Hi @matias.bjorklund ,
This thread that I moved your post to discusses your question.
You probably realise that W/m2 is measured on a flat surface horizontally which means the measurements from your Tempest if you live near the equator will read a greater amount of radiation than if you lived near the poles for the same amount of radiation from the sun.
edit. You can also use your tempest to help decide what angle to mount your panels. Rather than converting the horizontally recorded W/m2 values from the Tempest to the designed best angle for your panels you can mount your Tempest at the desired angle of your panels end record how much energy it receives. This will be much more accurate because it takes into account the cloudy times of the day at the correct angle to measure the true energy received. Ideally you would have more than one Tempest and compare the results to find the best location and angle for your panels.
cheers Ian 