Refrigerator amperage

[Longshhm]

I know this has been covered before, but I haven’t been able to nail down specific information that helps me figure out what I’d like to do. I’m looking at installing a residential fridge in place of my worthless absorption fridge. In considering this, I need to calculate expected amperage use by a 120v fridge from a 12v battery bank. I don’t have specific amperage or wattage loads for the fridge I’m looking at - all I have is the energy guide (it is Energy Star rated). With that, I’m trying to make the conversion. So, those of you who really have a handle on this, please check my math and tell me if I’m on track.

Energy Guide states annual energy consumption at 310 kWh. Therefore, 310 kWh per year/365 days per year = .849 kWh per day. .849 kWh per day x 1,000 watts per kWh = 849 watts per day.
849 watts per day/120v = 7.08 amps per day

Average run time for a residential compressor fridge is 8 hours per day. Therefore, 7.08 amps per day/8 hours per day = .885 amps per hour (average). Throw in brief start-up draws, and round up to 1 amp per hour.

For the inverter, the ratio of 12v amps to 120v amps is about 10:1. So, 1 amp per hour for 120v = 10 amps per hour for 12v. Take into account 85% efficiency for an inverter, then 10/.85 = 11.7 amps per hour - round it up to 12 amps per hour.

Now, battery bank with 2 6v batteries in series at 210 Ah each equates to 210 Ah at 12v. Not discharging below 50% leaves 105 Ah usable. 105 Ah/12 amps = 8.75 hours run time on that battery bank.

Am I close, or off base? How would a 12v compressor fridge compare?

Thanks.

Mike

[boyscout]

This may help. Google is your friend.

https://www.batterystuff.com/kb/tool...-inverter.html

We don't have a res fridge - touch wood our absorption fridge isn't worthless yet - but I've read about it. Until someone with actual experience comes along...

Figure on the res fridge needing about 725 watts of power (an average, according to some sources) which is about where you have arrived in your figuring. That translates to about 6 amps of AC current.

The calculator on the page above will help you translate that to required DC amperage, which it says is about 68 amps of battery current, again close to your figuring.

So, yes, it seems that you're on-track, but I'm just going on theory as you are. My reply will push your question back up the stack and hopefully someone with experience will tell you if the theory and the reality are close to each other.

I've seen very little discussion about 12V fridges, and a very few unfamiliar brand names mentioned. It doesn't seem to be a popular option. I don't remember all the reasons for that but one is price - even a very small one is very expensive.

Good luck.

[jleonard]

I had a 40 ft boat that came with a residential size fridge (same physical size as an RV fridge). It was set up to run off a 3000 watt inverter and a battery bank.
I upgraded the bank with 4 6V deep cycle GC batteries at 225 ah so I had an inverter bank of 450 ah.
I could easily go 2 days at anchor without thinking about the batteries. I'd still be above 50% charge.
After that I had to either travel to charge the batteries or start running the generator for a couple of hours a day.
The fridge was closer to 50% duty cycle not the 33% you are using in your calculations. But maybe yours is more efficient.
We typically didn't run anything else at anchor other than the fridge and occasional water pump so he fridge was the main load.

I did eventually replace that with a 12/120 V unit (unfortunately it was a "Nevercold") but at that time the batteries were getting old so I can't reliably say how long the batteries would last. A day anyway but not 2.

Hopefully this will help you sort it out.