- Traditional: a generator. Problem is that you need a fuel tank, but if you are aiming to get through a couple of weeks then that's possible.

- Grid attached battery backup: good because in some places (like the UK) electricity at night is much cheaper than electricity during the day... so you can charge the battery and then save money and get payback on your installation. Bonus point is that if there is a grid outage then you have power, for a day or so if you are careful.

- Grid attached with solar/wind: as above, but you can charge independently from the sun/wind... obviously in some places (like the UK) this has limited utility because the sun don't shine (like today), and the wind can be annoying and difficult as a power source (gearboxes, noise, aesthetics). Can cost in though - because of summer power my panels pay back in 11 years. Small wind turbines seem to be getting more practical but I am no expert.

- Non grid battery backup. These also come with solar now, so you can recharge if it's sunny. These are cheap (<~£1000), hold maybe 2Kw and so can power some devices while the grid is off for a little while. Definitely you can run your phones and computers for a few extra cycles, but people are often looking to run freezers or fridges with medicine with these.

Another thing that folks like to have is grid independent heating / cooking via a woodstove. This is frowned on in urban areas and by eco people but I live in the country and have done my reading and while other people will pop up and decry particulates to be honest I'm keeping my burner so my family will be warm and fed for as long as our food lasts or we don't get looted by the neighbours.