New Refrigerator Decision

[Meandering]

I am in the process of deciding which new refrigerator to buy for my boat. Having read threads about the advantages of 240v domestic refrigerators over 12v e.g. cheaper to purchase, use less power due to better insulation etc. I thought I would investigate, however, while I found A and A ++ rated refrigerators they express the annual power consumption in x KW/Annum. Could anyone tell me the calculations to convert KW/Annum to Ah/Day please (I intend to use 365 days as this is for a liveaboard). Your help and comments would be appreciated.

 

By the way there was a surprisingly big range of power consumption between the A and A++ rated refrigerators

[Keeping Up]

I think you will find almost universal agreement on here that 12v fridges generally use less power than 240v ones, by between 10% and 30%. When comparing specifications for 12v fridges in particular be certain that you are reading the average daily consumption rather than the peak current.

 

The consumption should be quoted in kiloWatt-hours per Annum (kWh/Annum). The conversion is quite easy; an average current of 1 Amp will correspond to 24 Amp-hours per day. On a 12v system, this same 1 Amp is 12 watts, so in one 24-hour day that will be 24x12 = 288 watt-hours. As there are 1000 watts-hours in one kilowatt-hour, that corresponds to 0.288 kWh/day. In a year that is 0.288x365 = 105 kWh/year, excluding leap years of course.

[blackrose]

If power consumption is your concern then have you considered a gas fridge?

[Rebotco]

I think you will find almost universal agreement on here that 12v fridges generally use less power than 240v ones, by between 10% and 30%. When comparing specifications for 12v fridges in particular be certain that you are reading the average daily consumption rather than the peak current.

 

The consumption should be quoted in kiloWatt-hours per Annum (kWh/Annum). The conversion is quite easy; an average current of 1 Amp will correspond to 24 Amp-hours per day. On a 12v system, this same 1 Amp is 12 watts, so in one 24-hour day that will be 24x12 = 288 watt-hours. As there are 1000 watts-hours in one kilowatt-hour, that corresponds to 0.288 kWh/day. In a year that is 0.288x365 = 105 kWh/year, excluding leap years of course.

 

 

 

I assume you mean average running amps?

Except a fridge does not run for 24 hrs a day. Say 50% of that, so half the stated consumption per year.

[Chalky]

The power consumption figures are based upon a standardised test regime that's supposed to represent real world usage (like the fuel consumption figures for cars). In practice every one uses their fridge differently so the only guarantee is they'll be wrong and you'll get something different. Where they are useful is comparing relative performance. I agree with the A & A++ comparison, but have a look at some others as well. I found a B rated fridge that had lower figures than an A rated one. The sales rep couldn't explain that one.

 

Which ever you choose will always use more than you expect - the choice depending on your usage profile as much as cost. We have the 240V option and turn it off at night. If we go on a cruising type of holiday it works well. If we stay in one place for a few days then we can get load balance issues.

[Meandering]

Thank you Allan for the quick reply. Building on your example a 98ltr Shoreline refrigerator is quoted at an average consumption of 1.42 amps/hour so the average annual consumption by the refrigerator would be 149 kWh/year. This does appear comparable to high compared to some of the high quality 240v refrigerators at considerably less initial cost.

[jelunga]

 

 

 

I assume you mean average running amps?

Except a fridge does not run for 24 hrs a day. Say 50% of that, so half the stated consumption per year.

 

 

Not if the quoted figure is AVERAGE comsumption. That takes into account periods where the compressor is runnng and when it is not.

[Meandering]

The power consumption figures are based upon a standardised test regime that's supposed to represent real world usage (like the fuel consumption figures for cars). In practice every one uses their fridge differently so the only guarantee is they'll be wrong and you'll get something different. Where they are useful is comparing relative performance. I agree with the A & A++ comparison, but have a look at some others as well. I found a B rated fridge that had lower figures than an A rated one. The sales rep couldn't explain that one.

 

Which ever you choose will always use more than you expect - the choice depending on your usage profile as much as cost. We have the 240V option and turn it off at night. If we go on a cruising type of holiday it works well. If we stay in one place for a few days then we can get load balance issues.

 

Chalky, could you explain "load balance issues" please?

[jelunga]

Thank you Allan for the quick reply. Building on your example a 98ltr Shoreline refrigerator is quoted at an average consumption of 1.42 amps/hour so the average annual consumption by the refrigerator would be 149 kWh/year. This does appear comparable to high compared to some of the high quality 240v refrigerators at considerably less initial cost.

 

 

But then it depends on where your 230v power is coming from. If from a landline at 9p/unit than yes it is cheaper to get a mains fridge.

 

 

If it is from a diesel propulsion engine alternator producing 12v that you store in batteries and then convert to 230v then no it is probably not cheaper since there are losses in converting from 12v to 230v and in the battery system.If you are running 12v then when the engine is running power with be drawn straight from the alternator, in practise.

 

Tell us where you will be getting power from and then we can be more specific about the best fridge.

[arpeeuk]

Thank you Allan for the quick reply. Building on your example a 98ltr Shoreline refrigerator is quoted at an average consumption of 1.42 amps/hour so the average annual consumption by the refrigerator would be 149 kWh/year. This does appear comparable to high compared to some of the high quality 240v refrigerators at considerably less initial cost.

 

Hi I think you may be looking at the wrong figures on the shoreline web site here. It's a very messy layout but I think it says that it uses an average of .95 amps/hour at 12v and the number you quote 1.42 amps is the rated current at 24v.

 

Also, don't forget, unless you are have a mains hookup, to take into account the additional power required to run your inverter.

 

Hope this helps

 

Rik

[Keeping Up]

If you are getting your 240v from an inverter there are a couple of further considerations.

 

Firstly the inverter will normally be only about 90% efficient while it is running, so add 10% to the effective consumption of the 240v fridge.

 

Secondly the inverter will still be consuming its standby power (maybe 1 amp) even when the compressor is not running. This can easily add 50% to the total energy consumed from the batteries. There is a work-round for this, using the fridge 'stat to turn the inverter on and off, but it can get complicated.

 

These factors don't apply if the 240v is from a shore line of course.

[Meandering]

But then it depends on where your 230v power is coming from. If from a landline at 9p/unit than yes it is cheaper to get a mains fridge.

 

 

If it is from a diesel propulsion engine alternator producing 12v that you store in batteries and then convert to 230v then no it is probably not cheaper since there are losses in converting from 12v to 230v and in the battery system.If you are running 12v then when the engine is running power with be drawn straight from the alternator, in practise.

 

Tell us where you will be getting power from and then we can be more specific about the best fridge.

 

Jelunga, the power would come from the domestic batteries. The lower cost I was referring to was the cost to purchase the fridges initially.

[Chalky]

Chalky, could you explain "load balance issues" please?

 

Electrical engineering term used in the automotive industry.

 

A positive load balance means that the over a usage cycle there has been more power power put into the batteries than taken out. A negative one means that you've taken more power out of the batteries than you've put in. The aim for a well designed electrical system is to stay in a positive or at worst neutral balance. In automotive design its common to start load managing the system if the load balance is negative this becoming more aggressive as the battery SOC (state of charge) starts to get worse. In the worst case the system will theoretically disconnect every load except the engine management, the starter motor and if required the glow plugs.

 

With our electrical system cruising generates a very positive load balance - I can produce far more power than I'm using. The system was designed to cruise for a day and then be able to stand for at least one with no additional charging required. In practice I can get 2 days standing before I need to run the engine and charge the batteries. To maintain load balance I need to run for 2 hours a day and then a long run to fully charge the batteries. I carried out a detailed system modelling / use case / statistical modelling exercise as part of the design exercise - gave me something to do between ordering the hull and having it delivered!

[Keeping Up]

Hi I think you may be looking at the wrong figures on the shoreline web site here. It's a very messy layout but I think it says that it uses an average of .95 amps/hour at 12v and the number you quote 1.42 amps is the rated current at 24v.

 

Also, don't forget, unless you are have a mains hookup, to take into account the additional power required to run your inverter.

 

Hope this helps

 

Rik

 

Indeed these figures from Rik are correct. The information on the Shoreline site is very poorly laid out, not helped by the fact that someone there obviously doesn't understand the difference between Amps and Amp-hours so quotes the utterly meaningless unit of "amps/hour" when they mean "amps" (or "amp-hours per hour" which would be the same thing)

[Chalky]

 

Firstly the inverter will normally be only about 90% efficient while it is running, so add 10% to the effective consumption of the 240v fridge.

 

 

Pseudo sine ones tend to be more efficient than pure sine due to the way the waveform is generated. A very good state of the art inverter can be +95% efficient, a poor one 80 - 85%. A lot depends on the quality of the design and also the waveform being generated.

 

One thing to consider is that the current needed to start a fridge motor is considerably greater than the current needed to keep it turning. You need to ensure that your inverter can supply this which usually means a generously sized inverter. Also the temperature at the back of the fridge can have a big effect on power consumption. If the temperature is high then the gas pressure in the fridge coolant circuits will be high which will require the motor to work harder - more power usage. Improving the cooling at the back of the fridge has been discussed on here before.

Edited April 27, 2012 by Chalky

[bowten]

If you already have an inverter buy the 240volt AA rated.

[Meandering]

Hi I think you may be looking at the wrong figures on the shoreline web site here. It's a very messy layout but I think it says that it uses an average of .95 amps/hour at 12v and the number you quote 1.42 amps is the rated current at 24v.

 

Also, don't forget, unless you are have a mains hookup, to take into account the additional power required to run your inverter.

 

Hope this helps

 

Rik

 

Rik, Just used your link and it says the 1.42 are Amp/hours and the .95 is Watts? Already clarified above

Edited April 27, 2012 by Meandering

[Meandering]

Pseudo sine ones tend to be more efficient than pure sine due to the way the waveform is generated. A very good state of the art inverter can be +95% efficient, a poor one 80 - 85%. A lot depends on the quality of the design and also the waveform being generated.

 

One thing to consider is that the current needed to start a fridge motor is considerably greater than the current needed to keep it turning. You need to ensure that your inverter can supply this which usually means a generously sized inverter. Also the temperature at the back of the fridge can have a big effect on power consumption. If the temperature is high then the gas pressure in the fridge coolant circuits will be high which will require the motor to work harder - more power usage. Improving the cooling at the back of the fridge has been discussed on here before.

 

The inverter(/charger) is a Victron 3000 Va so not anticipating a problem. Good points on the refrigerator inclosure temp. I'll check this out. TA!