Running fridge part time?

I don't think you would want to really. Most food does better at a stable temperature than one that swings wildly and your fridge only runs when it needs it so even if it is plugged in 24/7 it won't actually run 24/7. It actually is probably more efficient to run it 1-2 min every 30 mins at a difference of 1-2 degrees than to have to run it 30+ min at 20 degrees difference. You also need to consider that most appliances aren't meant to run like heat pumps so temperature control will be finicky.

Most appliances don't actually take all that much power to run so if you can add a battery bank you can kind of even out power demand and draw with that buffer. It is totally possible to do this with 12v fridges and some car batteries. Otherwise you would want an inverter and controller. If you can couple the source to batteries you get a much more efficient system and you won't have to run a generator all day to run an appliance but instead run it for an hour to charge up the batteries. Some of the really nice ones can be coupled to a controller to autonomously do exactly this and are quite efficient.

However if you really don't want to go the constant battery route then each fridge is different. Because there is no way to accurately know insulation values or mass or temperature or specific heat of every item in the fridge a mathematical model isn't much help and you are probably better off with empirical data. As a test take a thermometer and unplug the fridge/freezer hopefully fully stocked at hopefully the highest difference in temperature, measure it every hour or two and record how fast your particular fridge gains heat. Once you find a decent threshold you can start working on a timing schedule or a rate of heat gain that can be used to keep food at acceptable temps.

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