Night time wind - I suppose but I always charge during the economy 7 times and the Ampera ca be programmed to only charge during low rate. The trick is to set med rate to be from midnight to 00:30 or 01:30 winter/summer and then ignore med rate in charging options. Occasionally I get to add a bit of solar top up in the day but that will be more frequent when I retire. I only do that if I don't have to buy any full price as well as the solar, so 6A when I'm exporting 1.6kw I can have for free, and export drops off. Ecotricity seems to be lower than some at night. Got to be them or Good Energy IMO.