Domestic fridge

[stephen_h]

I am looking at using a domestic fridge/freezer and running it off an 12v inverter.
There is a lot more choice and a lot more cost effective!

Just want to check my thinking is correct -

As an example a fridge I saw in Currys shows an energy use of 207 KWh/annum.

So if I allow for an extra 10% for inverter loss that is 221 KWh x 83.33 (google! ) = 18416 amps /356 days = 52 amps per 24 hours therefore just over 2 Ah.

This is about the same as I have seen for 12v marine fridges.

Am I am missing anything??

 

[Telstar26]

I'm not sure what amps per hour measures, given that amps measures the current.

And don't forget to let it stand for 4 hours before switching on after it's been moved, to allow the refrigerant to settle. So it'll be fine as long as you don't leave the marina (in which case you won't need the inverter!)

 

[VicS]

I am looking at using a domestic fridge/freezer and running it off an 12v inverter.
There is a lot more choice and a lot more cost effective!

Just want to check my thinking is correct -

As an example a fridge I saw in Currys shows an energy use of 207 KWh/annum.

So if I allow for an extra 10% for inverter loss that is 221 KWh x 83.33 (google! ) = 18416 amps /356 days = 52 amps per 24 hours therefore just over 2 Ah.

This is about the same as I have seen for 12v marine fridges.

Am I am missing anything??

Your arithmetic is correct but you are in a muddle with your units and there are 365 days in a year

Correct answer is 2.1 Ah per hour. or an averaged current consumption of 2.1 amps

You should also check the actual power consumption ( in watts ) while running and calculate the actual current ( in amps) at 12 volts while running.

 

[stephen_h]

I'm not sure what amps per hour measures, given that amps measures the current.

And don't forget to let it stand for 4 hours before switching on after it's been moved, to allow the refrigerant to settle. So it'll be fine as long as you don't leave the marina (in which case you won't need the inverter!)

Amended .
It's on a catamaran so not heeling as a mono would.

 

[AngusMcDoon]

Am I am missing anything??

221 kWh per year is 0.61 kWh per day. A kWh is 3,600 kJ so 0.61 kWh is 2178 kJ. Over 24 hours the energy flow rate (i.e. power) is 25 J/s (i.e. Watts). You need 2.1 A at 12 V for that power.

 

[luke collins]

You will get some losses via the inverter so it will use more power. Also consider the door as it may come open so need a catch or child lock. Ensure the inverter is big enough to cope with the inrush of the compressor as they pull around 4 x power on start up than plated as have a start winding in the compressor.

Should work fine and most compressors ok for around 20 degree tilt. For the price if you have the space you don’t have much to loose.

 

[harvey38]

No lock on ours and it's never come open in some pretty heavy seas but probably worth fitting some kind of lock, better than finding the galley awash with milk?

 

[pandos]

I considered this for use until I found a small unit on Gumtree, but I am now considering doing so for a camper conversion.

It may be of interest to OP that there are many conversions of mains fridges to 12v on YouTube which seem to involve swapping out the compressor for a 12v unit...

Also some reference to a switching arrangements which turns on the inverter only when the compressor requires power.

 

[luke collins]

Good point ref turn on the inverter when required. May be easier to make the stat switch a 30 amp relay to turn on/off the inverter a child catch May work on the door. Nice simple solution.

 

[Rappey]

Am I am missing anything??

Your missing a very important point which VicS has pointed out.
Your appliance, let's say it is 2.1 amps

Volts x Amps = Watts. 2.1a x 240v =504w
You want to run a 504w appliance from 12v . watts ÷ volts = amps. 504w ÷12v= 42 amps (at 12v)

 

[lustyd]

Check the electric safety. A 12v fridge can’t kill you if it’s wet. A friend had this setup and it zapped you if you touched it wrong (fridges are often metal!). Earthing it will probably make it safe and the fridge itself is probably a better fridge

 

[luke collins]

Your missing a very important point.
Your appliance, let's say it is 2.1 amps

Volts x Amps = Watts. 2.1a x 240v =504w
You want to run a 504w appliance from 12v . watts ÷ volts = amps. 504w ÷12v= 42 amps (at 12v)

A domestic fridge does not pull 2.1 amps at 240v. That’s at 12v. They run at around 75 watts at 240v.

 

[superheat6k]

I think your 10% loss through the inverter is a tad optimistic. I would allow 25% losses, until you can show otherwise. i.e. don't undersize the inverter.

Also the 207 kWH per annum is an average energy consumption, and no way should it be used to assess the power consumption. The inverter must be sized to allow the start up and then continuous run current without any appreciable voltage drop on the output from the Inverter. So I would suggest allow for an Inverter continuous output at least 25% above the continuous required current consumption when the compressor is on. I had a look at Curry's range and none state the maximum run current nor kW power drawn (this would be marked on thee rear of the appliance), so without knowing this it is simply a guess to size an inverter, and I certainly would not attempt to do this based upon an energy per annum statement. Also be careful to check the inverter output will cope with an inductive (i.e. motor) load, and perhaps allow a further 10% when comparing an inverter's capability to run non inductive loads, e.g. lights or heaters.

But your idea is sound.. Don't underestimate your battery reserve capacity required, bearing in mind only ~40 - 50% of a batteries capacity is usable in practice.

 

[superheat6k]

A domestic fridge does not pull 2.1 amps at 240v. That’s at 12v. They run at around 75 watts at 240v.

I think your wattage is optimistic, but ...

At 75W + 25% inverter losses + 10% for motor rating = ~100W which at 12v will be ~ 8.5 amps into the inverter.

Assuming the compressor will run 50% of the time 8.5 x 24 x 0.5 = 102 AH. The battery serving this would need to be at least this capacity, and assumes you have a charger supply to match.

Fridges are considered low energy devices at home on the 240vac supply, but are generally the heaviest base load on a boat's 12vdc supply. Do not underestimate the draw down on your batteries.

 

[Rappey]

A domestic fridge does not pull 2.1 amps at 240v. That’s at 12v. They run at around 75 watts at 240v.

I did think it a little extreme at 12v but my post was to demonstrate the difference in current draw between 12 and 240v.
It's still 6.5 amps at what seems a rather low 75 watts .

 

[William_H]

As said estimation of a domestic fridge power consumption would be in ideal conditions inside a house. So not much use to you. As said it is actual power consumption when running that matters. 75w perhaps for a small fridge. More for a big fridge. Yes you need a pure sine wave inverter with added capacity for start up. Incidentally Danfoss compressors run on AC and have an inverter built in to controller though probably more efficient than a large 240vAC inverter. ol'will

 

[vyv_cox]

For info. My fridge with a Danfoss 35 compressor consumes 4 amps at start of cycle and 3 amps at end. 0.5 amps of this is the water pump. It runs for almost exactly 1/3 of the time.

 

[Boater Sam]

You need an inverter of at very least 1000w preferably 1500w to reliably start a 240v domestic fridge.
Think about using the thermostat to switch the inverter on only when required to eliminate the quiescent inverter current.
I reckon on 50Ah/day. The Danfoss compressor is not simply 12v, the electronics sequentially switch windings, its a AC motor not a DC brush motor.

 

[Mister E]

A company called Inlader12vfridges convert domestic compressor fridges to 12 volt.
Not the cheapest but they mainly sell to the Narrowboat market.

 

[stephen_h]

Thanks for all the replys and great information .

I am looking to get a 2000w inverter even if not for the fridge but the Currys fridge is only £165 as apposed to £700 for a 12v marine fridge.
I am sure with a bit of looking I can find a more efficient domestic fridge for similar money so not a lot to loose if it doesn't work .
I have 320 amps of battery with 600 watts of solar but looking to increase both in the near future.