How To Make a 72 Hour Battery Backup for Your Refrigerator

by Stephan on July 20, 2010

We have an old refrigerator with top mounted freezer and I was wondering how big and inverter it would take to run it if we setup an alternative power setup. Using a Kill-A-Watt meter we found:

Just sitting there takes about 15 Watts (W)

Running uses 282 W and it runs for about 10 minutes to cool it after opening the door.

Assuming a startup surge of double the running that would be 564 W for the first second or so.

Plus, give a 20% derating to not overload the inverter, we would nee one that can handle at least 677 Watts peak, just for the refrigerator.

So one of those 1000 W inverters you get at the home improvement store is more then enough. However you’ll have to hook it directly to the battery terminals under the hood, the wiring to the lighter plug would never be able to handle that, it would melt pretty quickly causing your car to catch fire. Besides inverters limit the power output when using the lighter adapter anyway, to keep that from happening.

Contractors also leave their trucks running when using something like this, because a starter battery like the ones in cars and trucks are designed for high current short use (that’s why they are rated in Cold Cranking Amps), not long term use like a deep cycle battery.

Since the frig is only using about 2.5 Amps while running how big of a deep cycle battery do we need to last 72 hours? We’d have to monitor the internal temperature of the frig with a probe thermometer and plug it in when it got to 40°F, assuming it takes 4 hours to reach that temperature from a typical 35°F  and that it takes 30 minutes to get back to 35°F. We can expect about 9 hours of time we need to run the frig. Multiply that by the 2.5 Amps and we need 22.5 Amp-hours worth of battery capacity. Since we don’t want to run a battery down by more then half its capacity; we want to get a 45 Ah deep cycle battery.

A 45 Ah battery is good sized and a quick search shows that would cost about $120-160, not including tax, shipping and handling.

A 1000 W inverter casts about $100 for a modified sine wave version (some electronics don’t like modified sine wave and modern refrigerators are full of them now) and a pure sine wave version goes for over $200.

A Battery Charger with smart or overcharge protection and battery maintainer technology costs less then $100. That way you can leave it plugged in until the power goes out and not worry much about the battery.

So for about $400 you can provide you refrigerator with power for 72 hours.

Caution: Electricity is dangerous stuff, read and understand the manufactures safety instructions before use. Work with one hand in your pocket. Ask for help before you need it. Never work alone, have a spotter in the room with you, preferably one that knows CPR. This is provided for entertainment purposes only.

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