Thanks Jonathan, I don't think the solar-powered option is feasible, I don't particularly want to drill a hole in the garage roof, especially as it's not my garage. The best option may be just to disconnect the battery for the winter, I just need to ask Porsche if there are any downsides to this (in terms of saved settings which will be lost). If you leave a car on idle until it is properly hot, e.g. for 20mins, I still don't see the potential for damage, but that seems to be the consensus....