# Positive perez parameter using gendaylit printing error sky

Hi all,
I’m getting an error using gendaylit. I have done a program to calculate irradiance on the ground at different hours. I used gendaylit with direct normal irradiance and diffuse horizontal irradiance values. The problem is that at the 4:00 pm I got the following error:

gendaylit 1 22 16 -a 37.799 -o 5.156 -m 15 -W 576.93 63.0
Warning: positive Perez parameter B (= 0.067705), printing error sky
Local solar time: 16.47
Solar altitude and azimuth: 4.7 60.3

void brightfunc skyfunc
2 skybright perezlum.cal
0
10 0.00 0.00 0.000 0.000 0.000 0.000 0.000 -0.865408 -0.494430 0.081291

Could you help me to understand the meaning of a ‘‘positive Perez parameter’’ and what cause it?

Thank you.

The reason you get this error is because the perez function cannot be fit this low sun angle to this high direct normal. In other words, these values are outside what is expected to be possible by the model. Where did you get your direct normal and diffuse horizontal values from? Is it from an epw file? If so, the direct normal for the 16th hour is the average over the previous hour so is best approximated by 15:30. If you repeat the parameters with the corrected time it works:

``````gendaylit 1 22 15.5 -a 37.799 -o 5.156 -m 15 -W 576.93 63.0
# gendaylit 1 22 15.5 -a 37.799 -o 5.156 -m 15 -W 576.93 63.0
# Local solar time: 15.97
# Solar altitude and azimuth: 9.7 55.3
# epsilon, delta, atmospheric precipitable water content : 3.3673 0.2567 2.0000

void light solar
0
0
3 3.153e+06 3.153e+06 3.153e+06

solar source sun
0
0
4 -0.810067 -0.561735 0.168064 0.533000

void brightfunc skyfunc
2 skybright perezlum.cal
0
10 1.932e+01 4.276e+00 -0.765799 -0.233179 13.481100 -3.262024 0.398945 -0.810067 -0.561735 0.168064
``````