I also live in a flat.
Lighting: I have gone the DC power route. I did think about getting a solar power station (Delta 3 for example) but the cost is high and where to put solar panels an issue.
Have lots of 20,000MhA power banks. Have usb powered lights, some have their own internal rechargeable battery.
Heat: This is where I have an issue.
I have a heated wearable blanket - which runs off a USB power bank.
I have sleeping bag, woollen blanket, various synthetic blankets.
I have a small pop-up tent which fits in my tiny flat, so my theory is that I could be inside that which may help reduce area needing heat.
I have USB rechargeable hand warmers - which could be used inside sleeping bag to help warm me.
Cooking:
Have a small gas stove. Gas canisters stored outdoors (in the bin store, in a plastic box which has a breathing hole so if gas was to leak it has a way out. Out of the way of direct sunlight.
There is a BBQ in the communal garden, so I also have firelighters, kindling, and when I go for walks in the wood I collect dry sticks.
Getting some charcoal would be useful but I am very short on storage space.
For Summer Cooling: I have USB fans.
I have several flasks. My idea that I would heat kettle using the gas stove, then keep the water hot by using the flasks, so can make several hot drinks. Already use this method day-to-day using electric kettle, so know the flasks keep the water hot for a good 6 hours.
Heating food:
Gas stove, BBQ, and I have tea light food warmer and have foil trays (this may be useful for warming things which don’t have to get to boiling, such as soup, baked beans).