It does state 3 hours, which is like with that very idea in mind. With a chest freezer as you say, not a problem even if the door is opened, with an auto defrost upright as I have said many times it depends on how close to the defrost cycle.
As I have said before a few years back I had a freezer fail with a bang, so knew it had failed as standing next to it. I have a fridge/freezer being used to brew beer in, so without opening the door, I went and switched on the spare, and gave it 20 minutes or so to get down in temperature. Thinking, I had a few hours to swap food.
Food in the majority of the freezer was OK, but top foot food had started to defrost. The whole idea of the frost free upright is the working parts are behind a panel
and a fan moves the cold air around so all the freezer is at the same temperature, there are no hot spots, and heaters in with the evaporator coils defrost the coils without defrosting the whole unit. The water goes to a tray on top of the motor so it will evaporate latter, if it is switched off right at the end of a defrost cycle then the top can defrost. Even when the main body remains frozen. The frost free freezer was not that popular in 1978 so we did not get the problem, we are told not to open the door so we don't see what has happened and all is refrozen, as to damage caused to the food, I don't know. Likely not that much? I keep lolly ices in the top, These will deform if defrosted, so I will know. And the freezer will display the highest temperature it reached until the door is opened after a power cut. But not all do that.
So having something in the top of an upright freezer to show if it has defrosted would be a good preparation for power cuts.
But in 1978 the problem for me was heating, back then non double-glazed windows, and we had picture windows either end of the living room, and once the hot air gas central heating failed, the room soon got very cold. There are I know in Shrewsbury some rental properties still with single glazing, permitted as they are listed buildings, and they are not allowed to change the windows.
Personally, I would cut a Perspex sheet to fit the window space for the winter, well personally I would not want a slopping floor in my flat either, the central heating has been removed, when built the cows were kept under the living area and the heat from the cows heated the house, that is not done any more. I could not see any signs of a fire place, likely they were originally with workshops above, not living quarters? But we did not have storage heaters in the 16th century.
I have one of these
or very similar if not the same, it has a built-in same fluorescent light in one end, and a 13 amp socket (300 watt) and cigarette lighter socket the other end, the dedicated jump start leads, the compressor has failed, and been removed, and battery (20 Ah AGM) has been replaced. Charges with a Walmart type charger, so slow to charge, but does auto turn off the charger when full.
Before replacing the battery, I bought a modern replacement. Yes it will jump start a car, and it has a light, but no 13 amp socket, or cigarette lighter socket, the USB outlet seems a bit hit-and-miss.

at, 6400 mAh

it is not going to last long as a stand by power supply, 3.2 hours if I could find a 10 watt USB lamp. I had expected a 12 volt unit and the warning "Don't continue Starting car more than 1 minutes. (Otherwise may short circuit and burnout machine)" does not fill me with confidence. And the LED may work wall as a torch, but not really going to light the room.