Residential (120v) fridge through inverter - check my math?

[eatSleepWoof]

Trying to figure out a fridge option for my trailer build and am finding that 12v fridge prices are insane for what you get, so I'm considering running a normal, residential 120v fridge through an inverter.

For example if we consider this fridge, with these energy guide specs, we see it has an annual use of 236 kWh. Double check my math please...

236 kWh = 236,000 Wh per year
236,000 Wh / 365 days = 647 Wh per 24hr period
647 Wh / 24 h = 27 W per hour
27 W / 12v = 2.24 amp draw per hour

My battery bank will be 200 ah of lithium. Assuming a 90% discharge before voltage gets too low, that means 180 ah of useable charge.

180 ah / 2.24 a = 80 hours (3.3 days) of runtime. (This is, of course, not counting power loss to inverter, other appliances, etc.)

Am I on the right track so far?

From a $$$ PoV:

This fridge will run me $720 CAD after tax and will give me 5.6 cubic feet of room, while the closest comparable 12v fridge would be around 3.3 cubic feet and cost around $2800 CAD. A $$ increase factor of 3.9.

The 12v fridges generally draw just under 1 amp per hour, so they are more efficient by a factor of ~2.3.

Meanwhile, an extra 100ah lithium battery (I've already ordered two for my 200ah bank) is $445 CAD.

Seems like it's a heck of a lot cheaper to use a residential fridge (of which there are millions of options - another plus) over an inverter, and spend more money on batteries, vs. having less battery capacity and a 12v fridge.

Any thoughts?

 

[eatSleepWoof]

Do you plan for your inverter to run 24/7 regardless if the 'fridge is running or not ?

Good question. I'd imagine the inverter would run 24/7 because it wouldn't know when to power on (or off) as-needed by the fridge. The exception being when I wouldn't need the fridge, and could then power down the inverter.

I can see the inverter being handy to have here and there, but by and large, I don't need it (other than potentially for the fridge).

 

[eatSleepWoof]

You probably dont want to run your inverter 24/7.
Consider installing another thermostat in the fridge which switches on the inverter.
Set that to your target temperature.
Leave the OEM thermostat set lower than your desired temperature.
Further,
Consider a dedicated inverter for the fridge.
Use a different inverter for whatever incidential AC needs.

Why not run the inverter 24/7? Parasitic draw?

 

[Hummelator]

Not sure what type of camping you do but one thing to consider is will the residential fridge hold up to the rough logging roads? ( if applicable)

 

[calicamper]

Not sure what type of camping you do but one thing to consider is will the residential fridge hold up to the rough logging roads? ( if applicable)

Having looked at the internals my self they are basically made side by side one gets 12v stuff the other gets 120v stuff. Would be interesting to look at the compressor to see if its the same and simply swapping out the power brick would it make 120v, a 12v fridge. ?

 

[burleyman]

You probably dont want to run your inverter 24/7.
Consider installing another thermostat in the fridge which switches on the inverter.
Set that to your target temperature.
Leave the OEM thermostat set lower than your desired temperature.
Further,
Consider a dedicated inverter for the fridge.
Use a different inverter for whatever incidential AC needs.

Well crap! I feel like an idiot going to the trouble to rewiring to always on when the inverter 120VAC shows up, then using the original fridge thermostat to turn on the inverter. Thanks. It'll be faster and easier if there's a next time.

Although it's not been an issue, that would also get the inverter up and running before the compressor load occurs.

 

[dstefan]

Why not run the inverter 24/7? Parasitic draw?

FWIW - I have a Victron Phoenix inverter that has an eco mode switch that cuts the parasitic draw (not sure how much) and comes on full when there's a demand. They make them in various capacities. Maybe thats a solution?

 

[eatSleepWoof]

Earlier this week I ordered this 12v/24v fridge, mistakenly thinking it draws 19.8ah/24hrs on 12v, when it actually draws that much on 24v.

I tried to cancel the order, but it looks like it shipped anyways, and I'll have to return it to HD myself. Looking at the specs again... if it's drawing 20ah on 24v, then it should (theoretically) draw 40ah on 12v, right? That's higher than I want, but realistically may still be fine given the other upsides (volume, convenience, separate, large freezer) of this fridge. However, the same list of specs also states:

Power Consumption: 52 W on 12V/24V - Average Run Current: 4.3A on 12V & 2.2A on 24V

52W / 12v = 4.3a, which matches the "average run current: 4.3A" statement. However, 4.3a * 24h = 103ah, which is a heck of a lot more than the 40ah/24h I'm assuming it should draw. Is the key part here the word "run"? In that, the fridge does draw an average of 4.3a, but since it won't be actively running 24/7, the cumulative consumption should still be around 40ah/24h? Am I missing anything?

 

[OldGreen]

Hey folks, I have used a 110v, 4.4 cubic foot fridge in my trailer for the last year and a half. It has never been shut off. The inverter is on 100% of the time. On the 12v side of the inverter, the fridge draws between 5 and 6.5 amps when running including .9 amps from the inverter. Cycle frequency depends on ambient temp. When traveling the fridge never touches the batteries. I have 300ah of solar and 420w of solar plus 30a DC to DC. Yes, the fridge can stand up to being off pavement. Zero issues. You do have to make sure that the glass shelves are captured and you put on a door lock, but it has been flawless for thousands of miles in Temps from 20 degrees to 113.