Modern AC fridges much more energy efficient than modern D/C fridges

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I am trying to work out whether to replace my oldish energy inefficient dc fridge, with small freezer compartment, with an energy efficient normal household fridge running through a 92% efficient sine wave inverter (which my boat already has).

The internet shows that a modern DC boat fridge uses an average of 1200 watts per day at 32 degrees. A modern AC fridge of similar size uses 0.21 kwhrs at 32 degress.
In the shops the best figure I have seen is 126 kilowatts per year for household ac fridges(with no temperature specs).
So.. by looking at these figures and taking into account the additional energy consumed by the inverter it still looks like the ac fridge is the way to go.
I just wondered what others think?
 
Just a small question.

0.21 kwhrs for even only ten hours running per day is 2100 watts and factoring in your (very efficient) figures for your invertor increases this to 2286 watts .
That sounds a bit higher than those you have quoted for a DC fridge to me - or have I missed something?

Cheers

Iain
 
Seems you have miscalculated a wee bit!! But of course how big is this AC fridge?
I use an AC fridge freezer, instead of 24volt ones, and my energy usage is very slightly higher than before, but I dont really care, the fridge is twice the capoacity of the "marine" fridge and a quarter of the price, so I'll stay with AC. By the way my new victron sine wave inverter is quoted at 96% efficiency.
 
Sorry I think you need to sort out the units.

Watts or kilowatts is the instantaneous power consumption. You cannot say "watts per day" although you can quote an average power consumption during a 24 hour period

A watt hour, or kilowatt hour, is the total quantity of energy consumed in any given period. So the time period must be specified, it could be kilowatt hours per day.

I take it that you are saying that:

<ul type="square">[*] the average power consumption of a modern DC fridge is 1200watts.
which works out at 24x 1200 = 28800 watt hours per day or 28.8 kilowatt hours per day

[*]A modern AC fridge has an average power consumption of 0.21 kilowatts which is 0.21 x 24 = 5.04kilowatt hours per day at 32° (by 32 degrees presumably you mean an ambient temperature is 32C or do you mean a cabinet temperature of 32F)

[*]In the shops you see, for household fridges, 126 kilowatts per year. WTF? 126 kilowatts average? Hardly every estate will need its own power station to supply that much. 126 kilowatt hours per year? would not think so, that's only 345 watt hours per day or an average power consumption of 14.3 watts. That would indeed be a very lower power. If only. Perhaps it means an average power of 126 watts which would be 126 x 24 = 3024 watt hours ( or 3.024 kilowatt hours) per day that's pretty low but realistic. [/list]

So I think you should be comparing

<ul type="square">[*]1200 watts or 28.8 kilowatt hours per day, for a DC fridge

[*]210 watts or 5.04 kilowatt hours per day, for an AC fridge

and

[*] 126 watts or 3.024 kilowatt hours per day, for a household fridge. [/list]


Concluding that while a AC fridge uses nearly twice as much power as domestic fridge a DC one uses nearly 10 times as much!

So I would agree even if your inverter is only 85% efficient an AC fridge is a much better bet than a DC one. ( 210 watts on an 85 % efficient inverter works out at 210/0.85 = 247 watts battery power consumption.

You must realise that these figures are averages over 24 hours. When actually on and running the instantaneous power consumption is much higher and you must ensure that your battery and inverter can supply that.

Note also that 247 watts works out at just over 20amps from a 12 volt battery thus requiring 24x 20 = 480 Ah of battery capacity per day. You will need the charging capability to match and exceed this. Based on 8 hours useful sunshine per day that's about 750 watts of solar panels.

If I have still misinterpreted your figures then my entire post is garbage /forums/images/graemlins/frown.gif
 
There's a mish-mash of units on this thread (and some dodgy calculations) so that it's difficult to figure out anything.

So, some basics...

A watt is a rate of energy, i.e. how much energy is used per period of time. So saying 1200 Watts per day or 126 kW per year doesn't help much. The Watt uint has time in it already. (If you used 126 kW at home all year constantly, you leccy bill would be over £100k per year).

A WattHour or kiloWattHour is a measure of a lump of electricity. On your domestic bill, you pay so much pence per kWh. 1 kWh is commonly referred to as a unit on domestic bills. So saying 0.21 kWh without a time frame over which that electricity is consumed also doesn't help much.

But, making some assumptions...

1) The DC fridge uses 1.2 kWh per day
2) The AC fridge uses 126 kWh per year, which is 0.345 kWh per day

So the AC fridge uses a lot less, even if running through an 80% efficient inverter...

0.345/0.8

0.431 kWh per day.

Edit...

Doh! Didn't get mine in quick enough. Looks like I got Vic'd to the post!
 
[ QUOTE ]
and some dodgy calculations

[/ QUOTE ] Of mine? Please pinpoint and I will correct.
 
Re: Modern AC fridges much more energy efficient than modern D/C fridg

Have you also factored in or do your wattage figures take account of how much your fridge is actually working?
For example I have a 12V 30 litre Waeco Fridge/Freezer rated at 55-60W and the specs say that at 32 degrees C ambient temperature outside it runs 30% of the time in Fridge mode, and the energy used is the rated figure multilped by the percentage running.
So for example my fridge's energy consumption in one day is 60Watts*24hours*0.3= 432 Wh or 0.432kWh

edit - changed some references to power to energy
 
[ QUOTE ]
Of mine? Please pinpoint and I will correct.

[/ QUOTE ]

No, not yours. I meant the calculation by iainsimpson because your post didn't exist when I started writing. We seem to have written pretty much the same thing, although we made slightly different guesses about what the OP's figures actually meant.

Regarding consumption of domestic AC fridges, I've not found one rated as low as 126kWh per year, but here's one not far above at 150...

Low consumption AC fridge
 
The key to who's mathematics is correct is how to interpret this phrase:
[ QUOTE ]
... DC boat fridge uses an average of 1200 watts per day ...

[/ QUOTE ]
If he means an average consumption of 1200 watts, then that's one inefficient fridge! Calculates as 28.8kWh each day (approx 29 "units", if you like), as has been shown.

If however he really means 1200 watthour, or 1.2kWh, then that's better (1.2 "units"). But still not as good as the quoted figures for a "domestic" fridge, even when you take into account the losses in the inverter.

The difference between kWh and kW have, I thought, been well explained elsewhere.
 
[ QUOTE ]
No, not yours

[/ QUOTE ] Thank goodness for that! It is very easy to make silly mistakes in posts though.

I think some people were away from school the day the rest of the class did science!
 
Ok, Mr Chemist. Here's another O' Level type question I just know you'll love...

It seems like the lowest consumption AC mains domestic fridge I can find uses 150 kWh per year.

My gas powered fridge I have in my house with no electricity uses a 4.5kg bottle of Calor butane in 30 days.

Compare and contrast the average power consumption of the 2 fridges /forums/images/graemlins/smile.gif



Edit

WikiClicky

I reckon it's about 90W. Not very efficient compared to the mains one at 17W!
 
According to calor:-
Butane = 7.8 kWH/Litre (of liquid)

There's apparently* 1.74 litres per kilo.
So, in the bottle you have 13.572KWh

30 Days is 720 hours, so your gas fridge is running at 18.85 Watt hours.

So they are similar, however gas is a very cheap source of energy.

*Not calor, some enviro site suggested that so could be wrong.
Calor give energy by volume but sell by weight!?!?
 
Vic's calculation is based on the quoted 1200 watts consumption of the DC fridge. However if the 1200 watts quoted was indeed the instantaneous consumption of this fridge as he suggests, at 12 volts it would be drawing 100 amps. I know boat fridges are heavy on the batteries, but surely never 100 amps?
This brings into question Vic's conclusions.
 
Having just checked on the Waeco sit a 40 cu ft fridge box is rated at 45 watts and runs for 22% of the time at 32 deg, pretty competitve in power consumption.
 
Sorry for the confusion, all figures are daily consumption in watthrs and kwhrs , to avoid confusion I will change it all to kwhrs.....so it should read like this....

The internet shows that a modern DC boat fridge uses an average of 1.2 kwhrs per day at 32 degrees. A modern AC fridge of similar size uses 0.21 kw/hrs at 32 degrees.
In the shops the best figure I have seen is 126 kilowatthours per year (kwhy) for household ac fridges(with no temperature specs). which calculates to 126/365 = 0.345 kw/hrs

My old fridge has the following calculation....it uses 2.5 amphr at 24volts which is 60 watthr, at 32 degrees C ambient temperature outside it runs 80% of the time if on the lowest setting otherwise it runs 100% of the time, and the energy used is the rated figure multilped by the percentage running.
So for example my fridge's energy consumption in one day is 60Watts*24hours*0.8= 1152 Wh or 1.152kWh
 
But that one is apples and pairs. A gas fridge uses amonia absorption process, which is fundamanetally very inefficient, while the mains ones uses a heat pump which is generally very efficient. Most of the energy of the gas frdge goes straight out the flue.
 
Think you might have missed out the 4.5 kg per bottle in your calcs...

1.74 litres per kilo, but I have 4.5 kilo, so 1.74*4.5 litres
=7.83 litres

So a bottle contains 7.8 * 7.83 kwH per bottle
=61.1 kWh per bottle

Divide by 720 hours gives 0.084 kW, or 84 Watts, which is near what I calculated.

Bit of Fred Drift here, but all good unit fun!
 
[ QUOTE ]
Having just checked on the Waeco sit a 40 cu ft fridge box is rated at 45 watts and runs for 22% of the time at 32 deg, pretty competitve in power consumption.

[/ QUOTE ]

I agree. I have one of their tiny 18 litre compressor coolboxes. I measured its consumption, and it uses 10 Ah per day at 12V. I don't have an inboard engine, so Ah are important to me. Outboard and solar panel only.
 
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