Lux and PV yield

Reading time 2 minutes

1 lux represents 1 lumen per m². In connection with the performance calculation of planned PV systems, values of solar energy measuring devices are in watts per m² reproduced. This value, multiplied by the planned square meters, gives the expected energy yield.

The so-called Solar constant describes the radiation output of the sun as 1,361 W/m² in space. For calculating the effective watts per m²that actually hit the PV modules, various damping factors (atmosphere, haze, dust pollution, etc.) must be taken into account, which also change constantly.

The steeper the incidence of light and the closer to the equator, the higher the yield. The relatively small difference in latitude between Germany and Austria alone results in a yield that is around 12 % higher.

If you want to derive a rough estimate of effectively usable radiation power from measured lux values, you can assume around lux / 900.

Lux measurement

There is one for Homematic and RaspberryMatic Light sensor, which records a measuring range from 0.01 lx to 83680 lx.

Variables

The following variables are of type Number must be created:

• LS_avg
• Wm_2
• Wm_2_g

Program

The program will be on Update of the sent measured value is triggered and the included script is executed.

script

The lux value is determined by the channel 1:AVARAGE_ILLUMINATION of the light sensor queried and in the variable LS_avg saved.

The conversion into any W/m² is done by dividing from LS_avg through 1300 and will be in the variable Wm_2 saved.

The approximate total yield, corresponding to the installed module area, results from multiplying the square meter output by the sum of the module areas (here 7,68 m²), stored in the variable Wm_2_g.

var avgLS = dom.GetObject("HmIP-RF.000D5F29ADD1DB:1.AVERAGE_ILLUMINATION").Value();
dom.GetObject('LS_avg').State(avgLS);

var Wm_2 = avgLS / 1300;
dom.GetObject('Wm2').State(Wm_2);

! 7.68 sqm total module area

var Wm_2_g = Wm_2 * 7.68;
dom.GetObject('Wm2_g').State(Wm_2_g);

Visualization

The visualization via AIO Creator then looks like this:

Be used as a value range for the display 0 and 83680, the maximum possible measured value of the light sensor, is entered.

The currently measured lux value, the average power per square meter, and the approximate power sum resulting from the total installed area are displayed.