Newbie Question about Refrigerators

[CoffeeGuy]

Okay, I'm sure this is basic stuff, but I just want to make sure I understand this.

Let's say I have a refrigerator with an energy rating that says 365 kWh estimated yearly electrical use. That means it uses 1 kWh per day, and 1/24 = .041 kW or 41 watts per hour.

Do I have this right?

Second question: if I'm running a 120v refrigerator at 1kWh/day from a 12v battery bank, how much loss should I anticipate at the inverter?

Third question: If I set up my battery bank as 24v versus 12v, would it be more efficient converting to 120v, or does it not matter?

I think I know the answers to these questions, but I could use some verification as I think about how to size and configure our off-grid solar. After all, the more energy I can save on other appliances, the more I can use to fire up the espresso machine!

Thanks!

[JDOnTheGo]

1: On average, yes, that is correct. Remember that a refrigerator work load varies based on outside temperature, contents, how often the door is being opened, etc... The manufacturer estimates seem to be pretty good but they cannot account for every scenario.

2: 10% is a reasonable estimate. Quality inverters include a chart somewhere in their documentation that documents this based on load.

3: I've not researched this but my GUESS is that there is very little difference.

[CoffeeGuy]

Originally Posted by JDOnTheGo

1: On average, yes, that is correct. Remember that a refrigerator work load varies based on outside temperature, contents, how often the door is being opened, etc... The manufacturer estimates seem to be pretty good but they cannot account for every scenario.

2: 10% is a reasonable estimate. Quality inverters include a chart somewhere in their documentation that documents this based on load.

3: I've not researched this but my GUESS is that there is very little difference.

Awesome! Thank you for this. Your answers are inline with what I thought.

[bp1791-unleashed]

Originally Posted by CoffeeGuy

Third question: If I set up my battery bank as 24v versus 12v, would it be more efficient converting to 120v, or does it not matter?

The thing to know about Watts is that it is Volts times Amps. In the real world in practical application, there is a little more that goes into it, like power factor corrections, conversion efficiencies, line losses, etc. But you can get close enough with just Watts = Volts times Amps.

Consider a 1200 watt load. At 120 volts, that's 10 amps. At 12 volts, that's 100 amps. At 24 volts, that's 50 amps. At 48 volts, that's 25 amps. The wattage remains the same. Taking it the other direction, at 240 volts, that's 5 amps.

There's no significant efficiency difference in a 12v vs 24v vs 48v battery bank that I am aware of, but the higher the voltage, the less power is lost in the cables. 1200 watts at 12 volts is 100 amps which is some massive wiring. You're looking at 8 gauge to 4 gauge wiring, depending on the length of the wire. At 24 volts, you'd be able to do the same wattage with 10 gauge to 4 gauge, depending on length.

Practically speaking, if your chassis battery setup is 12V, and if you stay 2000 watts or less, you will probably be fine with a 12 volt system, heavy gauge wire, and short wire runs. This remains a simple system that can also be charged by the alternator, and can be bridged to your starting system to help "jump start" yourself relatively easily.

If you're in the 2000 to 4000 watt range, I'd recommend bumping up to 24V, because at that point you will start to see a return on the investment.

Above 4000 watts, I'd personally recommend a 48V system.

Note, this is all my personal opinion, based on my own personal research. I've probably over simplified this, and left out something important, so do your own research.

I hope this helps,
jim

[BeNimble]

Good post Jim. Higher voltage is technically better for everything but safety.
New electric cars now are going 800 volts, so even that can be safe..or so they say.
For converting to 120vac, I don't see any difference, as the wire size is not an issue in a 20,000 lb bus and you can put the inverter close to the battery bank and run wire for the 120vac instead.

They only exception would be if you can make your battery bank be 160 volts, then it can be converted directly to AC without any voltage translation, as 120 volt AC has a peak of around 160v dc.

[CoffeeGuy]

bp1791-unleashed, thank you for taking the time to write a thorough explanation of the basics. I actually knew this stuff (and taught it) some 40 years ago, but my mind has been in lots of other places since then.

I have more or less settled on a 24v system, with a 3kw inverter, believing that will be more than enough to handle our needs with around 10kw of storage and the only major appliances a refrigerator, microwave, and (maybe) an espresso machine. I'm presently looking at the Growatt Off Grid Solar Inverter, but bummed that the MPPT is high frequency. Any ideas about a low frequency "all in one" that can run 24v/3kw? I'm trying to keep the off-grid system relatively simple, as we have plenty of more complicated things to deal with.

[CoffeeGuy]

Originally Posted by BeNimble

Good post Jim. Higher voltage is technically better for everything but safety.

I think 24v is still in the safe zone. 48v, maybe, but why push it?

[CoffeeGuy]

I’m still doing a crazy amount of research on all this, but so far I’ve decided on a 24v system, with my storage set up with 3 pairs of 12v 137ah LiFePo batteries. Next on the agenda is figuring out how much solar I want to collect with panels (I’m thinking 1.2kw with 4 300w panels) and the charge controller.

EJ has picked out a 10 cu. in. Top mount refrigerator/freezer that runs on around 285kw/yr. There isn’t anything else that we’ll be running 24/7, so I still need to sort out what more we’ll need, but I figure this will be a fairly robust system for the 2 of us with our laptops and iPads along with some LED lights. I could probably crank up the espresso machine for a couple hours in the morning without too much trouble.