Quick answer
Solar Insolation is the total amount of sunlight energy that lands on a surface over a period of time, usually given in kilowatt-hours per square metre per day (kWh/m²/day), and it sets how much electricity solar panels in that place can make.
The word is short for “incoming solar radiation”. It is a dose, not a moment: a whole day’s or month’s worth of sunlight added up. It rises and falls with the seasons, the weather and how far you are from the equator.
Quick facts
The key facts about solar insolation, with sources:
- Unit
- kWh/m²/day (or kWh/m²/year) 2
- Charleston, SC, yearly average on flat ground
- 4.70 kWh/m²/day 1
- Atlanta, GA, yearly average on flat ground
- 4.51 kWh/m²/day 1
- Richmond, VA, yearly average on flat ground
- 4.27 kWh/m²/day 1
- Best US resource
- The Southwest, per NLR maps shown by EIA 3
- Moment-by-moment version
- Irradiance, in W/m²
Key takeaways
- Insolation is sunlight energy added up over time. It is the fuel supply for solar panels.
- It is quoted in kWh per square metre per day. A higher number means more power from the same panels.
- In Charleston, Atlanta and Richmond, the sunniest month gets about 2.6 to 3.1 times the insolation of the darkest month.
- Clouds cut insolation in all three cities to roughly three-quarters of what clear skies would give.
- Insolation maps show places, not roofs. Tilt, direction and shade still decide what your panels see.
What insolation measures
Think of sunlight as rain. Irradiance is how hard it is raining right now. Insolation is how much water ends up in the bucket by the end of the day. Our irradiance page covers the right-now number. This page is about the bucket.
DOE notes that the sunlight reaching a spot changes with location, time of day, season, landscape and weather 2. Insolation rolls all of that into one figure for a day, a month or a year. EIA adds that places at lower latitudes and in dry climates generally receive more insolation 3.
For solar, the usual unit is kilowatt-hours per square metre (kWh/m²) 2. Divide by the number of days and you get kWh/m²/day, the figure on most maps and data tools.
From sunlight to your meter
- Sun → sunlight passes through air and clouds, losing some energy
- Ground or roof → insolation adds up over the day, in kWh/m²/day
- Panels → turn part of that energy into DC power
- Inverter → AC power your home uses, counted in kWh
Why insolation changes through the year
Sun angle. The Earth’s 23.5° tilt changes how high the sun climbs. DOE gives an example: Denver, near 40° latitude, gets roughly three times more solar energy in June than in December 2.
Day length. Summer days are longer, so there are more hours to add up.
Clouds and haze. DOE says the atmosphere can cut direct sunlight by 10% on clear, dry days and by 100% on thick, cloudy days 2. Some light still gets through as diffuse, scattered light.
Latitude. EIA notes that monthly swings grow larger the farther you are from the equator 3. That is why Richmond’s winter dip is deeper than Charleston’s.
Monthly insolation in Charleston, Atlanta and Richmond
| Month | Charleston, SC | Atlanta, GA | Richmond, VA |
|---|---|---|---|
| January | 2.76 | 2.75 | 2.35 |
| February | 3.45 | 3.28 | 3.10 |
| March | 4.60 | 4.38 | 4.10 |
| April | 6.01 | 5.59 | 5.22 |
| May | 6.54 | 6.16 | 5.82 |
| June | 6.52 | 6.19 | 6.35 |
| July | 6.32 | 6.02 | 6.13 |
| August | 5.68 | 5.49 | 5.45 |
| September | 4.82 | 4.82 | 4.47 |
| October | 4.08 | 3.97 | 3.45 |
| November | 3.09 | 3.09 | 2.66 |
| December | 2.49 | 2.37 | 2.07 |
| Year average | 4.70 | 4.51 | 4.27 |
Example: what the monthly table tells you
Simple sums on the NASA POWER figures above. Season averages are three-month means; yearly totals are the daily average × 365.
| Measure | Charleston | Atlanta | Richmond |
|---|---|---|---|
| Best month ÷ worst month | 6.54 ÷ 2.49 = 2.6× | 6.19 ÷ 2.37 = 2.6× | 6.35 ÷ 2.07 = 3.1× |
| Summer (Jun–Aug) average | 6.17 | 5.90 | 5.98 |
| Winter (Dec–Feb) average | 2.90 | 2.80 | 2.51 |
| Yearly total, kWh/m² | About 1,716 | About 1,647 | About 1,558 |
| Cloud effect: all-sky ÷ clear-sky (5.85, 5.86, 5.61) | About 80% | About 77% | About 76% |
Two things stand out. Charleston’s best month is May, not June, likely because summer afternoon clouds trim June and July. And Richmond, farther north, has the biggest winter dip. Expect bills in December and January to show far less solar than in May, wherever you live in these states.
Where you will see insolation figures
- Insolation maps from NLR (formerly NREL), built from the National Solar Radiation Database (NSRDB) 5.
- The NSRDB itself, which holds hourly and half-hourly sunlight data, including global horizontal, direct normal and diffuse horizontal values 4.
- PVWatts, which reports monthly solar radiation in kWh/m²/day for the tilt and direction you enter 6.
- An installer’s production estimate, often as a “solar resource” line.
- Our peak sun hours page, which turns the same daily dose into “hours of full sun”.
Kinds of insolation figures, and how to read a map
Most US maps you meet come from NLR and the NSRDB 5. EIA reproduces two of them, one for direct normal and one for global horizontal sunlight, both in kWh/m²/day 3.
Check three things before you trust a map number. First, the measure: “global horizontal” is sunlight on flat ground and is the one that suits rooftop panels best. “Direct normal” is for mirror-based solar plants.
Second, the period: annual maps hide the winter dip. Third, the colour bins. Maps group values into bands, so read the legend rather than guessing between shades.
A map gives the sunlight for an area, not your roof. Two homes on the same map square can differ a lot because of tilt, direction and shade.
What insolation data is good for, and its limits
Useful for
- Comparing places fairly: same unit, same method.
- Seeing the seasonal pattern before you size a system or a battery.
- Checking that an installer’s monthly production curve has the right shape.
- Spotting a bad month in your monitoring data: was it you or the weather?
Not useful for
- Telling you what your shaded or north-facing roof will receive.
- Predicting a single day; it is a long-term average.
- Capturing local haze or fog that a satellite grid smooths over.
- Replacing a full production model that adds heat, wiring and inverter losses.
Flat ground is not your roof
The table above is for a flat surface. Panels tilted toward the south catch more winter sun and a bit less summer sun, so the year evens out and the total usually rises. Panels facing east or west catch less overall.
That is why production tools ask for tilt and azimuth (direction). PVWatts returns plane-of-array radiation, the sunlight on the panel face, alongside its kWh estimate 6. Our panel tilt and panel orientation pages explain the trade-offs.
Averages also hide year-to-year swings. A cloudy year can come in well below the long-term mean, and a sunny one above it.
Sources: [6]
How insolation affects cost and payback
Insolation does not change what a system costs to buy; for prices see our cost per watt guide. It changes what you get back.
More insolation means more kWh from the same panels, so the same price buys more energy.
On the yearly totals above, a roof in Charleston gets roughly 10% more sunlight on flat ground than one in Richmond (1,716 vs 1,558 kWh/m²). Other things equal, that shortens payback.
But electricity prices and export rules often move payback more than sunlight does.
How installers use insolation in a design
- Pull long-term sunlight data for your location, usually from the NSRDB through a tool like PVWatts 6.
- Adjust it to your roof’s tilt and direction to get sunlight on the panel face.
- Subtract shade measured on site.
- Apply losses for heat, wiring, dirt and the inverter to get monthly kWh.
- Compare monthly kWh with your bills to choose a system size. A licensed installer signs off the final design.
You can try the first steps yourself with our system size calculator.
Using insolation to check your system over time
- Compare each month’s production with the monthly pattern above. A low December is normal; a low May is not.
- If a month falls well short, check whether it was unusually cloudy before calling for service.
- Watch for trees growing into the sun path; they cut the insolation your panels see even if the map stays the same.
- Keep panels clear of heavy dirt or pollen if your monitoring shows a slow decline in sunny months.
Warning signs and when to call a pro
- A quote uses one flat “sun hours” figure for every month. Ask for monthly numbers.
- Production in your sunniest months is far below the installer’s model two years running.
- A proposal quotes insolation for a different city or a much sunnier region.
- Monitoring shows zero on clear days. That is an equipment fault, not weather; see monitoring showing zero.
Rule of thumb
In these three cities, plan on winter months giving well under half the daily sunlight of late spring. Size batteries and expectations for December, not May.
Standards and data behind insolation figures
No law sets insolation values. They come from scientific datasets. In the US the main one is the National Solar Radiation Database, now run by the National Laboratory of the Rockies (NLR), which publishes hourly and half-hourly sunlight values and the maps built from them 4, 5.
NASA’s POWER project publishes satellite-based climate averages worldwide 1.
When a state program or utility uses sunlight in a rule, it names its own method. Ask which dataset a tool or quote used, and for which years. Our methodology page lists the datasets we use.
Insolation vs related sunlight terms
| Term | What it is | Unit |
|---|---|---|
| Solar insolation | Sunlight energy added up over time | kWh/m²/day or kWh/m²/year |
| Solar irradiance | Sunlight power at one moment | W/m² |
| Peak sun hours | Insolation restated as hours at 1,000 W/m² | Hours per day (same number as kWh/m²/day) |
| Capacity factor | Actual output ÷ output at full power all year | Percent |
| Solar production | Electricity your system made | kWh |
Common misconceptions
- Myth Insolation and insulation are the same.
- Reality Insolation is incoming sunlight. Insulation keeps heat in a building.
- Myth Hot places always have the most insolation.
- Reality Clouds matter more than heat. EIA points to low latitude and dry climate as the drivers 3.
- Myth Cloudy days give zero solar.
- Reality Diffuse light still gets through; DOE says clouds cut direct sunlight by up to 100%, not all light 2.
- Myth June is always the best month.
- Reality In Charleston, May edges out June in the NASA POWER averages 1.
Insolation in South Carolina, Georgia and Virginia
On flat ground, Charleston averages 4.70 kWh/m²/day, Atlanta 4.51 and Richmond 4.27 (NASA POWER, 2001–2020) 1. All three see winter sunlight drop to between about 2.1 and 2.8 kWh/m²/day in December and January. The southern coast gets the most; Virginia has the deepest winter dip.
When insolation matters to you
- You are judging whether your area is sunny enough. Anything in the 4 to 5 kWh/m²/day range, like these three cities, supports rooftop solar well.
- You want backup through winter. Size a battery plan around December insolation.
- You are checking a quote. Ask for monthly production and compare its shape with the table above.
- Your system seems weak. Compare the month with its normal insolation before calling for service.
- Next step: get designs based on your own roof from vetted installers through Get Solar Options.
Questions about solar insolation
What is solar insolation in simple terms?
It is how much sunlight energy lands on a surface over a day, month or year. Think of it as the total dose of sun, not the brightness at one moment. It is usually given in kilowatt-hours per square metre per day, and higher numbers mean more power from the same panels.
What is the difference between insolation and irradiance?
Irradiance is the power of sunlight at one instant, in watts per square metre. Insolation is that power added up over time, in kilowatt-hours per square metre. Irradiance tells you how bright it is right now; insolation tells you how much solar energy arrived over the day.
What does kWh/m²/day mean?
It is the sunlight energy, in kilowatt-hours, that falls on one square metre in an average day. Charleston averages about 4.7 kWh/m²/day on flat ground. The same number also equals peak sun hours, since one peak sun hour is 1 kWh/m² of sunlight.
What is a good insolation value for solar panels?
Around 4 kWh/m²/day or more is solid for rooftop solar. Charleston, Atlanta and Richmond all average between about 4.3 and 4.7 kWh/m²/day on flat ground. The US Southwest is higher, but solar works well across most of the country.
Why is insolation lower in winter?
The sun sits lower in the sky, days are shorter and sunlight passes through more air. In Charleston, Atlanta and Richmond, December averages roughly 2.1 to 2.5 kWh/m²/day on flat ground, against 6 or more in late spring. Tilting panels toward the south narrows the gap.
Where can I find insolation data for my address?
Use NLR’s PVWatts, which pulls long-term data from the National Solar Radiation Database and reports monthly solar radiation for your tilt and direction. NLR also publishes insolation maps. NASA’s POWER site gives satellite-based monthly averages for any point on Earth.
Does insolation tell me how much power my panels will make?
It tells you the fuel supply, not the output. To get kWh you also need panel size, tilt, direction, shade, temperature and system losses. A production model like PVWatts combines these. As a rough guide, more insolation means proportionally more output from the same system.
Do clouds stop solar panels working?
No, they reduce output. DOE says the atmosphere can cut direct sunlight by up to 100% on thick cloudy days, but scattered light still reaches the panels. In our three cities, cloudy weather brings the yearly average to about three-quarters of clear-sky levels.
Sources
- NASA POWER Climatology API v2.10.0, ALLSKY_SFC_SW_DWN and CLRSKY_SFC_SW_DWN, 2001–2020, points for Charleston SC, Atlanta GA and Richmond VA (Charleston query shown), retrieved .
- US DOE Solar Energy Technologies Office, Solar Radiation Basics, retrieved .
- US EIA, Energy Explained: Where solar is found and used, retrieved .
- National Laboratory of the Rockies, NSRDB: National Solar Radiation Database (C-MIX / Open EI record), retrieved .
- National Laboratory of the Rockies, Solar Resource Maps and Data (C-MIX / Open EI record), retrieved .
- NLR (formerly NREL), PVWatts V8 API documentation, 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: Monthly figures were pulled on 8 Oct 2026 from the NASA POWER Climatology API (2001–2020, horizontal all-sky and clear-sky shortwave, community RE) at the coordinates stated.
We first tried NLR’s PVWatts API, but it returned a rate-limit error all day, so NLR values were not used in the table. Season averages, ratios and yearly totals are our arithmetic.
Seasonal and cloud effects are from DOE SETO; regional patterns and map units from EIA; dataset descriptions from NLR’s C-MIX records and PVWatts documentation.
Suggest a correction. We fix errors and say what changed.