Power cuts are a problem. Living in a village it's not unusual for us to have power cuts. Obviously it would happen while we were away for a weekend, tripping the circuit breaker so none of the heaters came on. -14C outside, -7C inside…
Electric heaters do seem to be the best bet though. My glasshouses are 18ft x 8ft, 12 x 8, 6 x 8 and the new one 8x8. A 2kw heater manages to keep the biggest one above -2C, even in the worst we've had recently. I probably could do with smasller ones in the others?