Ah, I would not call that 'a boiler' - do I take it that you are talking about an 'instant water heater' (which my parents and grandparents would probably have called "a geyser"!)?The main 7 was for domestic hot water, not central heating, it had a pilot flame to turn it on, and the sensor in the flame generated to only electric power used. I think it was around 18 kW worked shower OK, but took ages to fill the bath.
My (true) boiler also still has "a pilot flame" but that doesn't alter the fact that it needs electricity to work the gas valve. Both the 'enabling' and the local thermostatic control are 'electrical'. As with your Main 7, the thermocouple in the flame generates (a tiny amount of) electricity which opens the 'flame failure solenoid' in the valve, but that's a totally different matter, and requires no electricity supply. (it's always amazed me that a little thermocouple can generate enough electricity to do that!)
Although I've had my (petrol) genny for about 20 years, it has only really been used 'in anger' a very small handful of times (far more often 'tested' than 'used in anger'). Although we have a lot of 'power cuts', they rare last for more than a small number of minutes (and, most often, only seconds).For me a petrol/TVO generator would be great, the 28 sec gas oil is nearly TVO so engine should run on that for months with a 600 gallon tank, but it is well down on the priority list.
Kind Regards, John
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