GenDayMtx values seem low

Hi all,

I have a question about GenDayMtx values. We ran as follows:

  • gendaymtx -m 1 -A -c 1 1 1 singapore.wea

The weather file we used was created from a standard Singapore EPW weather file.

The values that we get seem a bit low. For Singapore, we expect the total annual irradiance on an unobstructed horizontal surface to be around 1650 kWh/m2 (based on measure data). We did a calculation using the GenDayMtx values, and came to 1030 kWh/m2. That is 38% too low.

The calculation we did is shown in the following spreadsheet:

Basically for each patch radiance value, we converted it to kWh/m2 and then apply the Lambert cosine weighting based on altitude. We then calculated the sum, which should be the total annual irradiance for a horizontal unobstructed surface.

(We have previously done the same thing using the GenCumulativeSky, and in that case, the result is 1666 kWh/m2, which seems very close to what we were expecting.)

Please can you let us know if we are doing something wrong?


you miss the -O1 option to get output in solar irradiance rather than the irradiance only in the visible range (default). See also the manual page of gendaymtx:

The −O1 option specifies that output should be total solar radiance rather than visible radiance.


Thanks Jan, that fixed it. I now get 1672 kwh/m2.

For others how might be on this path, I have updated the CSV file.