I live in the PNW and when we lose power, my water heater is gas, so I make hot hot water, put it in jars, wrap a towel around one and place it with the tort. He will cuddle up to the towel and keep reasonably warm.
I've spent my fair share of time advising people to do this when the weather turns. We live in a rural area so we've already warned the neighbors with the wood stove that we will be visiting with a bucket of tortoises if the power is out for more than an hour or two. All she said was BYOBB (Blankets and booze) and we'll make it a party.