Solar Irradiance
Read all about it here...

Solar irradiance is measured by the Pyranometer mounted on top of the ERU Control Room, R111

About the Pyranometer

Solar irradiance (sunlight) on a horizontal plane is measured by a silicon pyranometer .

The pyranometer measures solar irradiance [W/m2] using a silicon photodiode and therefore its response matches the output from a photovoltaic (PV) panel. It has a 180 degree field of view and is mounted on the horizontal plane so that it measures the solar irradiance received from the entire hemisphere.

Where possible, PV panels used for power generation are mounted facing south, and at an angle to the horizontal (usually equivalent to latitude), in order to maximise total energy production. However it is possible to mount the panels closer to horizontal to take advantage of the all-sky diffuse radiation prevalent in the UK, and they may be mounted facing closer to west to provide peak power at a later time of day. Ideally they should be mounted at different angles to the horizontal at different times of the year; however this is usually not justified.

Where a particular PV generator is being monitored, a pyranometer should be mounted in the plane of the PV panels to measure the solar irradiance directly incident on the panels.