I've got a solar panel, rigged up to a voltage regulator (designed for charging car/bike battery's). I haven't got a mains power supply to the garage. Last winter my previous bike was hooked up to it, hadn't started the bike for over two months but it started first time. Measured the battery voltage, about 12.3V before I started the bike. After the initial cost of buying the panel it's free to run, it even puts out a good charging rate on dull days. It won't charge a dead battery but it keeps mine nicely topped up.