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Old 09-12-2019, 11:28 AM   #1
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DC Watts vs AC Watts

I'm trying to do an electric budget estimate. Being as pessimistic as possible, rounding up everywhere, assuming I'm always in 120 degree weather trying to keep an uninsulated bus at 62 degrees, while constantly running the fridge with the door open, and drying kiddie paintings with the hair dryer.

I know that the only real way to do this is by measuring. But I'm trying to figure out if I'm even on the right track at all. It looks like 24V DC appliances are insanely more efficient than their AC counterparts.

Take these identical drawer refrigerators:

https://www.defender.com/product.jsp?id=2127996
https://www.defender.com/product.jsp?id=2799262

One runs on 12VDC * 6A = 72W
The other runs on 120V * 2.5A = 300W

Are DC appliances just that much more efficient? Is there a secret formula for converting Direct Current watts to Alternating Current watts? Am I comparing school buses and motorcycles here? (Apples and oranges..bad joke).
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Old 09-12-2019, 11:32 AM   #2
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It's worth noting there's a good deal of hyperbole happening in my post...just in case that wasn't _abundantly_ clear.
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Old 09-12-2019, 12:19 PM   #3
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So ultimately a watt is a watt. It's a unit of work. 745 of them equal 1 horsepower, regardless of how that watt was delivered. But there is a big difference between AC and DC that can manifest in things taking more watts one way than the other. DC can often be much more efficient than AC, and higher DC voltages can be more efficient than lower DC voltages.

But that's not really answering the question, you're likely comparing apples and oranges in a way that doesn't really help you. You should care much less about how many watts a refrigerator draws at a given moment and more about how many watts it draws over a period of time. This is the watt/hour which is the unit of measurement that matters for electric consumption, That information is totally lacking in the product descriptions linked.

This is why major appliances like these have energy guides estimating their power consumption per month or year. Ultimately i expect those units to consume very similar amounts of power, and your decision would be based more on how feasible it is in your build to give it one or the other. I use a DC refrigerator in my bus because I run everything on DC. Many people tie their battery bank directly to a huge inverter and run everything from AC power. so the DC option wouldn't be feasible for them.
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Old 09-12-2019, 12:23 PM   #4
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If already understood, please ignore.

Watts are the power - what is needed to run a device. Amperage/current is how fast it flows and voltage is like the supply.

By the data provided, it appears that the DC fridge requires less power to run than the AC fridge.

If you were to compare apples to apples, then you would need to convert one type to the other. Eg:

12vdc@6amp=72w or
120vac@0.6amp=72w

The power consumed by the same device is the same, just the voltage and amperage changes.

It seems strange that the DC fridge consumes so little power. It might be average or marketing. Best to test it.
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Old 09-12-2019, 12:28 PM   #5
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Excellent posts above.

You need to know the duty cycle (% of time when the refrigerator actually runs) to compare apples to apples.

That being said, there will be a 5-10% loss when converting DC to AC in an inverter.
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Old 09-12-2019, 12:33 PM   #6
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A watt is a watt. Itís how you use it.

Those refrigerators have the same compressor. It uses a DC motor so if you want to run it off AC there needs to be an additional circuit known as a rectifier between it and the power supply. A lot of energy is wasted making the conversion. Same goes for inefficiencies converting DC to AC using an inverter.

Itís best not to convert if you can help it
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Old 09-12-2019, 12:47 PM   #7
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@Pizote, Thanks! I do understand the basics pretty well - I even included a brief synopsis in my first draft of my question. That's why I was confused.

It makes sense that DC components work significantly differently, I guess the best bet for figuring out the sizing of the electric system is to split my budget into AC and DC sides. And if the DC appliances are that much more efficient it's a double win: no inverter losses and more efficient appliances...
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Old 09-12-2019, 12:51 PM   #8
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OK Follow up question from this then...what's the secret to googling for 24V appliances...I keep getting 24" wide appliances which is nice since I'm also space constrained, but not at all what I'm looking for...
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Old 09-12-2019, 12:59 PM   #9
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Quote:
Originally Posted by Crisfole View Post
OK Follow up question from this then...what's the secret to googling for 24V appliances...I keep getting 24" wide appliances which is nice since I'm also space constrained, but not at all what I'm looking for...
Use:
"24 Volt" refrigerator
(with the quotes)

I also use sometimes the keywords RV, marine and solar.

If you are looking for an energy efficient fridge, exclude the ones that also run on propane. They use an absorption cycle driven by heat and are electricity hogs when run on AC or DC.
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Old 09-12-2019, 04:36 PM   #10
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The refrigerator specs list 6 amps draw at 12 volts and 3 amps at 24 volts.



"Current Draw: 6.0 Amp @ 12 Volt DC (half at 24 Volt DC)"


watts = amps * volts so your draw is 72 watts on either 12 or 24 volts. A 12v system is much more convenient to set up.
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Old 09-12-2019, 07:03 PM   #11
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12v is super convenient if your solar needs are small. When you look into running an air conditioner or other high draw appliance, higher voltage starts to make more sense really fast. Twice the voltage means half the amps, which are your primary limiter for your charge controller, wiring, and a major factor in the lifespan of your batteries.
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Old 09-14-2019, 10:45 PM   #12
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Quote:
Originally Posted by Crisfole View Post
It's worth noting there's a good deal of hyperbole happening in my post...just in case that wasn't _abundantly_ clear.

Thank you for the clarification. Starting to wonder if we had another one of "those type of people"
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Old 09-15-2019, 12:53 PM   #13
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One pro engineer at using DC for everything goes by Electrodacus.
He has developed cutting edge charge controllers and BMS. Taken it to the next level. See:
ElectroDacus
If I had not already bought mppt and AC technology I would consider it. His controllers will also work with a inverter and AC coupled sytems.
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Old 09-15-2019, 04:18 PM   #14
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Aside from inverter losses (which you're not discussing), there's nothing inherently more efficient about DC vs AC in this context. But some of the refrigerators that run off DC are more efficient, because they're designed to be used in off-grid applications where sparing power is a key consideration. So ultra efficient electric-only fridges will generally A) be DC, because then you don't have inverter losses, B) run Danfoss compressors, because they're very efficient, and C) be extremely well insulated.

24V is just a design consideration for matching up with your home bank's voltage. Many of the fridges you'll find in 24V will also have 12V counterparts, and will pull exactly the same watts in juice.

I assume a 50% duty cycle for fridges. I think this is likely a liberal estimate unless you're really sloppy about leaving the door open.

There are some AC/DC fridges which are designed to run off AC primarily, and have DC as a backup. You can tell them from the ones described above because on DC, their efficiency SUCKS. I'd stay away from those.
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Old 09-15-2019, 05:04 PM   #15
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Quote:
Originally Posted by TheHubbardBus View Post
Aside from inverter losses (which you're not discussing).
The two refrigerator examples posted by the OP are essentially the same fridge. They both have a Danfoss DC compressor. The AC model has a rectifier to run it off AC. Thatís where the big loss is.
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Old 09-15-2019, 05:24 PM   #16
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Ok, gotcha Danjo. Didn't even look at his examples.
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Old 09-15-2019, 08:31 PM   #17
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By using a efficient computer fan blowing on the coils a fridge gains efficiency. Especially if there is tight space and minimal ventilation.
Did anyone see a movie called The Mosquito Coast. Classic movie involving a DIY ice maker. I wonít spoil it.
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Old 02-29-2020, 07:05 PM   #18
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The DC units will always be more efficient as the compressor and other electronics are DC so the AC version will always have conversion losses.


But that's likely not the reason for the huge discrepency, if you look at the two fridges carefully you will see the 110v unit has an icemaker the 12/24v unit does not.
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Old 02-29-2020, 07:24 PM   #19
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Just a couple points and clarifications:



Quote:
Originally Posted by TheHubbardBus View Post
Aside from inverter losses (which you're not discussing), there's nothing inherently more efficient about DC vs AC in this context

AC is not inherently less efficient BUT in practice, in a mobile or offgrid system, it almost always is. For 2 reasons, first as you stated, inverter losses (both conversion inefficiency, and the power the inverter itself consumes when switched on), and second, almost all electronics and appliances are DC internally, so there is a second conversion loss (even with "AC" appliances) as they are converting from AC back to DC internally. If you can power an appliance using DC you are eliminating two conversion losses, one at the inverter, one at the appliance itself.



Quote:
There are some AC/DC fridges which are designed to run off AC primarily, and have DC as a backup. You can tell them from the ones described above because on DC, their efficiency SUCKS. I'd stay away from those.

This might be true in some cases, but the logic behind many fridges running off AC first and DC as a backup is to preference shore power or generator power (AC) first and then fall back on battery power (DC) when external power is not available. These fridges are still more efficient running off of DC as their compressors use DC.
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Old 03-01-2020, 06:30 AM   #20
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One thing not mentioned here. It is quite easy to safely wire AC from one end of your bus to the other to run any 120V appliance. Twrlve gauge will run pretty much anything rated for 120V at that distance.
With DC you have to run increasingly larger wire as power requirements and distance increase. And the size requirements go up quite rapidly.
My fridge, microwave and induction cook tops are about 20 feet from the power source. Wouldn't have been feasible on DC.
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