In my small greenhouse (6x4) at home, I have an electric heater with a thermostat, and in the lottie greenhouse I have a gas heater. But I don't use either of them over the whole winter (the few salads etc that I grow do ok with no heat). The heaters come into their own from March onwards, when the tomatoes, chillis & peppers all come out of propagators. They move to the small greenhouse first, then when that gets crowded I move them down to the allotment. The temperature has to be 10°c for toms etc, so it could get exhorbitant if it was for longer than a couple of months of the year
