45W sounds like overkill for an RPi - I live in Scotland (55.95N) and I have a 50W panel charging a 110Ah battery in my garden shed. The battery is used to power a 1kW Invertek 250V inverter (150A fuses are impressive BTW) which I use to run a lawnmower, power tools, soldering station and lights (not all at once!).

The lawnmower is the biggest drain (900W) and is really pushing the envelope - but in the summer I can cut 55 sq. metres of lawn once a week without any charge problems. In the winter the grass doesn't grow :) but I can run the other tools just fine. I was very surprised at just how well this system worked, but then I did splash out on a high end 50W Kyocera panel when I designed this. My only alternative would have been to bury an SWA cable across common ground and somehow run it to my consumer unit in a 1st floor flat, so I was prepared to spend £300 (as it was then) on the panel. They're a lot cheaper now.