You can't just leave generators outside in the garden, they aren't weatherproof. They're noisy and it has been pretty much impossible to buy petrol for the last couple of days because obviously with no power, and panic buying all the petrol stations in the area are shut.
It's possible to get battery packs for UPS systems to increase the backup time to take standard 7ah/9ah batteries (external chargers required for a few packs) but how much power and for how long would you seriously want?
If you bought a 3kVA online UPS with 5 additional battery packs you'd probably get about 12 hours of run time at 1/4 load, but that would be 18U of rack height, over 200kg in batteries and cost £1000's.
UPS's are designed for short term use either for short power outages, stabilising power, safe shut down for long term outage or back up power to allow a generator to start up. In some African countries it's quite common to have a home inverter wired up to large capacity batteries but their power requirements are much lower.
Due to how infrequent we get power cuts, for home use it's probably cheaper to book in to a hotel for a few days.
The impact of power cuts is severe though so even if they don't happen often they are worth mitigating against, 3 days without having internet, telephones, or computers makes communication virtually impossible, I can't work or do my coursework, due to the power cuts I had to contact my uni to get an extension because I missed the submission date because I couldn't upload my coursework.
I've actually measured my power use and done the maths and my computer equipment is using approx 180W in low usage mode so a single £75 120Ah battery would last 8 hours ignoring inefficiencies. I don't know where you got the figure of thousands from?
You will struggle - as generally UPSes aren't designed for long term use, in most cases UPSes are just used to smooth the period between mains power failure and a generator kicking in.
Guessing it's a BackUPS series, in which case I would say upgrading the battery is almost certainly out of the question (The inverter etc are likely not rated to run 24/7)
Personally I'd just buy a much larger high end unit - 3000VA is about the biggest you can go without requiring hard wiring iirc. Worth looking on ebay as you can often get larger ex enterprise UPSes that have been refurbed with new batteries.
To clarify I'm not talking about running the UPS at max load for 24 hours, I'm talking about using the computer at a low level of intensity for a few hours at a time, currently this would be @ half load with my 400W PSU, I'm open to upgrading to a better PSU with a better inverter if necessary, since I can use an external battery it doesn't matter what state the battery of a second hand unit off ebay is in either.