We own and use a 'briefcase cooker':

Briefcase Cooker

£9-£10 here and Butane refills £1 each. Very easy and very safe to use. Also an additional gas ring in the kitchen when needed, Christmas dinner, big BBQ, cooking at the table, picnics.

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A large (huge) vacuum flask may be used as a slow cooker, Pre-warm, fill with rapidly boiling stew, seal and only open when ready to serve.

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I have found it easy to get large 3-4kWh computer UPSs for nothing. Companies often replace the whole thing rather than just rebattery every couple of years and throw out the old one!!!. I have the heating boiler wired to a standard plug which is simply plugged into the UPS which is then plugged into the wall. It will run our system for some days easily, either bridging the blackout or giving time to set up an alternative