OK time for some maths!
I’ve gone and picked up 2 18Ah SLA batteries for a pretty decent price (trade prices) so now the fun begins!
18Ah x2 @ 12V is 432Wh
The panel I’m looking at is 100W, and a nice size which will fit perfectly on my parcel tray, which leads to this calculation:
432Wh / 100W = 4.32hrs
My research has suggested that you need to multiply this by 2.5-3 to get a reasonable value. That leaves me with:
4.32 x 3 = 12.96hrs
I’m not going to get that much sunlight but I don’t think I’ll ever run the battery down, so it should work fine.
The next step is to test the power draw on the Arduino Mega and android phone I want to use. The other option is to to use a GSM shield which will draw less power.