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.
HyreSolar tools
Solar System Size Calculator
Turn a year of electricity use into a system size, a panel count and a roof area.
System size in kW DC is annual kilowatt-hours divided by peak sun hours, days in the year and a derate factor. It is a planning figure. It is not a shade-aware design and it does not know what your roof looks like.
What this returns at the defaults
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, roughly 27 panels at 400 W, or 25 at 440 W, occupying about 511 square feet before any setbacks. Move to a sunnier 5.5 sun hours and the same house needs 8.72 kW. Move to a cloudier 3.5 and it needs 13.70.
Last updated . Data as of 16 August 2026.
System size model
Annual usage in, kW DC and panel count out.
— System size
— Panels
— Panels
— Module area
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 kWA 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 kWThe ordinary residential range. Most single-family roofs can physically carry this if the orientation is reasonable.
Over 14 kWLarge. 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.
How to use this calculator
Every input below is a number you can find, not one you have to guess. This is where each one comes from.
- 01
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.
- 02
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.
- 03
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.
- 04
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.
- 05
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 this calculator works
Start from a year of consumption
Annual kilowatt-hours, from twelve months of bills rather than one.
Divide by daily energy per kW
Peak sun hours × 365 gives the annual kWh one kilowatt of DC capacity would produce with no losses.
Apply the derate
One multiplier for inverter, wiring, soiling, temperature and mismatch losses.
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.
Inputs
- 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. A proposal can change any of these without saying anything untrue, so these are the inputs to check first.
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.
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 agonising over.
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.
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.
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 mobilisation 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.
Important: this is a planning estimate
- 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.
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 mobilisation, 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.
The research behind these numbers
Every assumption in this calculator is argued from primary sources somewhere in our research library. These are the pages that matter for this one.
- Shade and solar output The input this calculator does not have. Two independent sources both exclude roof area below 80% of unshaded output.
- What panel efficiency actually buys Why higher-wattage modules change the panel count and the area but not the system size.
- How your exports are credited Whether sizing for full annual offset is the right goal on your tariff.
- Ground mount versus roof mount What to do when the array will not fit on the roof, and the 10-foot perimeter the fire code imposes instead.
- Electricity consumption by state Where your annual kilowatt-hours sit against your state, computed from EIA-861.
- Interconnection screens Why system size interacts with your utility’s review, and what changes above certain thresholds.
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Nominal 25-year totals for cash (or financed) purchase versus an escalating lease.
Want a more accurate estimate for your home?
Actual results depend on roof, usage, utility rules and a real proposal. Matching is still being built. The form is an enquiry, not a dispatch line.
Written and audited by
HyreSolar Research
Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the statute, the tariff, the code section, the federal filing or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it. Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. That rule has cost us whole sections, and it is the reason the rest can be trusted.
- 160
- primary sources read and cited
- 220
- figures with a retrieval date
- 115
- federal and state government sources
- 66
- researched pages published
How this desk works
- Primary sources only. Statutes from the legislature’s own publishing system, federal data from the agency that collects it, code text from the adopted edition, manufacturer claims from the data sheet. We do not cite an article that cites a source; we go and read the source.
- Every figure carries its provenance. A named document and the date we retrieved it, so you can check it and so you know how old it is. Retrieval dates are not decoration: an EIA rate from May is a different fact from an EIA rate from August.
- We publish what we could not verify. Every research page carries a section naming the things we tried to establish and could not, and why. A paywalled standard, a state website that refused the request, a manufacturer that publishes no figure at all.
- We separate measurement from modelling from our own reasoning, and label which is which on the page. A laboratory measurement, an assumption inside a modelling tool and our own inference are three different kinds of claim and they are never presented as one.
- We do not sell solar, and we take no payment for placement, ranking or a favourable mention. Nobody buys a position on this site.
Data as of 16 August 2026. Authorship on this site is organisational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Data and sources
- PVWatts Calculator (NREL, retrieved 2026-08-16. The right source for peak sun hours and system losses at your address.)
- PVWatts Version 5 Manual, NREL/TP-6A20-62641 (NREL, retrieved 2026-09-03. Documents the system loss assumptions behind a conventional derate.)
- Electric Power Monthly (EIA, retrieved 2026-08-16. Context for typical residential consumption.)