Installation

Solar Irradiance

Solar irradiance is the power density of sunlight striking a surface, measured in watts per square meter (W/m²). It quantifies how much solar energy is available at a specific location and time, and it is the fundamental input variable for predicting how much electricity a solar panel system will produce.

At the top of Earth's atmosphere, solar irradiance is approximately 1,361 W/m² — the solar constant. By the time sunlight passes through the atmosphere, scattering, absorption by gases, and cloud cover reduce this to roughly 1,000 W/m² under clear skies at sea level with the sun directly overhead. This 1,000 W/m² figure is the irradiance level used for Standard Test Conditions (STC) panel ratings.

Real-world irradiance varies continuously throughout the day and year. It peaks around solar noon, drops toward zero at sunrise and sunset, fluctuates with cloud cover, and varies seasonally with sun angle. Geographic location plays a massive role — the US Southwest receives roughly 5.5-7.0 kWh/m²/day of total daily irradiance, while the Pacific Northwest receives 3.5-4.5 kWh/m²/day.

Two types of irradiance matter for solar design: Global Horizontal Irradiance (GHI) measures total sunlight on a flat surface, while Global Tilted Irradiance (GTI) measures sunlight on a surface tilted at the panel's angle. Solar designers use irradiance databases like NREL's NSRDB, PVWatts, and SolarGIS to estimate location-specific production using decades of satellite-derived irradiance data.

Understanding your location's irradiance profile is essential for accurate system sizing. A 6 kW system in Phoenix produces roughly 50% more annual energy than the same system in Seattle, entirely due to irradiance differences.

Frequently Asked Questions

How do I find the solar irradiance for my location?

Use NREL's PVWatts Calculator (pvwatts.nrel.gov) — enter your address and it provides monthly and annual irradiance data along with estimated production for a system of any size. The Global Solar Atlas (globalsolaratlas.info) provides worldwide irradiance maps with high resolution.

Does cloud cover eliminate solar production?

No. Clouds reduce irradiance but do not eliminate it. On an overcast day, irradiance might drop to 100-300 W/m² — roughly 10-30% of clear-sky levels. Panels still produce power, just at reduced output proportional to the available light. Diffuse light (scattered by clouds) still reaches the panels from all directions.