. HyreSolar sells and installs nothing.

This solar system size calculator gives an estimate from your own numbers in under a minute, with every assumption listed under the result.

System size in kW DC is annual kilowatt-hours divided by 365, by peak sun hours and by a derate factor. A house using 14,000 kWh a year at 4.5 sun hours and a 0.80 derate needs about 10.65 kW: roughly 27 panels at 400 W, covering about 511 square feet before setbacks.

At 5.5 sun hours it needs 8.72 kW; at 3.5, 13.70 kW.

System size is the number every solar quote starts from, so it is worth knowing before an installer tells you. The calculation needs a year of electricity use, not a single month, because usage swings with heating and cooling.

Most utilities show twelve months of kilowatt-hours on the bill or in an online account; add them up. The other two inputs, peak sun hours and the derate factor, describe your location and the losses between the panel and the outlet.

Peak sun hours are the equivalent number of hours a day of full-strength sun your area gets, averaged over the year. NREL’s PVWatts tool gives a site-specific figure free; the default here is a middle-of-the-country value. The derate factor covers wiring, inverter, soiling and temperature losses.

A system sized for 100% of use is not always the right target: under net billing, exports earn less than imports, and a smaller system that matches daytime use can return more per dollar.

The result also gives a rough panel count and roof area, which is the first check on whether your roof can hold the system at all. Fire-code setbacks from ridges and edges reduce usable area, so leave margin.

Take the kilowatt figure to the cost estimator for a price, or to the panel count calculator to test different module sizes. If the size fits, the form under the result sends it to an installer for a design on your actual roof.

System size model

Annual usage in, kW DC and panel count out.

Example values The filled-in numbers are an example, not your home. Type over them with your own.

— System size

— Panels

— Module area

How this result was worked out

Working it out from the numbers above.

Planning size for full annual offset. Not a shade-aware design.

PVWatts beats these defaults for your roof. HyreSolar does not sell systems.

What this size actually means

The number is DC nameplate capacity, which is what panels are sold in and what proposals quote. It is the sum of the module wattages, measured under standard test conditions your roof will almost never see. Your inverter will be smaller, and that is normal rather than a mistake.

This sizes for a full offset of the usage you entered. It assumes you want to generate as many kilowatt-hours in a year as you consume.

That is a choice, not a law, and under a tariff that pays poorly for exports it is often the wrong choice.

Sizing to your daytime load rather than your annual total is a legitimate and sometimes better strategy.

Roof area is the constraint that most often overrules the answer. The square footage shown is modules only. It excludes fire access pathways, setbacks from the ridge and eaves, vents, hips, chimneys and anything shaded. Usable roof is routinely a fraction of total roof.

Under 6 kW: A small system, a small house, or an unusually sunny site. Check the annual kWh figure is a full twelve months rather than a summer bill multiplied up.
6 to 14 kW: The ordinary residential range. Most single-family roofs can physically carry this if the orientation is reasonable.
Over 14 kW: Large. Worth checking whether your electrical service can take it, whether the roof has the area, and whether your utility’s interconnection screens treat it differently.

Where to find each number

Where each input comes from, so nothing is a guess.

1. Enter twelve months of kilowatt-hours

A full year, not a month multiplied by twelve. Consumption is seasonal and a single bill will mislead you in either direction depending which month you pick.

Where to find it: Most utilities publish twelve months of usage history in the online account. Failing that, add up twelve bills. If you have a new heat pump or an electric car arriving, add its expected annual kWh before you size anything.

2. Enter peak sun hours for your location

Not hours of daylight. Peak sun hours is the equivalent number of hours per day at full test-condition irradiance, and it already accounts for the sun being low in the sky.

Where to find it: NREL PVWatts reports the irradiance figure for your address. Continental US values typically fall between about 3.5 and 6. Using daylight hours here will undersize the array by a factor of two or more.

3. Set the derate factor

A single multiplier covering everything between the module label and the meter: inverter losses, wiring, soiling, temperature, mismatch and downtime. The 0.80 default is a conventional planning value.

Where to find it: PVWatts uses a default system loss of about 14%, which corresponds to a derate near 0.86. Older or shaded installations run lower. Moving from 0.75 to 0.85 changes this house from 11.36 kW to 10.03.

4. Read the panel count against your roof

The tool gives counts at 400 W and 440 W because module wattage changes the count substantially for the same system size.

Where to find it: The proposal names the module. If you do not have one yet, 400 to 450 W is the common residential band in 2026. Our panel count calculator takes this further with a specific wattage and module area.

5. Treat the area figure as a floor

It is module area only. Real installations need access pathways and setbacks, and no roof plane is a perfect rectangle.

Where to find it: Measure the usable plane on a satellite image, subtract obstructions, and expect to lose more than you think. If the array will not fit, read ground mount versus roof mount.

How the calculator works it out

1
Start from a year of consumption
Annual kilowatt-hours, from twelve months of bills rather than one.
2
Divide by daily energy per kW
Peak sun hours × 365 gives the annual kWh one kilowatt of DC capacity would produce with no losses.
3
Apply the derate
One multiplier for inverter, wiring, soiling, temperature and mismatch losses.
4
Convert to panels and area
Divide by module wattage and round up, then multiply by module area.

The formula, in full. kW DC = annual kWh ÷ (peak sun hours × 365 × derate). panels = ceil(kW × 1000 ÷ module watts). area = panels × 1.9 m² at 440 W, converted to square feet.

A worked example, start to finish

A household consuming 14,000 kilowatt-hours a year, at a site with 4.5 peak sun hours, using the conventional 0.80 planning derate.

Step	Value
Annual consumption	14,000 kWh
Peak sun hours	4.5 / day
Days	365
Energy per kW, no losses	1,642 kWh / kW / yr
Derate	0.80
Energy per kW, after losses	1,314 kWh / kW / yr
Result	10.65 kW DC

27 panels at 400 W, or 25 at 440 W. About 511 square feet of modules before a single setback or access pathway is subtracted. If your usable roof plane is under roughly 700 square feet, this system will not fit as drawn.

How the answer moves

The same 14,000 kWh house, one input changed at a time. Every figure is computed by the calculator on this page.

Change	System size	Panels at 400 W	Module area
Base case4.5 sun hours, 0.80 derate	10.65 kW	27	511 sq ft
Cloudier site, 3.5 sun hours	13.70 kW	35	654 sq ft
Sunnier site, 5.5 sun hours	8.72 kW	22	409 sq ft
Pessimistic derate, 0.75	11.36 kW	29	532 sq ft
Optimistic derate, 0.85	10.03 kW	26	470 sq ft

Location moves the answer by 4.98 kW across the range shown. The derate, which people argue about, moves it by 1.33. Get the sun hours right first.

What moves this number most

Ranked, largest first. A proposal can change any of these without saying anything untrue, so check these inputs first.

1. Peak sun hours

The dominant input and a property of where you live. The same house needs 8.72 kW in a 5.5-sun-hour location and 13.70 kW in a 3.5-hour one, a 57% difference in system size for identical consumption.

2. Your annual consumption

Linear: double the kilowatt-hours, double the system. It is also the input most likely to change after you install, because heat pumps and electric cars are large new loads. See what an EV does to a house.

3. The derate factor

Covers inverter, wiring, soiling, temperature and mismatch in one number. Between 0.75 and 0.85 the answer moves by about 12%. Worth getting roughly right, not worth agonizing over.

4. Module wattage

Does not change the system size at all, only the panel count and the area. The same 10.65 kW is 27 modules at 400 W or 25 at 440 W. Higher wattage matters when roof area is the binding constraint.

5. Shade, which is not in this model at all

This calculator has no shading input. A plane that cannot produce 80% of an unshaded system at the same orientation is excluded outright by the federal methodology. Read shade and solar output before trusting any size on a shaded roof.

6. Whether you want full offset

The model assumes you do. Under net billing or a buyback tariff, generating far more than you consume during daylight can be worth much less than it looks. See how your exports are actually credited.

Common mistakes with this calculation

Using daylight hours instead of peak sun hours
The most common and most damaging error here. Peak sun hours is an irradiance equivalent, typically 3.5 to 6 in the continental US, not the 12 or 14 hours the sun is above the horizon. Entering daylight hours undersizes the array by half or more.
Sizing from one bill
Consumption swings seasonally. A January bill in a heating-electric house and a July bill in an air-conditioned one describe different houses. Use twelve months.
Sizing to today’s usage when tomorrow’s is larger
An electric car adds a large annual load, and a heat pump replaces gas consumption with electric. If either is coming, size for the house you will have, because adding panels later is a second mobilization and often a second interconnection application.
Treating module area as roof area
The square footage here is panels only. Fire access pathways, ridge and eave setbacks, vents, hips and chimneys all take space. Usable area is routinely far less than the plane looks.
Assuming a bigger system is always better
Under a tariff that credits exports below retail, oversizing buys kilowatt-hours worth a fraction of what they cost you. The right size depends on the tariff as much as on the roof.
Ignoring the electrical service
A large array interacts with your panel’s busbar rating. If the service is 100 A, size may not be the binding constraint at all. Our load management page covers the arithmetic.

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What this calculator leaves out

The result is a planning estimate. A real proposal accounts for these; this page does not.

Not a shade-aware design.
There is no shading input in this model at all.
Sizes for full annual offset, which is not always the right target under net billing.
Area is module area only.
Setbacks, access pathways and obstructions are excluded.
Does not check whether your electrical service or roof structure can carry the array.
Peak sun hours and derate are planning inputs.
PVWatts is better for both.

The research behind these numbers

Shade and solar outputThe input this calculator does not have. Two independent sources both exclude roof area below 80% of unshaded output.Read the research What panel efficiency actually buysWhy higher-wattage modules change the panel count and the area but not the system size.Read the research How your exports are creditedWhether sizing for full annual offset is the right goal on your tariff.Read the research Ground mount versus roof mountWhat to do when the array will not fit on the roof, and the 10-foot perimeter the fire code imposes instead.Read the research Electricity consumption by stateWhere your annual kilowatt-hours sit against your state, computed from EIA-861.Read the research Interconnection screensWhy system size interacts with your utility’s review, and what changes above certain thresholds.Read the research

Questions this calculator answers

What size solar system do I need?: Divide your annual kilowatt-hours by peak sun hours, 365 and a derate factor.; A house using 14,000 kWh a year at 4.5 peak sun hours and a 0.80 derate needs about 10.65 kW DC, which is roughly 27 panels at 400 W.; The answer is driven far more by where you live than by anything about the equipment.
What are peak sun hours and how do they differ from daylight hours?: Peak sun hours is the equivalent number of hours per day at full test-condition irradiance, typically 3.5 to 6 across the continental US.; Daylight hours are how long the sun is up, which is 12 or more.; Using daylight hours in this calculator will undersize your array by half or worse, and it is the most common mistake made with this formula.
What is a derate factor?: A single multiplier covering every loss between the module label and your meter: inverter conversion, wiring resistance, soiling, high cell temperature, mismatch between modules and downtime. The 0.80 default is a conventional planning value. PVWatts models about 14% system losses, corresponding to roughly 0.86.
How many solar panels is 10 kW?: It depends entirely on module wattage. Ten kilowatts is 25 modules at 400 W, 23 at 440 W or 20 at 500 W. System size in kW is what determines production; panel count determines how much roof it covers. Our panel count calculator handles the conversion with a specific module.
How much roof area do I need?: The area figure here is modules only, about 511 square feet for a 10.65 kW system at 440 W. Real installations need fire access pathways and setbacks from ridges and eaves, and every roof has vents, hips and chimneys. Plan on needing substantially more usable plane than the module area suggests.
Should I size for 100% of my usage?: Not necessarily. This model assumes full annual offset, but under net billing or a buyback tariff the kilowatt-hours you export are credited below retail, so generating a large surplus can be worth much less than it costs. Sizing closer to your daytime load is a legitimate strategy and sometimes the better one.
Should I size for an electric car I do not own yet?: If it is genuinely coming, yes. Adding capacity later means a second mobilization, possibly a second interconnection application and sometimes a panel upgrade that a single co-ordinated design would have avoided. Add the vehicle’s expected annual kilowatt-hours to your usage figure before sizing.
Does this calculator account for shade?: No, and that is its most important limitation. There is no shading input.; A federal methodology excludes roof area that cannot produce 80% of an unshaded system at the same orientation, so a shaded plane may not be usable at all regardless of what this size says.; Read our page on shade before trusting a number for a shaded roof.
Why is my inverter smaller than my system size?: Because DC nameplate is measured under standard test conditions that a real roof rarely reaches. Deliberately pairing a smaller inverter with a larger array is normal practice and costs very little annual production, since the array only exceeds the inverter’s rating for a few hours in the year.
My roof cannot fit the size this gives me. What now?: Three options, in order of how often they work. Use higher-wattage modules, which cuts the count and the area for the same capacity.; Accept a smaller system and a partial offset, which is usually fine economically.; Or move the array off the roof entirely, our ground mount page covers what the code requires when you do.

Sources and methodology

Figures dated August 16, 2026. Last reviewed September 3, 2026 by the HyreSolar research desk, which writes and checks every tool on this site.

PVWatts Calculator (NREL, retrieved August 16, 2026). The right source for peak sun hours and system losses at your address.
PVWatts Version 5 Manual, NREL/TP-6A20-62641 (NREL, retrieved September 3, 2026). Documents the system loss assumptions behind a conventional derate.
Electric Power Monthly (EIA, retrieved August 16, 2026). Context for typical residential consumption.

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