Solar Irradiance Calculator
Average daily kWh/m² by city, panel tilt, and azimuth — with monthly breakdown.
| Month | Daily avg (kWh/m²) | Monthly total | |
|---|---|---|---|
| Jan | 3.63 | 112 | |
| Feb | 4.46 | 125 | |
| Mar | 5.60 | 174 | |
| Apr | 6.19 | 186 | |
| May | 6.09 | 189 | |
| Jun | 6.48 | 194 | |
| Jul | 6.40 | 198 | |
| Aug | 6.17 | 191 | |
| Sep | 5.77 | 173 | |
| Oct | 4.86 | 151 | |
| Nov | 3.62 | 109 | |
| Dec | 3.39 | 105 |
How to Use This Calculator
Select your city
Choose the city closest to your installation. The calculator uses NREL Global Horizontal Irradiance (GHI) data — measured solar energy hitting a flat, horizontal surface. Real monthly averages account for cloudy days, seasonal variation, and atmospheric conditions across decades of measurements.
Set tilt angle
The panel tilt angle adjusts the irradiance from horizontal to your panel's actual orientation. A 30° south-facing tilt receives more irradiance in winter (when the sun is low) and slightly less in summer (when the sun is high). The calculator applies monthly tilt correction factors to convert horizontal GHI to plane-of-array (POA) irradiance.
Enter azimuth
Azimuth is the compass direction your panels face. 180° is true south — the optimal direction in the northern hemisphere. East-facing panels (90°) capture morning sun; west-facing (270°) capture afternoon sun. Deviation from south reduces annual irradiance: 15° off = ~3% loss, 45° off = ~12% loss, 90° off (east or west) = ~15-20% loss.
Read the monthly breakdown
The table shows average daily irradiance in kWh/m² for each month and total monthly kWh/m². Multiply these values by your panel efficiency and panel area to estimate panel output. Alternatively, use the Solar Production Calculator which combines all these factors automatically.
The Formula
GHI (Global Horizontal Irradiance) is the total solar radiation received on a horizontal surface, including direct beam and diffuse sky radiation. POA (Plane of Array) irradiance is what your tilted, azimuth-adjusted panels actually receive.
The tilt correction is largest in winter — a 30° tilt increases winter irradiance by 14-17% vs. flat because panels directly face the lower winter sun. In summer, that same tilt marginally reduces capture because the sun is nearly overhead.
Example
30° south-facing panels in Phoenix vs. Seattle
This comparison shows the dramatic difference in solar resource between the two US extremes:
The same 8 kW system in Phoenix captures 77% more annual solar energy than in Seattle. Yet Seattle's summer irradiance approaches Phoenix levels — the difference is almost entirely driven by Seattle's dark, cloudy winters. Seattle homeowners with solar still benefit substantially in summer months but need realistic expectations about November-February production.