Thanks again for the speedy answer:
You can use a small solar panel with a good specification and decay rate for watts/meter over time. Then infer solar radiation using charge rate.
That exactely the way I want to do it:
I have an Arduino connected to Solar Panels that measure the KWHour produced: it is enough to use in ET0 formulas the ratio between max daily power to the actual one.
I will also try the PAN EVAPORATION method applied to Swimming Pool!
I will tell you the results!
Paolo