Figuring how much electricity a refrigerator uses

[wrenchtech]

I am looking at 120V refrigerators on websites like Home Depot, and all I can find about electric consumption is the energy usage certificate that talks about kilowatt hours per year . Most that I’m looking at use about 330 kWh per year. I’m trying to figure out how this relates to the capacity of my solar system and battery. Is it as simple as dividing the kilowatt hour per year figure by 365 to get the daily kilowatt usage and then dividing that by 24 hours to get a kilowatt hour figure for example 330÷365= 0.904 kWh/day ÷24 hours = 0.038 kWh or 38 watts per hour..Then divide the watts by volts to get amp hour, which I can use to gauge the effect on my 480 amp hour battery bank. Am I understanding this correctly?

Of course this doesn’t represent the maximum draw for the refrigerator which runs intermittently.

[Pizote]

Refrigerators don’t run 24x7. Likely they only run about two minutes out of every 30. Also those 38 watts are watt hours. So if you are running for only 4
Minutes per hour, the watt consumption for time
Used will be somewhere around 570 watts.

But more important than the watts are the draw or amperage. How many amps is the fridge rated at? This would be the running amps. The starting amps can be up to 10 times or more for a millisecond. So your converter/inverter will need to be able to handle these spikes every 30 minutes.

Eg: my rooftop AC unit draws 8.9 amps and when running. But it draws 170amps on startup. My
Inverter can’t handle that so I have to get a larger inverter.

[PNW_Steve]

I have a similar fridge. 10,8cf and energy star estimate of 336 kwh.

I put my Kill-A-Watt meter on it for a month and averaged less than .8 kwh daily.

If you want to know what a 120 load actually consumes, don't go by nameplate numbers, measure actual consumption with a Kill-A-Watt meter or simila

http://www.p3international.com/products/p4400.html

[john61ct]

Standard test protocol, units are Ah @12V per 24hrs

If AC plug into inverter, Watts Up or equivalent coulometer, placed in between inverter input and battery,

for apples to apples, use 25°C/77°F ambient, target temp 4°C/40°F, no added insulation nor added ventilation

only open as required for testing

A. Start out fridge off, packed full of room temperature water bottles. Note the time, start measuring for the cooldown period

B. Once contents are fully at target temp, start another 24hr Ah count

C. optional and looser: in regular use, full of cooled down contents, opening door at least a dozen times

[Ronnie]

Originally Posted by john61ct

Standard test protocol, units are Ah @12V per 24hrs

If AC plug into inverter, Watts Up or equivalent coulometer, placed in between inverter input and battery,

for apples to apples, use 25°C/77°F ambient, target temp 4°C/40°F, no added insulation nor added ventilation

only open as required for testing

A. Start out fridge off, packed full of room temperature water bottles. Note the time, start measuring for the cooldown period

B. Once contents are fully at target temp, start another 24hr Ah count

C. optional and looser: in regular use, full of cooled down contents, opening door at least a dozen times

This is basically what I did once I owned a fridge for our bus. But not useful to pick one.

I forgot how much mine uses at this point but my 400 watts of solar and 220 ah of battery will keep it running about two days without sun, rainy days that is. I bought a 4.7 cu ft dorm room fridge from Lowes.

[john61ct]

If more people documented their setup, full model #s then that would be very useful

[wrenchtech]

Originally Posted by Ronnie

This is basically what I did once I owned a fridge for our bus. But not useful to pick one.

I forgot how much mine uses at this point but my 400 watts of solar and 220 ah of battery will keep it running about two days without sun, rainy days that is. I bought a 4.7 cu ft dorm room fridge from Lowes.

I have 400 W worth of panels and 480 Ah worth of batteries which I haven't put into a system yet. For the rest of the winter I'm going to be tied to shore power. After that I hope to move around a little bit and will have to rely on my solar at least part of the time. The refrigerator I was investigating was about 9 ft.³, 60 inches tall and 23 inches wide if I remember correctly, with separate freezer and refrigerator compartments and frost free operation. This was the refrigerator I referred to which had the rating of 338 kW per year. That and some HVAC equipment are my main concerns.

[JDOnTheGo]

One of the reasons this is a challenging is that everyone has a different style of use. The power of my mostly empty fridge/freezer that is sitting in 60 degree ambient temperature and opened about three times a day does not compare well to your full unit sitting in 90 degree temperatures that is opened 50 times a day.

I can tell you that my Samsung 23 cu. ft. (Counter Depth 4-Door Refrigerator with FlexZone Drawer) is advertised to consume 665 kWh/yr.

I measured it in actual operation over a two month period during late spring/early summer (with ice maker running), it actually consumed:
84 kW - 1304 hours elapsed (54.3 days)
64.4 W/hour
1546 Wh per day
5.12 A/hour @ 12.5VDC (123.6 Ah per day)

It appears to me that the advertised power consumption is fairly accurate - at least in my case. No doubt that actually measuring the unit/device is the best approach but that is impossible without buying it first. So, in my limited experience, I'd say the advertised energy number is a reasonable place to start for the calculations that you are performing.

Remember to include inverter losses in your calculations.

[dzl_]

Originally Posted by Pizote

Refrigerators don’t run 24x7. Likely they only run about two minutes out of every 30. Also those 38 watts are watt hours. So if you are running for only 4
Minutes per hour, the watt consumption for time
Used will be somewhere around 570 watts.

But more important than the watts are the draw or amperage. How many amps is the fridge rated at? This would be the running amps. The starting amps can be up to 10 times or more for a millisecond. So your converter/inverter will need to be able to handle these spikes every 30 minutes.

Eg: my rooftop AC unit draws 8.9 amps and when running. But it draws 170amps on startup. My
Inverter can’t handle that so I have to get a larger inverter.

This is a misunderstanding of Energy Star ratings. They give you a Kilowatt PER year figure. Think of this like Kilowatt hours but replace hours with year as the unit of time. Duty cycle and on time are irrelevant here, they have already been factored in. 330 kw/year is the consumption including all the time the compressor is running and all the time the compressor is just sitting there idle.

[john61ct]

And using a standard set of values for the test factors, as with solar panels, so apples to apples.

Which may or may not have anything to do with those factors in **your** use case

[TheHubbardBus]

Even measuring actual use can be problematic, if you're doing so during a period of low ambient temps when you'll eventually travel to hotter climes.

I just assume a 50% duty cycle, & base calculations on the specs. Unless you're opening the door every 5 minutes, I'd think that would provide enough wiggle room. If you're running your compressor more than that, you probably need to look into more insulation / different use habits anyway.

Some things just don't lend themselves to accurate measurement.

[dzl_]

Based on what I've read over at expeditionportal.com, most people using an efficient 12/24/120 volt chest style fridge freezer experience between a 1/4 and 2/3 duty cycle depending on the season. With 1/3 to 1/2 being most common in 'normal' conditions outside of peak summer in hot climates. So your 50% duty cycle estimate is probably as close to spot on as you can get.

[john61ct]

Using it as a freezer can increase Ah per 24hr consumption by 3-5 times or more.

Nothing is more accurate than actual measurement.

But yes of course the various factors need to match your actual use case.

To be conservative hotter ambients and colder setpoint, opening more frequently.

See post #4 for comparing two or more units.

If more people do this the community builds up a database others can use and add to.

[dzl_]

John, I like what you are trying to accomplish with the 'standardized' test you outline in post 4. I wonder if it might make more sense to test in conditions where the temperature fluctuates AM to PM, as (at least for those of us out west or in the mountains) this better simulates real world conditions. Might be more difficult to standardize though.

[john61ct]

Yes just like the solar panel ratings, simple easy rigid standardisation is the key

"real world conditions" aren't relevant for apples to apples, vary too much anyway,

that's what ongoing in-use reporting is about will be all over the map.

I've seen the same unit average 14Ah/24hrs, go up to over 60Ah in different conditions, both temp-delta and usage patterns.

[dzl_]

Originally Posted by john61ct

"real world conditions" aren't relevant for apples to apples, vary too much anyway,

that's what ongoing in-use reporting is about will be all over the map.

I've seen the same unit average 14Ah/24hrs, go up to over 60Ah in different conditions, both temp-delta and usage patterns.

Well I agree and disagree. "real world conditions" can't be the test conditions, but test conditions should be as close to real world as possible while still being "standardized." I think you would probably agree with that.


But I do agree that standardized tests and real world reporting, or real world testing are useful for different reasons.

[john61ct]

The issue is variability.

The real world can't be replicated for a broad audience.

The point of the standardized test is reducing the variables' impacts AMAP

and to simplify implementation for regular noobs without specialized gear.

[PNW_Steve]

My observations:

My refrigerator used a bit less power while sitting in my garage with ambient temperatures in the 50's than it does installed in the travel trailer.

Once installed the refrigerators power consumption is the same parked in my driveway or at my favorite camping spot up on the mountain. It behaves just as I expected.

[dzl_]

Originally Posted by john61ct

The issue is variability.

The real world can't be replicated for a broad audience.

The point of the standardized test is reducing the variables' impacts AMAP

and to simplify implementation for regular noobs without specialized gear.

Very true. But still the closer the better if it can be standardized.


Even your test conditions, seek to emulate real world to a degree by choosing 25C (77f) which is a reasonable average summer temperature as opposed to 35c or 10c or 0c. But I think we pretty much agree it should be easy, standardized, and as close to real world as possible while still being standardized and easy to do.

[john61ct]

Exactly.

In the real world, temps do fluctuate,

we may not often fill the space with thermal mass,

of course doors get periodically opened, and

warm contents get inserted as cold stuff is consumed.

But it would IMO be silly to try to build variability in those factors into the protocol.