Quick answer
Solar Irradiance is the power of sunlight striking a surface at a given moment, measured in watts per square metre (W/m²); insolation is that irradiance added up over time, usually in kilowatt-hours per square metre per day.
Irradiance tells you how strong the sun is right now. Insolation tells you how much solar energy a place gets in a day or a year, which is what a solar production estimate is built on.
Quick facts
The key facts about solar irradiance, with sources:
- Irradiance unit
- Watts per square metre (W/m²) 2
- Insolation unit
- kWh/m² per day (or per year) 1
- Panel test irradiance
- 1,000 W/m² at standard test conditions 6
- Components
- Direct (beam) + diffuse = global 2
- Clouds and direct beam
- Direct beam cut about 10% on clear, dry days and up to 100% under thick cloud 2
- US reference dataset
- National Solar Radiation Database (NSRDB), from the national lab now called the National Laboratory of the Rockies (NLR) 3
- Shortcut
- 1 kWh/m² of daily insolation = 1 peak sun hour
Key takeaways
- Irradiance is how strong sunlight is right now, in W/m².
- Insolation is how much sunlight adds up over a day or year, in kWh/m².
- Panels are rated at 1,000 W/m². Real roofs see that only in strong sun.
- Panel output rises and falls roughly in step with irradiance.
- Production estimates are built on decades of local irradiance data.
What irradiance means in plain words
Sunlight carries energy. Irradiance measures how much of that energy lands on one square metre each second. A higher number means brighter, stronger sun. A lower number means haze, cloud, low sun or shade.
The DOE notes the units change with the job. Solar electric work uses W/m² and kWh/m². Solar water heating sometimes uses British thermal units per square foot. They all describe the same sunlight.
Sources: [2]
Irradiance vs insolation at a glance
| Irradiance | Insolation | |
|---|---|---|
| What it is | Power: how strong the sunlight is at one instant | Energy: total sunlight received over a period 1 |
| Unit | W/m² | kWh/m²/day or kWh/m²/year |
| Analogy | Speedometer | Odometer |
| Where you see it | Panel data sheets (STC 1,000 W/m²), weather-station feeds, monitoring apps with a sensor | Solar resource maps, production models, "peak sun hours" |
| Also called | Solar flux, solar power density | Solar irradiation, solar exposure, peak sun hours (numerically) |
How irradiance becomes kWh on your bill
- Sun → light passes through air, cloud and dust
- Irradiance on the panel → depends on tilt, direction and shade
- Panel → turns a share of it into DC power
- Heat, wiring and the inverter → trim some of that power
- Over the year → total insolation sets your solar production in kWh
GHI, DNI and DHI
| Abbreviation | Name | What it measures | Matters for |
|---|---|---|---|
| GHI | Global horizontal irradiance | All sunlight on a flat, horizontal surface: direct plus diffuse | Rooftop PV estimates (a model tilts it to your roof angle) |
| DNI | Direct normal irradiance | The direct beam on a surface pointed straight at the sun | Trackers and concentrating solar plants |
| DHI | Diffuse horizontal irradiance | Sunlight scattered by clouds and air, arriving from the whole sky | Output on cloudy days; shaded roof faces |
Which kind of sunlight each technology uses
The EIA explains that PV panels and flat solar water heaters use global radiation, both direct and scattered. That is why rooftop solar still works under thin cloud.
Mirror-based solar power plants need direct sunlight. That is greatest in dry areas with few cloudy days, so these plants sit in deserts. Tracking systems that follow the sun also collect more than fixed panels.
Sources: [1]
Example: converting the numbers you will see
Illustrative arithmetic. The 435 W panel rating and the 1,000 W/m² test irradiance are from a manufacturer data sheet 6; the irradiance and insolation values are round inputs for the math, not a forecast for any place.
| Question | Math | Answer |
|---|---|---|
| A sensor reads 800 W/m². Roughly what can a 435 W panel make, ignoring heat? | 435 W × (800 ÷ 1,000) | ≈ 348 W |
| A map shows 5 kWh/m²/day. How many peak sun hours is that? | 5 kWh/m² ÷ 1 kW/m² | 5 peak sun hours |
| What does that day give a 6 kW DC array before losses? | 6 kW × 5 h | 30 kWh, then less for heat, wiring and inverter losses |
Output scales roughly with irradiance, which is why panels make less on hazy days and in winter. Real estimates also subtract temperature and system losses, so use a production model rather than this shortcut for a purchase decision.
What changes the irradiance on your roof
The EIA lists the drivers: latitude, climate and weather; clouds, dust, volcanic ash and pollution in the air; buildings, trees and hills that shade a site; and the time of day. Intensity is greatest around solar noon, when the sun is highest.
Seasonal swings grow with distance from the equator. The DOE gives Denver as an example: it gets nearly three times more solar energy in June than in December. Earth’s 23.5° tilt causes this.
Your roof adds two more: the direction it faces and its tilt, which decide how squarely sunlight hits the panels. See panel orientation and panel tilt.
Where irradiance data comes from
In the US, most solar production estimates trace back to the National Solar Radiation Database (NSRDB). It was built by the national lab now called the National Laboratory of the Rockies (NLR). It holds hourly and half-hourly values of GHI, DNI and DHI plus weather data, modelled from satellite measurements.
NLR’s free PVWatts calculator uses this kind of data. It says its results are based on 30 years of actual weather at your location. It also warns that it makes many assumptions and does not capture every difference between PV products.
The EIA’s maps show the Southwest gets the strongest sunlight in the country. When an installer quotes annual production for your array, their software has almost always pulled this kind of data, tilted it to your roof and subtracted losses.
Why irradiance data helps you, and where it falls short
What it helps with
- Lets you compare sites and roof faces fairly.
- Turns a panel rating into a yearly kWh estimate.
- Helps spot underperformance: low output on a bright day points to a fault.
- Based on decades of public data, not a sales pitch.
Where it falls short
- A typical year is an average; any one year can be sunnier or cloudier.
- Maps miss local shade from your own trees and buildings.
- Satellite data is modelled, not measured on your roof.
- Models simplify; PVWatts itself notes its assumptions 4.
Limits of irradiance numbers
Irradiance tells you about sunlight, not about your equipment. Two homes with the same irradiance can produce very different kWh because of shade, heat, dirt, wiring and inverter choices.
Short-term readings swing a lot. A passing cloud can drop irradiance by most of its value in seconds. Judge your system on monthly and yearly totals, not one moment. See our guide to solar underproduction.
How irradiance affects cost and payback
Irradiance does not change what a system costs to build. It changes how much energy that cost buys. More sunlight means more kWh from the same panels, so the cost per kWh falls and payback gets shorter.
Local power prices matter just as much. A sunny place with cheap power can pay back slower than a less sunny place with expensive power. Compare with the payback calculator and our cost by system size guide.
Panel life does not depend much on irradiance. Heat and weather matter more. The panel warranty is your protection.
How installers use irradiance in a design
- Pull a typical weather year for your location from a dataset like the NSRDB.
- Tilt and turn that sunlight to match each roof face.
- Subtract shade, using a site survey or 3D model.
- Apply panel ratings, temperature effects and system losses.
- Report annual kWh, often month by month.
Ask each installer which tool and loss assumptions they used. Our site assessment page covers the survey step.
Using irradiance to check your system over time
Some monitoring systems include an irradiance sensor. Most do not. Without one, compare each month to the same month last year and to the installer’s monthly estimate.
If a sunny month comes in well below the estimate, look for new shade, dirt or a fault. Trees grow, and a few years can add real shade. Keep branches trimmed by a pro.
Warning signs and when to call a pro
Call your installer if:
- Output on clear days is far below the same days last year.
- One roof face or string lags the others under the same sun.
- Monthly production is below the estimate for several months running.
- The app shows normal irradiance but low or zero power.
Rule of thumb
Judge output against the same month last year, not last month. Seasonal changes in irradiance can be large.
Standards behind irradiance numbers
Panel ratings use a fixed test irradiance of 1,000 W/m², a 25 °C cell temperature and the AM 1.5 spectrum. The example data sheet lists these test conditions according to IEC 60904-3. NLR’s record cell chart also requires IEC 60904-3 or ASTM G173 spectra.
No code tells you how much sunlight your roof must get. Utilities and lenders may ask for a production estimate, though, and some programs set their own sizing rules based on it.
Misconceptions
- Myth Panels need direct sun to make power.
- Reality PV uses global radiation, direct plus diffuse 1.
- Myth Hot places always have the best solar.
- Reality Dry, clear places have the most sunlight, but heat lowers panel output.
- Myth A 435 W panel makes 435 W whenever the sun is out.
- Reality It makes that only at 1,000 W/m² and 25 °C. Real roofs usually see less.
- Myth Peak sun hours are the hours of daylight.
- Reality They are total daily energy restated as hours of full-strength sun.
A South Carolina rule of thumb built on insolation
Berkeley Electric Cooperative sizes member systems by dividing annual kWh use by 1,800, which implies its territory expects about 1,800 kWh a year from each kW of solar 7.
That divisor folds local insolation and typical losses into one number; it is specific to that utility’s area. Georgia and Virginia utilities set their own sizing rules; Virginia caps net-metered systems by expected annual use 8.
When irradiance matters to you
Pay attention to irradiance when you:
- Compare two quotes with different kWh estimates for the same size.
- Choose between roof faces, or a roof vs a ground mount.
- Decide whether shade from trees is worth fixing.
- Check whether your system is underperforming.
Use it
- Solar system size calculatorTurns your usage and local sun into a kW size
- Shade and solar outputHow shade cuts the light your panels see
Questions about solar irradiance
What is the difference between solar irradiance and insolation?
Irradiance is power at an instant, in W/m². Insolation is energy over a period, in kWh/m², the irradiance added up across the day or year. Think of a speedometer and an odometer. Production estimates use insolation.
Is insolation the same as peak sun hours?
Numerically, yes, for daily figures. Peak sun hours restate daily insolation as the number of hours at 1,000 W/m² that would deliver the same energy. So 5 kWh/m²/day equals 5 peak sun hours.
What is the irradiance at standard test conditions?
It is 1,000 W/m², with a cell temperature of 25 °C and the AM 1.5 spectrum. Panel wattage ratings are measured under these conditions. Real rooftops see that strength only in bright sun, so real output is usually lower.
How is solar irradiance measured?
At ground stations with pyranometers, which measure global sunlight, and pyrheliometers, which measure the direct beam. Across the US, satellite-based models such as those behind the NSRDB fill in the map.
Does irradiance affect solar panel output directly?
Yes. Output rises and falls roughly in step with the irradiance on the panel. Then it drops a little further as the panel heats up. Half the irradiance means roughly half the power.
How much do clouds reduce solar irradiance?
A lot for the direct beam. The DOE says direct beam sunlight is cut by about 10% on clear, dry days and by up to 100% under thick cloud. Scattered light still reaches the panels, so output falls but rarely to zero.
Which US region has the highest solar irradiance?
The Southwest, according to the EIA. Dry air, few clouds and lower latitude help. Solar still works across the country, and power prices often matter as much as sunlight for savings.
Where can I find irradiance data for my home?
NLR’s free PVWatts calculator uses local weather data, based on 30 years of records, to estimate production. Enter your address, roof angle and system size. Use it to sanity-check installer estimates.
Does solar irradiance change with the seasons?
Yes, and more so farther from the equator. The DOE says Denver gets nearly three times more solar energy in June than in December. Your app will show the same seasonal swing.
Sources
- US EIA, Solar explained: Where solar is found and used, retrieved .
- US DOE Solar Energy Technologies Office, Solar Radiation Basics, retrieved .
- Data.gov catalogue, National Solar Radiation Database (NSRDB), National Renewable Energy Laboratory (now NLR), retrieved .
- National Laboratory of the Rockies (NLR), PVWatts Calculator, retrieved .
- National Laboratory of the Rockies (NLR), Best Research-Cell Efficiency Chart, retrieved .
- Qcells, Q.TRON BLK S-G3R.12+/BFG 435–450 W data sheet (2025-08 Rev04), retrieved .
- Berkeley Electric Cooperative, Renewable energy (Renewable Surplus Rider), retrieved .
- Code of Virginia §56-594 (net energy metering), 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: Definitions, units, cloud effects and drivers are from the EIA and DOE pages cited; NSRDB description from its federal catalogue entry; PVWatts behavior from NLR’s tool page.
The worked example uses the data-sheet test irradiance and panel rating with round illustrative inputs, labelled as such. SC and VA facts are from sc-local/facts.js and the verified Virginia statute row.
Suggest a correction. We fix errors and say what changed.