[tnjm]

I installed a backup system (in the greenhouse rather than the house... priorities) to cope with these kinds of problems.

Using a Victron Multiplus 1.6kVA system with 2x 110Ah 12V Gel batteries was remarkably cheap and simple. The whole lot came to just over €1k and took ~1h to set up, mostly setting jumper switches.

It keeps the pellet boiler, circulation pumps, backup gas heaters and control gear going all night when necessary. The automatic switchover from mains (<20ms) is so fast that none of the PLCs, Arduinos, Raspberry Pis or heating equipment notices. Upgrading its capacity simply involves plugging in another 2x batteries (it's a 24v unit) in parallel.

The only thing I wasn't thrilled about was that it doesn't protect against power surges, and a storm fried the boiler logic board AND the gas valve in the backup heater simultaneously in midwinter. (Victron unit was fine though!) Lesson learned.

A similar installation in the house could easily power our (non-electric) central heating system, LED lighting and wifi for plenty long enough to stay civilized, and we're being prepared for 2 hour rolling blackouts here this winter.