Fridge power consumption

[pessimist]

I have an old compressor driven fridge which works inasmuch as it keeps the wine and beer cold. I am, however, suspicious that the power consumption is high. To confirm this I'd like to monitor its consumption over a 24hr period. Would something like this do the trick? They claim a capacity fof 50amps continuous but then burble about radio control models. They appear to be connected in series with the "load". Go for it or will there just be lots of fizzing and popping?

 

[jwfrary]

Most stuff is made in China these days so not much point in shying away from it!

what I would say is to use an ammeter of known accuracy to check the thing is accurate first. depending on your requirements of accuracy of result. I would expect +-10-15% for 7 pounds!

make sure the appropriate fuses are in place that will protect you from the fizz bangs!

 

[Mistroma]

I have something similar to measure everything in & out of the battery. However, it has sensor over the very thick battery wires and also feeds data to an app via Bluetooth. The model you show is connected in-line and the wires don't look thick enough for 100A.

However, you are only talking about measuring a fridge and it will probably only pull 5-6A and won't be running all the time. I can't see any reason why it wouldn't be up to that job, assuming you connect it into the fridge supply. I wouldn't want to connect it directly to the main battery posts to monitor everything but should be fine for just the fridge circuit.

Another option would be to use a cheap hour meter wired to run just when the compressor runs. You'd need to measure the compressor current with the motor on or just accept the figures quoted on the compressor.

e.g. Leave it on for 24 hours and hours use reads 6 hours with a compressor rated at 6A. This would mean it is using 36Ah per day.

Obviously not the most accurate method but probably good enough. It would probably be the same cost as the device you mention and only worth it if you happen to have a spare one lying around.

 

[William_H]

All you need is an amp meter to check current drain when running. Then it is simply a question of time running versus time not running. This of course depends on ambient temperature and temperature set on thermostat. Actual consumption over 24 hrs is current drawn reduced by the on off ratio. Do don't need accurate results to know if you ahve a problem. Typically (hopefully) in mild conditions for a just fridge temps you will only be running about 30% of the time. If it is running full time you have problem of bad insulation or more likely a gas leak. I am not aware of more modern compressors being more efficient than older ones if operating correctly. ol'will

 

[vyv_cox]

All you need is an amp meter to check current drain when running. Then it is simply a question of time running versus time not running. This of course depends on ambient temperature and temperature set on thermostat. Actual consumption over 24 hrs is current drawn reduced by the on off ratio. Do don't need accurate results to know if you ahve a problem. Typically (hopefully) in mild conditions for a just fridge temps you will only be running about 30% of the time. If it is running full time you have problem of bad insulation or more likely a gas leak. I am not aware of more modern compressors being more efficient than older ones if operating correctly. ol'will

Yes, that's what I do. Every few weeks I sit in the saloon watching the ammeter and timing the fridge on and off with a stopwatch. In normal use the fridge plus its little seawater pump draws 4 amps at startup, gradually reducing to just over 3 amps before it cycles off again. The period on is almost always about one third of the total, with cooling seawater temperature 25 - 28C. Interestingly though, the actual cycle time varies quite a lot, sometimes only about 5 minutes, at other times 15. I have no idea why this is as it does not seem to have anything to do with conditions.

 

[Neeves]

If your compressor is in an enclosed space, a locker with no ventilation - then it will be inefficient (not because it is old but because it only has warm air,, from itself - the compressor - and cooling something with warm air is really not very effective).

Before trying to establish that the system is good, or not, give it half a chance to start with.

It must be cold enuff in dartmuff currently so make sure that cold air is available to the cooling fan - and then test. If you end up being 'happy' then consider ways of encouraging a flow of fresh air to the cooling fan (and maybe adding more insulation to the fridge box itself (if its old the insulation might be a bit malnourished, - lots of theads on fridge cooling and insulation).

Jonathan

 

[pessimist]

Thanks everyone. It's not an option to increase insulation - it's a built in front opening cabinet. The compressor appears to be well ventilated. I'm just trying to get an empirical measure of total consumption over a 24hr period. I can't stay on board the boat at the moment so the plan was to connect the device and leave it for 24/48 hours and check the consumption. I suspect I need to upgrade both batteries and solar to be able to run the fridge 24/7 but I'd like to know what I'm aiming at.

 

[Zing]

You could something this and measure amps used and hours running:
DC12V-36V Generator Sealed Hour Meter Counter For Boats Trucks Tractors Car s | eBay

Boat fridges are typically very inefficient compared to good domestic equipment. If your unit is tested as working ok and efficiency is your goal, either massively insulate a custom installation, so you hardly need your inefficient cooling or find a way to install a good domestic model.

 

[Mistroma]

Yes, that's what I do. Every few weeks I sit in the saloon watching the ammeter and timing the fridge on and off with a stopwatch. In normal use the fridge plus its little seawater pump draws 4 amps at startup, gradually reducing to just over 3 amps before it cycles off again. The period on is almost always about one third of the total, with cooling seawater temperature 25 - 28C. Interestingly though, the actual cycle time varies quite a lot, sometimes only about 5 minutes, at other times 15. I have no idea why this is as it does not seem to have anything to do with conditions.

I used to do that as well, easy to do but a bit boring. The cheap eBay device I fitted has Bluetooth and I sometimes look at the Android app with a graph showing current before going to sleep. It's easy to see the point when the compressor turns and off on the timeline. Nothing else running and the current change is all related to the fridge/freezer.

 

[simonfraser]

My victor shunt shows 6A draw when on
outside temp 12C
at the mo it’s on 50% of the time
so average amps 3 I’d say
more during the summer when there is more solar

 

[PaulRainbow]

Boat fridges are typically very inefficient compared to good domestic equipment. If your unit is tested as working ok and efficiency is your goal, either massively insulate a custom installation, so you hardly need your inefficient cooling or find a way to install a good domestic model.

Any evidence to support these claims ?

Why would a cabinet fridge in a hot kitchen be more efficient than a cabinet fridge on a boat, where there is access to better airflow ?

 

[PaulRainbow]

I have an old compressor driven fridge which works inasmuch as it keeps the wine and beer cold. I am, however, suspicious that the power consumption is high. To confirm this I'd like to monitor its consumption over a 24hr period. Would something like this do the trick? They claim a capacity fof 50amps continuous but then burble about radio control models. They appear to be connected in series with the "load". Go for it or will there just be lots of fizzing and popping?

Why not fit a battery monitor, this well tell you the current draw of the fridges and all sorts of other useful stuff. A cheap hour metre, left on in parallel with the fridge will tell you how long is runs for in a 24/48hr period. Leave the hour metre connected, wired to a switch, and you can check it cycling ay time you want. Handy to see how it differs in Summer/Winter.

What battery capacity and solar do you have ?

 

[CliveF]

I use one of the type you mention on my RC aircraft to match the current draw from different prop sizes to the capacity of the speed controller for the very reason of stopping magic smoke syndrome.
I find that it works very well. I think as long as you MAKE SURE YOU HAVE VERY GOOD CONECTIONS , and that they can not short out on anything else, then it should do what you want effectively and with little risk of anything going wrong unattended.

Its only £8 and you can always sell it on again.

Regards Clive

 

[Zing]

Any evidence to support these claims ?

Why would a cabinet fridge in a hot kitchen be more efficient than a cabinet fridge on a boat, where there is access to better airflow ?

Sure. A typical medium to large sized boat fridge will have a 50w compressor, run 2/3 of the time and a similar sized 3* or 4* rated fridge will have a rated consumption at about 100Kwh p.a.. That works back to a fraction of the marine fridge power consumption on a similar duty cycle - 7W. They work hard at maximising insulation and minimising refrigerant temperature differentials, which is how they achieve higher efficiency. I have tested the figures on my own boat and domestic equipment and they are correct.

 

[PaulRainbow]

Sure. A typical medium to large sized boat fridge will have a 50w compressor, run 2/3 of the time and a similar sized 3* or 4* rated fridge will have a rated consumption at about 100Kwh p.a.. That works back to a fraction of the marine fridge power consumption on a similar duty cycle - 7W. They work hard at maximising insulation and minimising refrigerant temperature differentials, which is how they achieve higher efficiency. I have tested the figures on my own boat and domestic equipment and they are correct.

Your figures are distorted. if the fridge on your boat runs 2/3 of the time, it's a poor installation, mine runs for about 1/3 of the time. A modern 12v cabinet fridge on a boat is rated at about 100kwh pa, that's without adding additional insulation or cooling fans. A domestic fridge will use much more than 100kwh, unless it's really tiny. It's practically impossible to add insulation to a domestic fridge and nigh-on impossible to add cooling fans, because it's sitting in a centrally heated room, typically stuck under a worktop.

After you have installed your "good domestic model", how will you power it, as it's going to be 240 VAC ?

 

[Zing]

Your figures are distorted. if the fridge on your boat runs 2/3 of the time, it's a poor installation, mine runs for about 1/3 of the time. A modern 12v cabinet fridge on a boat is rated at about 100kwh pa, that's without adding additional insulation or cooling fans. A domestic fridge will use much more than 100kwh, unless it's really tiny. It's practically impossible to add insulation to a domestic fridge and nigh-on impossible to add cooling fans, because it's sitting in a centrally heated room, typically stuck under a worktop.

After you have installed your "good domestic model", how will you power it, as it's going to be 240 VAC ?

I don't agree. The figures are not distorted and I have quite a good fridge installation as far as marine fridges go. A fridge should be designed to not cycle frequently as cycling is a cause of considerable efficiency losses, so to run longer at lower pump speeds is the optimum situation. I would suggest a fridge running at 1/3 duty cycle is incorrectly set up and needs a lower pump speed. You may also be comparing with an unheated boat in the UK, but which is set up to cope with warmer conditions, so that would be not optimally set up and 'distorted'.

With my figures, I compared like with like on fridge size. I suspect your comparisons are not.

A domestic fridge is 230VAC not 240VAC in the UK.

I would use a decent inverter (92% efficient). What else were you thinking of?

Here's another data point for you, a marine fridge:
Waeco MDC65 Caravan Motorhome Fridge
A domestic fridge, 50% larger in capacity:
UR4584S | Silver | Beko Refrigerator | ao.com

The marine fridge uses 45W average, the domestic unit works out at 15W average and it's 50% bigger.

 

[PaulRainbow]

I don't agree. The figures are not distorted and I have quite a good fridge installation as far as marine fridges go. A fridge should be designed to not cycle frequently as cycling is a cause of considerable efficiency losses, so to run longer at lower pump speeds is the optimum situation. I would suggest a fridge running at 1/3 duty cycle is incorrectly set up and needs a lower pump speed. You may also be comparing with an unheated boat in the UK, but which is set up to cope with warmer conditions, so that would be not optimally set up and 'distorted'.

Make your mind up. Lower pump speed ? What are you talking about ? It's a standard Danfoss DC compressor.

With my figures, I compared like with like on fridge size. I suspect your comparisons are not.

A domestic fridge is 230VAC not 240VAC in the UK.

Don't be a pedant.

I would use a decent inverter (92% efficient). What else were you thinking of?

I was thinking of not throwing 10% of my fridge power away on a 240V AC fridge.

Here's another data point for you, a marine fridge:
Waeco MDC65 Caravan Motorhome Fridge
A domestic fridge, 50% larger in capacity:
UR4584S | Silver | Beko Refrigerator | ao.com

The marine fridge uses 45W average, the domestic unit works out at 15W average and it's 50% bigger.

The domestic fridge is using 15w average, if it's on a 1/3 duty cycle that's 50w all of the time it's running. 139 kwh for the year.

The marine fridge isn't going to be using 45w continuously, or it would be using 394 kwh pa, totally ridiculous.

As i said, distorted figures.

Try this marine fridge : Dometic CoolMatic CRX 65S - Compressor refrigerator, 57 l, stainless

109 kwh

 

[Mistroma]

I don't agree. The figures are not distorted and I have quite a good fridge installation as far as marine fridges go. A fridge should be designed to not cycle frequently as cycling is a cause of considerable efficiency losses, so to run longer at lower pump speeds is the optimum situation. I would suggest a fridge running at 1/3 duty cycle is incorrectly set up and needs a lower pump speed. You may also be comparing with an unheated boat in the UK, but which is set up to cope with warmer conditions, so that would be not optimally set up and 'distorted'.

Boats systems are set to work over a wide range of conditions as you mention. My system is set to run at about 1/3 duty cycle for much of the year. However, it gets up to about 80%-100% during the day when air temp hits 40C. Fortunately, that doesn't happen too often and is is more often in the 60%-80% range for temperatures in mid-high 30s. Around 30% is the normal duty cycle for most of the time (cooler days and overnight).

I could adjust the compressor speed to improve efficiency but it is a lot easier to accept that it works well over a wide range of conditions. The speed adjustment option is hidden away in the documentation and many people don't know about it. I don't think all models allow speed adjustment.

Happy days in Scotland when it would run about 25% of the time or I could just leave stuff in the cockpit overnight and put it into the fridge during the day to warm up a bit.

I suppose I could make up a board with a few resistors and a multi-way switch to change the speed.

Update:
I found my crib sheet for newer versions of Danfoss BD35 and BD50 models shows:
2,000 rpm with no resistor in the thermostat circuit
2,500 rpm with 227 ohm
3,000 rpm with 693 ohm
3,500 rpm with 1.5 kohm

 

[dslittle]

Yes, that's what I do. Every few weeks I sit in the saloon watching the ammeter and timing the fridge on and off with a stopwatch. In normal use the fridge plus its little seawater pump draws 4 amps at startup, gradually reducing to just over 3 amps before it cycles off again. The period on is almost always about one third of the total, with cooling seawater temperature 25 - 28C. Interestingly though, the actual cycle time varies quite a lot, sometimes only about 5 minutes, at other times 15. I have no idea why this is as it does not seem to have anything to do with conditions.

Me too. There’s nothing quite as satisfying as an hour or so with the lap timer on the iPad monitoring the on/off cycle. Perhaps I need to get out more...
Coincidentally, my measurements seem very similar to yours.
As Mistroma states, it was a bit different on the West Coast of Scotland, we hardly needed the fridge to cool things at all - ah happy days!!!

 

[Zing]

Make your mind up. Lower pump speed ? What are you talking about ? It's a standard Danfoss DC compressor.

There is no changing of mind. I don't know what you are on about. On most boat systems you can turn down the pump speeds to reduce cycling energy wastage and I am suggesting you could improve things if you did that.

I was thinking of not throwing 10% of my fridge power away on a 240V AC fridge.

Yet you would rather use a DC compressor, which then converts power internally to AC to run the motor? You are going to waste power that way anyway.

The domestic fridge is using 15w average, if it's on a 1/3 duty cycle that's 50w all of the time it's running. 139 kwh for the year.

Yes that's what I said also and is what the tech spec says and that's a lot less than the marine fridge. This is my point.

The marine fridge isn't going to be using 45w continuously, or it would be using 394 kwh pa, totally ridiculous.

They give it as the average consumption. (It's a similar figure to your example below from Dometic fridge manual). Even at 50% duty cycle it would be a huge consumption for such a little fridge.

As i said, distorted figures.

Nope. I've also checked things in the real world too and they are worse than for these examples.

Try this marine fridge : Dometic CoolMatic CRX 65S - Compressor refrigerator, 57 l, stainless

109 kwh

It ought to be better than the component built marine fridges, but it's still not very good. That fridge is nearly half the size of the Beko but 20% less power, but I doubt the power quoted is correct as it works out at a 27% duty cycle. I suspect these off the shelf fridges don't quote the EC standard test results.