Fridge power consumption math help

TT-Tacoma

Observer
So I know for a fact I have seen the calculation on how to convert power consumption for a fridge on the forum before but for the life of me I cannot find it and have been searching for some time.

I have been running a test using my old Norcold fridge in an attempt to how much solar I need and give me an estimate on how much power it consumes. I am currently at the following numbers:

Norcold MRFT-630
100% empty
40 degree's in garage
knob turned to 1.5
16 hours
123.4 VAC
0.10 kW used

I was able to calculate out that it should burn 12 amps to run it for 24 hours on the 123.4 VAC.

How do I know what the 12 VDC consumption is now?

I am going to order a meter so I can test the fridge inside and off a battery next.
 

DaveInDenver

Expedition Leader
You know you used 0.10kW over 16 hours that means your fridge is consuming 100 watts over 16 hours or 6.25 watt-hours. At a nominal 12V that means 0.5208 amp-hours. Power (watts) = volts * amps or rearranging amps = watts / volts. Therefore 6.25 W-hr / 12V = 0.5208 A-hr.

To know more you need to know the conversion efficiency. It's likely the fridge runs either more efficiently on one voltage than the other so it's something you really need to test on a battery instead of wall power (I assume you mean 123.4 VAC, as in volts A.C. on your wall plug and not VA volt-amps, the measure of complex power).

But for a rough guess 0.5 A-hr is a start. That's about what I'd expect my Engel MT45 to do, give or take, under those conditions (40F, about 25% duty cycle, it would be a very small temperature differential).

The next step is to know your battery capacity. A 100 A-hr battery will do 0.5 A-hr for 200 hours. But you can't use 100% of a battery's capacity, so you'll actually get 1/2 of that. So a 100 A-hr battery run to 50% state of charge (which is in itself a deep discharge) will run the fridge for 100 hours in this theoretical example. That's about typical, most people get about 3 or 4 days on a large deep cycle running only a fridge in mixed warm days, cool nights (about 33% average duty cycle).
 
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SoCal Tom

Explorer
You know you used 0.10kW over 16 hours that means your fridge is consuming 100 watts over 16 hours or 6.25 watt-hours. .
I think he means 6.25 watt-hours per hour.
100 watts is pretty low, I suspect the fact it was in a 40F garage helped it stay low. If it was on fridge mode, then it was probably set at about 40F. For comparison If it helps, my CF35 uses about 150 -180 watt hours between solar off and solar on ( about 16 hours). That is 8-10 watt-hours per hour. That includes opening and closing and adding some warm bottles of water to replace the cold ones. This is on a days with a high of ~70F and an overnight low of ~45F.
Tom
 

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TT-Tacoma

Observer
Awesome!

Thank you both for helping me out. I should have known I could have just used the consumption and done the math with 12 volts in place of the 123.4 volts. I am going to move the fridge inside tonight and start another test after it has been inside for an hour or so. I am very impressed with the bugger so far and cannot wait to be able to use the old girl when out camping this year. Although you would never even think it will come around when you look outside our house right now!

FWIW my current battery is a Exide group 31 AGM which is a 100ah capacity battery. I am planning on adding a 100w solar suitcase to the mix to put the power back in when camping.

I hadn't been able to find any power consumption tests done on an old fridge like mine to compare off of so hopefully with some good data anyone out there that comes across a cheaper older unit like mine will have an idea as to what they are getting.

I'll do my best to keep this thread going to get some good documentation.
 

plh

Explorer
Yea, the 40 deg garage has a LOT to do with the power consumption in refrigerator mode. Once the temperature equalized, I doubt the compressor even ran.
 

TT-Tacoma

Observer
Ok. More testing has been done overnight.

70 degree house, overnight down to 68
Empty fridge
Pre cooled
122 VAC
0.14 kw
10.5 hours
Knob set to #1
Thermostat inside reads 36 degrees

So if I'm doing the math correctly and rounding to the tenth I get this

0.14 kw equals 140 watts
140/10.5= 13.3 watts per hour
13.3/12= 1.1 amps per hour

So in these conditions I will burn 1.1 amp per hour. Which with my battery gives me 45 hours to 50% capacity.

So with the solar panel I am looking at being a 105w and if putting in 70% of it's capability being 70 watts even at 12v it's capable of doing 5.5amps and let's say in Minnesota I get 5 hours of good sun I can expect 25 amps into battery and that should be sufficient.

What do you guys think?

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DaveInDenver

Expedition Leader
I think he means 6.25 watt-hours per hour.
Watt-hour is the consumption rate at 25% duty cycle (estimated). How you use that figure is up to you. In an average hour that would mean 6.25 watts are consumed. In an average 16 hours it's 100 watts, in an average 10 minutes it's 1.042 watts, etc.
100 watts is pretty low,
Yeah, I agree, it's exceptionally low.
13.3/12= 1.1 amps per hour
Approximately 1 amp-hour is a more realistic expectation, which probably closer to 50% duty cycle. Any way you look at it you can arrive at the same place. My Engel draws about 2.25A while it's running, which is probably typical. This is instantaneous current.

So if it runs continuously (100% duty cycle) for 60 minutes it will have used 2.25 amps per hour or 2.25 amp-hour is the rate. So at 25% duty cycle, meaning it runs 1 minute, turns off for 3, it will be roughly that 0.5A per hour. IOW, based on a 25% duty you can estimate 2.25 A-h * 0.25 = 0.5625 amps per hour. Watts are directly related to current, so that same 2.25 A * 12V = 27 W-hr or 6.75W burned at 25% duty cycle in a hour.
So in these conditions I will burn 1.1 amp per hour. Which with my battery gives me 45 hours to 50% capacity.

So with the solar panel I am looking at being a 105w and if putting in 70% of it's capability being 70 watts even at 12v it's capable of doing 5.5amps and let's say in Minnesota I get 5 hours of good sun I can expect 25 amps into battery and that should be sufficient.
Check out the many excellent discussion about solar and average power harvested. A 105W panel will rarely deliver 105W and when it does it will be for a very short period. So 70% is probably a fine expectation for a couple of hours every day of full sun. You are better off estimating something quite a bit lower, perhaps 50% per 8 hour of exposure and figure in cloudy days where you might get very little or none.

My $0.02 is that ~100W extends your time but may not allow you to go indefinitely. My fridge in typical summer use consumes about 300 watts per 24 hour period and a 100W panel will collect about that over an average day. If you get good sun (I figure about 5 to 6 sun hours) you'll do it but throw in an overcast day and there's not enough capacity to recover.
 
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DLTooley

Observer
Multiple folks, including myself, have observed much lower power consumption under D.C. operation - as much as 40% - but not specifically a Norcold.

Actual operating temps will make a huge difference. 70 degrees Fahrenheit is a good average for either conditioned or unconditioned space.

I use mine as a freezer and use my cooler as the refrigerator.

A 12v cooler is a big power draw, enough to interfere with the crucial absorption phase of solar charging. Turning it off for a couple three hours mid morning will help extend your battery life if you are an extended user. (More than a weekend.)

Tracking your actual power charge and load is an important part of the learning curve.
 

TT-Tacoma

Observer
Ah the learning is occurring gents trust me. I spent a good amount of time yesterday researching a solar panel and have been debating which type of controller to use.

There is sooooooooo much information out there. As I had said earlier on I'm really looking forward to using my current setup and am seriously enjoying the research and build phase.

Sometimes I wonder what I like more, research and building or using the stuff!

Once I get my 12v power monitor I'm going to conduct my test again using my old battery I took out of the truck.

After that I am planning on putting it in-line to my fridge to monitor usage in the field.

I'm waiting hear back from the manufacturer on if their controller is ok to use with my AGM battery. If it is I may buy another monitor to keep track of solar input to the battery.

I wish my day job was building solar systems and testing them on vehicles. I really enjoy this stuff.

Sent from my Nexus 5X using Tapatalk
 

SoCal Tom

Explorer
Ok. More testing has been done overnight.

70 degree house, overnight down to 68
Empty fridge
Pre cooled
122 VAC
0.14 kw
10.5 hours
Knob set to #1
Thermostat inside reads 36 degrees

So if I'm doing the math correctly and rounding to the tenth I get this

0.14 kw equals 140 watts
140/10.5= 13.3 watts per hour
13.3/12= 1.1 amps per hour

So in these conditions I will burn 1.1 amp per hour. Which with my battery gives me 45 hours to 50% capacity.

So with the solar panel I am looking at being a 105w and if putting in 70% of it's capability being 70 watts even at 12v it's capable of doing 5.5amps and let's say in Minnesota I get 5 hours of good sun I can expect 25 amps into battery and that should be sufficient.

What do you guys think?

Sent from my Nexus 5X using Tapatalk
So here is my experience, I tracked the power in, and power out using a digital meter like this. https://smile.amazon.com/gp/product/B013PKYILS/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

Last trip, the first night we used 169 watt hours between 4:30 pm and about 6:30 am. Peak charge amperage, that I saw was 5.2 amps, with the charge voltage set to 14.8 and it got there about noon. When the sun went down 214 watt hours had gone to the battery. Once voltage got to 14.8, amperage dropped off significantly. Battery read 12.7+ volts once charging was stopped. Next night we used 245 watt hours overnight, voltage was still above 12.3 in the morning. Packed up and went home, so I didn't use the panels. ( used a lot of battery charging ipads etc the 2nd night)

I only have the one meter, so while the panels are charging, it doesn't read any discharge wattage ( which means the solar panels covered any power needed while the sun was out as well as charging the battery). This was with 2x50W solar panels ( home made suitcase) and a cheap PWM solar controller. I'm using a Marine/Deep cycle battery with 85Ahs.
 
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TT-Tacoma

Observer
So here is my experience, I tracked the power in, and power out using a digital meter like this. https://smile.amazon.com/gp/product/B013PKYILS/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

Last trip, the first night we used 169 watt hours between 4:30 pm and about 6:30 am. Peak charge amperage, that I saw was 5.2 amps, with the charge voltage set to 14.8 and it got there about noon. When the sun went down 214 watt hours had gone to the battery. Once voltage got to 14.8, amperage dropped off significantly. Battery read 12.7+ volts once charging was stopped. Next night we used 245 watt hours overnight, voltage was still above 12.3 in the morning. Packed up and went home, so I didn't use the panels. ( used a lot of battery charging ipads etc the 2nd night)

I only have the one meter, so while the panels are charging, it doesn't read any discharge wattage ( which means the solar panels covered any power needed while the sun was out as well as charging the battery). This was with 2x50W solar panels ( home made suitcase) and a cheap PWM solar controller. I'm using a Marine/Deep cycle battery with 85Ahs.
That is the same meter that I ordered. Once I get my hands on it I'll be able to run a battery test.

Question for ya. If you only have the one how did you know that the solar panel had put in the 214 watt hours if the meter was wired to monitor power out of the battery and not in? Oh and if so how do you have it wired up to do both?
 

SoCal Tom

Explorer
you can use a DPDT switch to flip the wires. When I'm charging I monitor the input, then when the panels are done, I flip the switch, push the reset button and it watches the discharge rate etc.
Tom
 

dwh

Tail-End Charlie
Redo the math at a variety of voltages, such as 12.8v (full charge resting), 13.6v (full charge floating on solar), etc.

At 12v, the battery is already down to around 50% capacity, so that's "worst case", not "normal operation".
 

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TT-Tacoma

Observer
you can use a DPDT switch to flip the wires. When I'm charging I monitor the input, then when the panels are done, I flip the switch, push the reset button and it watches the discharge rate etc.
Tom
Does this sound correct for your wiring then?

Solar -- one side of switch
Battery -- other side of switch
Meter -- constant terminals of switch

And I would assume both leads to switch from solar and battery are just leads off each? I was under the impression the meter had to be wired in-line from solar controller to battery and from battery to devices.


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SoCal Tom

Explorer
Does this sound correct for your wiring then?

Solar -- one side of switch
Battery -- other side of switch
Meter -- constant terminals of switch

And I would assume both leads to switch from solar and battery are just leads off each? I was under the impression the meter had to be wired in-line from solar controller to battery and from battery to devices.


Sent from my Nexus 5X using Tapatalk
The shunt is bidirectional, there are 4 wires that connect to the meter. Power, ground, north and south end of the shunt. You just wire the switch to flip the north /south wires


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