[danner]

The wires you are using are short enough to not need to be twisted at all. Maybe it's better not do it.
A reading of 12w/m2 in the dark is to high. If possible try to measure the voltage output of the SP-110 in the dark, it should be zero (testing things before having them installed its easier, however I know that sometimes isn't possible). Even with a 0V input, can happen that the output voltage of the OP drifts from 0 to 3 mV, but will never reach a value of 7mV (which is needed to produce a reading of 12W/m2).

OK

Simply it isn't calculated.
If you find a way to calculate it with an Arduino, without consuming a lot of memory, please let me know, I will be very happy to include it.
The WeatherDuino Pro2 emulates the Davis communication protocol, and some aspects of a Davis weather station like the data logger, but it isn't a Davis Weather Station.

OK that answers it. Thanks.