Quick definition
Irradiance is a snapshot measurement: how intensely the sun is shining on a given spot at a given moment, in watts per square meter. It changes constantly throughout the day, rising from zero at sunrise, peaking around solar noon, and falling back to zero at sunset, with clouds and atmospheric conditions adding real-time variation on top of that basic pattern.
It's the raw input behind more familiar solar planning numbers like peak sun hours, which are calculated by adding up irradiance data over an entire day.
At a glance: standard test conditions for panel ratings assume 1,000 watts per square meter of irradiance, a useful benchmark figure though real conditions vary considerably throughout any actual day.
Why it matters
Irradiance is the fundamental physical quantity underlying essentially every solar production estimate, even though homeowners rarely see the raw number directly. When a production estimate accounts for your location's solar resource, it's ultimately built from historical irradiance data collected for your specific region, often going back years or decades, capturing typical seasonal and daily patterns. In the US, much of this data traces back to NREL's National Solar Radiation Database, which combines satellite-derived estimates at roughly 4-kilometer resolution with ground-based measurements to build the long-term irradiance record production tools draw from.
This is exactly why the same panel, same tilt angle, and same azimuth can produce meaningfully different annual output in two different cities. A location with consistently higher irradiance, more direct, intense sunlight on average across the year, simply has more raw solar energy available to convert, independent of anything about the equipment itself. To see Irradiance in a real example, the quick 5-minute system size estimator in our System Sizing section applies it to a working system.
Irradiance also explains why panel output doesn't move in a perfectly smooth curve through the day. A passing cloud causes a real, immediate irradiance drop, and panel output responds accordingly in real time. This is part of why production monitoring can show choppy, variable output on a partly cloudy day even though the panels themselves are working perfectly normally. The U.S. Energy Information Administration's solar explainer describes how sunlight becomes electricity, starting with the solar resource itself.
Where you'll see this
The peak sun hours calculator converts your location's irradiance data into the daily peak sun hours figure commonly used in production estimates.
If you're comparing solar potential across different regions, the peak sun hours by latitude tool shows how irradiance-derived solar resource varies across different locations.
This term comes up in production estimate methodology, in solar resource maps, and any technical discussion of why a location's underlying sun exposure varies from another's, separate from equipment choices. Natural Resources Canada publishes solar photovoltaic information for Canada, including resource data used to judge a site's potential.
Related terms
Frequently asked
Irradiance is an instantaneous measurement, the intensity of sunlight at one specific moment. Peak sun hours is a daily total, adding up the equivalent hours of standard peak-intensity sunlight a location receives across a full day. Peak sun hours is derived from irradiance data collected over time.
Generally yes, more irradiance means more instantaneous power potential. But panel temperature also affects output, and very high irradiance often comes with higher cell temperatures, which can pull some efficiency back down even as available light increases.
No. Irradiance varies by latitude, season, time of day, cloud cover, atmospheric conditions, and local altitude, which is exactly why solar production estimates rely on location-specific historical irradiance data rather than a single universal number.
Ground-based sensors called pyranometers measure irradiance directly at specific weather stations, and satellite-derived models fill in irradiance estimates for locations without a nearby ground sensor, together forming the historical datasets solar production tools rely on. NREL's National Solar Radiation Database, a widely used US source, combines both approaches to produce a gridded irradiance record at roughly 4-kilometer resolution across the country.