I have noticed that generally one of my ac180 units will back up my refrigerator for about 14 hours.
So if I plug, a fully charged AC180 into another fully charged AC180 can I assume about 2 * 14 = 28 hours of backup time for my refrigerator❓
In other words: Will plugging one AC180 unit into another AC180 unit double the backup time for the first unit (assuming of course nothing else is plugged in to either of the two units)
I’m not sure how much less, but it will be less due to overhead of the units running. If I had to take a wild guess, it probably would be closer to 25 hours, but again, that’s just a quess.
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Any risk of doing harm to either one of the units?
No harm to either. My AC180 runs my home fridge for around 13 hours, I have 3 I can daisy chain if needed.
However, I also have a spare 100Ah LiFePo4 deep cycle and used that feeding the AC180 DC input and can get approx 30 hours total with that method. There will be the usual efficiency loss, but the actual methodology is akin to pass thru UPS charging. i.e.
If I daisy chain all 3 AC180s - AC180-1, AC180-2 & AC180 3, with the fridge plugged into AC180-3, then AC180-1 will power the fridge passing thru AC180-2 & 3 until it depletes. Then AC180-2 takes over until it depletes, then AC180-3 continues to run the fridge. Not ideal and I would unplug each depleted 180.
My preferred method is to use 1 AC180 until flat, then plug in the next. Whilst number 2 is running the fridge, I can take the first outside and recharge from solar or generator. This leaves a third for other use.
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The above option is also not my preferred method, just an example. I have 1 x AC70, 3 x AC180 & 1 x AC200P.
The 200P is not UPS, or Bluetooth and has a 240VAC power brick. I use my power stations as follows;
AC70 - Fibre optic box & WiFi Router = 28 hours.
AC180-1 Home fridge = 13 hours
AC180-2 Main TV 55" Oled and my desk - laptop, small LED desk light & mobile charging. = 10 hours
AC180-3 I keep for other sporadic and portable use around the house.
AC200P - This is used to top up AC180-1 & 2, then is recharged via solar or generator. I keep this on a “dolly” trolly to wheel around when using or charging as it is near 30kg (60+lbs).
Hope my posts confirm your need and gives options for use. 
NOTE - The above times are for my appliances, yours may differ. They were tested to check capacity by switching grid power off. That way I know what to expect in a grid outage, rather than guess.
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Thank you, your reply it is very helpful.
Thank you for responding.
Thank you for your input.
No harm, but each unit will have internal energy losses so you double the losses when you daisy chain power stations. It is better to use one at a time. And if the battery is empty, plug your refrigerator in the next power station.
This requires you to be at the location however.
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Thank you for your response.