What Is Solar Irradiance and How Does It Affect Your System?
Last updated: 2026-04-08 · Solar Benchmark
What Is Solar Irradiance and How Does It Affect Your System?
Solar irradiance is the power of solar radiation striking a surface, measured in watts per square meter (W/m²). At ground level on a clear day, peak irradiance reaches 900-1,000 W/m². Your system's hourly output is directly proportional to how much irradiance arrives at the panel surface, making it the single most important physical input in any solar performance model.
Definition and How It Works
Solar irradiance is not a single number. Scientists and engineers split it into three components:
- GHI (Global Horizontal Irradiance): Total solar radiation hitting a flat, horizontal surface. This is the standard reference for comparing solar resources across locations.
- DNI (Direct Normal Irradiance): The beam component arriving directly from the sun's disk, measured perpendicular to the sun. DNI is dominant in clear, dry climates.
- DHI (Diffuse Horizontal Irradiance): Radiation scattered by clouds and the atmosphere, arriving from all directions of the sky dome.
For a tilted residential solar panel, the relevant quantity is GTI (Global Tilted Irradiance): the sum of the direct beam, diffuse sky, and ground-reflected radiation reaching the panel at its actual angle. Physics-based models use the Perez transposition model to convert GHI, DNI, and DHI into GTI for any given tilt and azimuth.
A day with an average irradiance of 400 W/m² produces half the energy of a day with 800 W/m² average, all else being equal. This linear relationship is what makes accurate irradiance data essential for any performance comparison.
Annual GHI Benchmarks for US Cities
| City | Annual GHI (kWh/m²/year) |
|---|---|
| Las Vegas, NV | 2,340 |
| Phoenix, AZ | 2,310 |
| Denver, CO | 2,140 |
| Los Angeles, CA | 2,020 |
| Houston, TX | 1,900 |
| Atlanta, GA | 1,880 |
| Washington, DC | 1,720 |
| Chicago, IL | 1,580 |
| Seattle, WA | 1,310 |
Source: Open-Meteo ERA5 weather data. Values represent long-term annual averages.
What Affects Solar Irradiance at Your Location
- Latitude: Locations closer to the equator receive higher annual GHI because the sun angle is more favorable year-round. Seattle's solar resource is roughly 56% of Las Vegas's.
- Seasons: At northern latitudes, winter irradiance can be less than 30% of summer irradiance on a daily basis. This creates the wide month-to-month swings that make annual averages misleading for month-level performance analysis.
- Cloud cover: Clouds are the dominant variable for locations like Seattle or Chicago. A single overcast day can reduce GHI to under 100 W/m².
- Aerosols and air quality: Smoke, dust, and pollution scatter and absorb sunlight before it reaches the panel. Wildfire smoke events in the western US can reduce GHI by 10-30% for days at a time.
- Altitude: Higher elevations receive more irradiance because there is less atmosphere to absorb solar radiation. Denver's high elevation partially offsets its northern latitude.
ERA5 reanalysis from ECMWF provides hourly GHI, DNI, and DHI at 9 km global resolution from 1940 to the present. This is the irradiance dataset used in physics-based solar performance models to answer what your system should have produced on any given day.
Frequently Asked Questions
What is the difference between irradiance and insolation? Irradiance is a rate: power per unit area (W/m²) at a specific moment. Insolation is energy: irradiance accumulated over time, expressed in kWh/m². A location receiving 6 hours at 800 W/m² average has a daily insolation of 4.8 kWh/m², sometimes called 4.8 peak sun hours.
Does cloud cover always mean low production? Not always. Thin, high clouds can diffuse sunlight without dramatically reducing total irradiance. On partly cloudy days, the diffuse component increases even as DNI drops. Edge-of-cloud effects can momentarily boost irradiance above clear-sky levels. However, dense overcast cloud cover reduces production substantially.
Why do physics-based models use ERA5 instead of a weather station near my house? Weather stations measure irradiance only at their precise location and many gaps exist in the station network. ERA5 reanalysis combines satellite observations, weather balloons, surface stations, and atmospheric models to produce a consistent, gap-free hourly record at every 9 km grid point globally. For most residential locations, ERA5 matches ground truth within 2-5% mean bias error.
Data: pvlib physics modeling + Open-Meteo ERA5 weather data | Last updated: 2026-04-08 | Solar Benchmark