PV solar panel is producing more than is possible according to its datasheet?

Question

I have a client in Belgium that has 16 DM400-M10-54HBB on his roof. On Saturday the 2nd of December 2023, the PV output was 3.95 kW, according to the log of the Huawei SUN2000 4.6KTL-L1 5 kVA inverter. This means 247 W per panel.

This is the house, with the panels directed to the south-east:

However, the irradiance was only 250-300 W/m² that day, according to multiple measurements accross the country (clear blue skies according to my naked eye). This is to be expected because we only get around 1100-1200 W/m² in summer and now it is winter.

However, according to the datasheet, there is approximately 600 W/m² needed to produce this amount of power (yellow line is drawn by me to indicate the produced peak power on that day). How is this possible?

Datasheet source: https://www.elektrototaalmarkt.nl/amfile/file/download/file/3868/product/41786/

The data in the Huawei logs seems accurate..

Answer 1

I checked with the RMI (Royal Meteorology Institute in Belgium) and the sensor of the pyranometer is mounted horizontally. So this can only be compared with a solar panel that is also mounted horizontally. This explains the difference :)

Answer 2

The answer from fromwastetowind is correct that solar irradiance is published for a flat horizontal surface. I entered the time, latitude, and logitude for Brussels into the NOAA ESRL Solar Position Calculator. This gave a solar elevation of 16.87 degrees. Not knowing your customer's panel angles I just assumed a perfect angle 90 degrees to the incoming radiation (the best possible production). The best way to think about this calculation is that 0.29m^2 of solar radiation is spread out over the 1m^2 that the pyronometers measured.

The 861 watts per meter squared is a reasonable number because the radiation has to travel through more atmosphere at the high angle. And the roughly 600 watts per meter squared you figured from the spec sheet is reasonable for the not perfect angle on the fixed array.

Also note that in some cases mixed cloud cover (like shown in the OP photo) can cause an increase in local irradiance. The light that was headed to other locations on the earth's surface gets diffused and a portion of that light can end up at the solar array being analyzed. This can be a big problem for closed loop solar hot water systems and additional safety factory has to be considered when sizing the thermal load dump. Article on mixed cloud cover irradiation spikes.