Quick answer
Peak Sun Hours are the number of hours a day that sunlight would need to shine at 1,000 watts per square metre to deliver the same energy the site actually receives.
One peak sun hour equals 1 kWh of sunlight per square metre, so a location averaging 5.35 kWh/m²/day has 5.35 peak sun hours. Multiply by system kW and a loss factor to estimate daily production.
Quick facts
The key facts about peak sun hours, with sources:
Key takeaways
- A peak sun hour is one hour of full-strength sunlight, 1,000 W/m². It is not an hour of daylight.
- SC, GA and VA all average about 5 peak sun hours a day on a south-facing roof.
- Rule of thumb: kW × peak sun hours × 365 × 0.75 ≈ yearly kWh.
- Winter has far fewer peak sun hours than summer. Columbia drops from 6.5 in June to 3.9 in December.
- Shade and roof direction can matter more than which state you live in.
Not the same as hours of daylight
A long summer day in South Carolina has plenty of daylight, but the light is weak at dawn and dusk and strongest around solar noon. Peak sun hours squeeze that uneven curve into the same amount of energy delivered at full strength, 1,000 W/m².
That benchmark is not random. Panel watt ratings are measured under the same 1,000 W/m² standard test light, so a 425 W panel in one peak sun hour produces roughly 425 Wh before real-world losses. The unit you see in solar data, kWh/m²/day, is the same number written another way.
Sources: [4]
How peak sun hours are measured and modelled
Sunlight data comes from NREL’s National Solar Radiation Database. It blends satellite images and weather data into hour-by-hour sunlight for a “typical year” at each location. The DOE says NREL’s PVWatts tool uses that data to estimate the output of grid-connected solar for any address.
PVWatts reports sunlight on the tilted panel surface, in kWh/m²/day. That is the peak-sun-hour figure for your roof angle and direction. It then runs an hour-by-hour model that adds heat, wiring and inverter losses to get kWh.
Roof angle and direction change the number. A south-facing roof in the US catches the most. East- or west-facing roofs catch less, and flat panels catch less in winter.
Kinds of sunlight numbers you may see
| Name | What it measures | When it is used |
|---|---|---|
| GHI (global horizontal) | Sunlight on a flat, level surface | Weather data, maps |
| POA (plane of array) | Sunlight on the tilted panel face | PVWatts solrad, your roof’s peak sun hours |
| DNI (direct normal) | Direct beam only, facing the sun | Trackers and large plants |
| Annual average | Mean daily peak sun hours over a year | Yearly sizing |
| Monthly value | Mean daily peak sun hours in one month | Winter planning, bill credits |
Peak sun hours in our three focus states
Worked estimate: a 6 kW system near Columbia, SC
An example using the NREL figures above. The formula is the rule of thumb installers use; the last row is NREL's full model for comparison.
| Step | Math | Result |
|---|---|---|
| Sunlight-only ceiling | 6 kW × 5.35 h × 365 days | 11,716 kWh/yr |
| NREL PVWatts modelled output (14.08% system losses, plus temperature and inverter effects) | Hourly simulation 1 | 8,765 kWh/yr |
| Implied derate | 8,765 ÷ 11,716 | ≈ 0.75 |
| Rule of thumb from here on | kW × peak sun hours × 365 × 0.75 | Within a few percent of the model |
Run it backwards to size a system: an average home's 10,791 kWh a year 5 ÷ (5.35 × 365 × 0.75) ≈ 7.4 kW in Columbia. Shade, a steeper or east-facing roof, or a hot attic-mounted inverter all lower the result; the system size calculator applies those adjustments.
Peak sun hours compared with related terms
| Term | Measures | Unit |
|---|---|---|
| Peak sun hours | Daily sunlight energy, as hours at full strength | h/day = kWh/m²/day |
| Solar irradiance | Sunlight strength right now | W/m² |
| Daylight hours | Sunrise to sunset | Hours |
| Capacity factor | Real output ÷ output at full rating all year | % |
| Solar production | Energy the system makes | kWh |
Why peak sun hours are useful, and where they fall short
Useful for
- A quick, honest first estimate of yearly kWh.
- Comparing places on the same basis.
- Checking whether a quote’s production claim is in the right range.
Falls short on
- Ignores your own shade, trees and chimneys.
- An annual average hides the winter dip.
- Does not include heat losses unless you add a derate.
Where peak-sun-hour estimates go wrong
- Using a horizontal-surface (GHI) number for a tilted roof, or the reverse. Ask which one a quote used.
- Applying a state average to a shaded lot. Trees and chimneys can remove hours that no map shows; the shade guide explains why one shaded panel can drag down a string.
- Ignoring losses. Multiplying kW by sun hours with no derate overstates output by roughly a third in the Columbia example.
- Comparing one year of real production against a typical-year model. Weather varies; judge underperformance over a full year (see solar underproduction).
Why the annual average hides a winter dip
Columbia's 5.35 annual average comes from months as low as 3.91 (December) and as high as 6.51 (June). PVWatts models the same 6 kW array at 584 kWh in December and 830 kWh in June.
Under monthly netting or credits that bank forward, summer surplus can cover part of the winter gap. Under rules that credit exports at a lower rate, it cannot. See kilowatt-hour for how those kWh show up on a bill.
Sources: [1]
How peak sun hours affect cost and value
Sun hours do not change what a system costs to install. They change how many kWh you get for that cost. More peak sun hours means more kWh per kW, so each dollar buys more energy.
But value also depends on your electricity rate and export credit. A sunny place with cheap power can pay back more slowly than a cloudier place with costly power. See the payback period and payback calculator. We do not list prices here.
Panel output slowly declines with age, so the same sun hours give a bit less each year. Qcells warrants at least 90.58% of rated power after 25 years 4.
How installers use peak sun hours
- They look up your address in a tool such as PVWatts, with your roof’s tilt and direction 6.
- They run a shade check on site or from aerial images. See site assessment.
- They size the system from your yearly kWh and the modelled kWh per kW.
- The quote lists expected yearly kWh. Ask which data source and losses they used.
- After install, they compare first-year production with the estimate.
Getting full value from your sun hours
Upkeep here means keeping the panels able to use the light they get. Trim trees that start to shade the array. Watch for heavy pollen, bird droppings or leaves that rain does not clear. Our guide on cleaning panels covers when it is worth it.
Heat also cuts output. The DOE notes that hot cells lose voltage. That is one reason a clear spring day can beat a hot summer day. Panels mounted with an air gap underneath stay cooler.
Sources: [7]
Warning signs your system is not using its sun hours
- Yearly kWh well below the estimate after a full year.
- Output on clear days much lower than the same month last year.
- One section of panels lagging the rest in the monitoring app.
- A quote that promises far more kWh per kW than PVWatts shows for your area.
- If you see these, ask your installer for a site check. Never climb onto the roof yourself.
Is there a standard for peak sun hours?
No law sets peak sun hours; they are measured data. The 1,000 W/m² benchmark comes from the standard test conditions used to rate panels. The Qcells sheet names IEC 60904-3 as the test basis: 1,000 W/m², 25 °C and an AM 1.5 light spectrum.
Where rules do touch sunlight, it is through sizing. Some utilities cap system size by your use. Berkeley Electric in SC sizes systems from annual kWh ÷ 1,800 or average monthly kWh ÷ 150. For other rules, see your utility’s solar rider.
Common myths
- Myth "Twelve hours of sun means twelve peak sun hours."
- Reality Morning and evening light is weak. A long day may give only 5 or 6.
- Myth "Solar does not work in winter."
- Reality It works, just less. Columbia still averages 3.91 peak sun hours a day in December 1.
- Myth "My state’s number tells me my output."
- Reality Your roof’s shade, angle and direction matter as much.
In South Carolina, Georgia and Virginia
All three states sit in the 5-hour range on a south-facing 25° roof, so the production gap between them is smaller than the gap between a sunny and a shaded roof in the same town. What changes more by state is how exported kWh are credited. The state pages cover each one.
When peak sun hours matter to you
- Getting quotes: check each production claim against PVWatts for your address.
- Choosing a roof face: south first, then west or east.
- Planning for winter bills: look at the December figure, not the annual one.
- Judging performance: compare a full year, not one month.
Questions about peak sun hours
How many peak sun hours does South Carolina get?
NREL's typical-year data via PVWatts gives about 5.35 peak sun hours a day for a south-facing panel tilted 25° near Columbia, ranging from about 3.9 in December to 6.5 in June. Other parts of the state are similar.
Is a peak sun hour the same as an hour of sunlight?
No. It is an hour of sunlight at 1,000 W/m². A day with 12 hours of daylight may deliver only 4 or 5 peak sun hours because morning and evening light is weak. Clouds lower it further.
How do I calculate solar output from peak sun hours?
Multiply system size in kW by daily peak sun hours, then by a loss factor of about 0.75. A 6 kW system with 5.35 peak sun hours makes roughly 24 kWh on an average day, or about 8,800 kWh a year.
Where can I find peak sun hours for my address?
NREL's PVWatts calculator returns monthly and annual solar radiation in kWh/m²/day for any US address, using the National Solar Radiation Database. Enter your roof tilt and direction for the best figure.
How many peak sun hours do you need for solar to be worth it?
There is no single cutoff. Value depends as much on your electricity rate and export credit as on sunlight. A sunny state with low rates can pay back more slowly than a cloudier state with high rates.
How many peak sun hours does Georgia get?
About 5.18 a day near Atlanta for a south-facing roof tilted 25°, per NREL PVWatts. It ranges from about 3.65 in December to 5.93 in June. A 1 kW system there makes about 1,414 kWh a year in the model.
How many peak sun hours does Virginia get?
About 4.97 a day near Richmond for a south-facing roof tilted 25°, per NREL PVWatts. December drops to about 3.30, and June rises to 6.28. A 1 kW system there makes about 1,361 kWh a year in the model.
Do east- or west-facing roofs get fewer peak sun hours?
Yes, in the US a south-facing roof catches the most sunlight over a year. East and west faces catch less, mostly in the morning or afternoon. Run your actual roof direction in PVWatts to see the difference for your address.
Sources
- NREL (National Laboratory of the Rockies), PVWatts v8 API, NSRDB PSM V3 TMY; 6 kW, 25° tilt, south, 14.08% losses, Columbia SC (34.0, −81.03), retrieved .
- NREL PVWatts v8 API, same inputs, Atlanta GA (33.75, −84.39), retrieved .
- NREL PVWatts v8 API, same inputs, Richmond VA (37.54, −77.44), retrieved .
- Qcells, Q.TRON BLK M-G2+ series data sheet (STC: 1,000 W/m², 25 °C, AM 1.5), retrieved .
- US EIA, FAQ: How much electricity does an American home use? (2022 data), retrieved .
- US DOE Solar Energy Technologies Office, Homeowner’s Guide to Going Solar, retrieved .
- US DOE Solar Energy Technologies Office, Solar Performance and Efficiency, retrieved .
- Berkeley Electric Cooperative, renewable energy (Renewable Surplus Rider), retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Three PVWatts v8 API runs on 7 Oct 2026 (NSRDB PSM V3 TMY, 6 kW, standard module, fixed open rack, 25° tilt, 180° azimuth, 14.08% losses). solrad values are plane-of-array kWh/m²/day. kWh per kW = ac_annual ÷ 6.
The rule-of-thumb derate is derived from the Columbia run, not assumed. STC from the Qcells data sheet (re-read 2026-10-08).
Suggest a correction. We fix errors and say what changed.