interestingly we use, what we call in Delhi . an Inverter . which is simply a very powerful electric power pack . in many households and offices which is a precharged up battery pack to kick-in when the power clicks off as it so often does in much / most of Delhi from time to time
Just recharged from the ordinary electric plugs in the home whatever . had them for years in my offices there ........... just repurposing, in your case, an old effective EV battery maybe ..costs about equiv £100 / 150 in Delhi I think and can last for many years ( often a week or more wages tho' ) . and a freshly charged Inverter can last about 4 hrs in constant use .... one aircon, a few computers, telephony, internet and some lights maybe
Malc