I've been thinking about this too although we haven't had a power cut for years and never one lasting for more than two hours, mostly during summer peak load times too. We'd need a 3-phase generator to power the boiler and its auxilliary stuff, there's all sorts of motors connected to it (pellet feed auger etc.). I guess that makes it a diesel unit. If we get it permanently installed we can run it on fuel oil (diesel fuel is taxed much higher than chemically identical fuel oil in most European countries but legal to use in anything stationary). The biggest issue is that we currently have a circulator pump in every apartment so we'd need to devise a way to get backup power into every bathroom. Now if the planner and plumbers had had any sense they'd have installed a system without local pumps and saved us all kinds of headaches.
