Car Alarm trickle charge.
Ultimately a Prius is a 1500cc hatchback that is very heavy. The hybrid bit distributes the power from that engine, maybe marginally better than a conventional transmission. Drivers on the A34 have been quoting about 38 MPG, about what you get if you drive a 1500cc car sensibly. In urban stop-start driving the hybrid bit should come into its own a bit more but ultimately the engine still has to do all the work to push it along the hybrid bit changes the route to the wheels a bit.
If you could plug it in at night you might be able to make a saving using economy 7 electricity but the sandal-wearing brigade will complain that's not 'green' either. Its made really for the American market so they probably won't try a diesel version either.
At the moment too much compromise for too little gain unless maybe you are a city driver when a Citroen C2 diesel would be more economical and kind to the environment
The weight seems to affect the Prius' handling as well. I watched one overtaking a truck on the A34 and it behaved like it had sacks of cement in the boot, seemed very reluctant to change lanes and seemed to need wrestling back in afterwards as well. The rest of us started giving them plenty of room... Then again maybe people who buy Eco-cars are not good drivers...
The solar panel's real utility (if any) will come if its parked up at the airport or other long term parking when it may be able to keep the battery alive to stop the alarm going off. Ask anyone who has worked at Heathrow parking. To be fair some cars, particularly luxury models could do with some help under those conditions.