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Kilowatts, amps and other hard stuff


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I am looking to replace the fridge on board with a small 240v domestic unit.

I have two good quality inverters on the boat - the two kilowatt one uses 1.5 amps to power itself and the one kilowatt one uses 0.6 amps to power itself. The fridge I am considering at is supposed to be very energy efficient and the blurb says it uses 96 kilowatts per year. I am assuming that I could run it off the small inverter. From the above is it possible to calculate how many amps the fridge will suck out of my batteries per hour?

At the moment I have an ancient 240v Electrolux fridge that uses an eyewatering 12 amps so am looking for a little improvement here.

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I would have thought 96kw was very low, I work it out to about 21.9w ph! The inverter is using more power than the fridge!? I am doing something wrong here? Our DC 12v fridge uses about 4AH, 48w so thats a bloody remarkable fridge!

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Interesting fridge comparison between dedicated 12V and 230V thru an inverter

I was surprised that the conversion losses thru the inverter did not significantly affect overall power consumption

the 2 important points if you run a fridge thru an inverter

1. Its stand by power draw

2. Its ability to start the fridge compressor - in rush current at startup is typically 4-6 x the normal operating current for a second or so. Most inerters have some surge capacity but you need to check this.

 

ps the inverter needs to be pure sine wave, modified sine wave will struggle to run the motor

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Checked it out again, 96kw per annum but unfortunately on a Dutch website.

The fridge is around 115 litres and is an AEG, model RTB 91431 AW.

BUT agree, sounds a bit too good.

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A friend who had a big ferro 'Shady Lady' did what you are doing for 13 years before he went landlubbing and I remember he said it worked very well when we were looking at getting a new fridge. Was going to do the same but not enough room! A two KW inverter would work as MH said, startup sag in the AC side needs to be minimised but even if the fridge was pulling 10 amps to start that inverter will do it. If you have a clamp meter and can rig up 2 separate wires for phase and neutral, only for test purposes, on a 2 or 3 pin plug you can see how many amps its pulling. Be careful, its mains you are playing with. If the clamp meter has DC then even easier, just clamp one wire feeding 12v to the inverter. It will show you total amps when running.

1535229807756143736061.jpg

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Thanks for the input.

This AEG fridge seem to only be available in either Holland or Germany and I can't find any info in English. However I have checked the info on 4 different sites and they all state either 95 or 96kwh per year.

I don't want to spend my money only to find i've bought a lot of exaggeration..

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Ok, let me re-consider. Yes those figures are possible on a small fridge, not allowing for inverter losses.

My boat runs a 190 ltr fridge and a 190 ltr freezer with a single Ozefridge compressor driven, water cooled unit. It draws 5.5 amps, and runs about 30% of the time. That’s 5.5a x 8 hours = 44 a/h per day. At 12v that’s 528 Watt hours. Divide that by 24 hours gives an average of 22 watts.

So it can be done.

Just remember inverters are not 100% efficient, and will cause additional power use, some 20% or even more.

To be 100% up front, that’s what my boat used to do. The cabinet is old, and the insulation is not what it was. It’s closer to a 40% run time now, and it’s on my work list to rip out the insulation and re do it. After all, it’s 30 years old next month....

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Ok, let me re-consider. Yes those figures are possible on a small fridge, not allowing for inverter losses.

My boat runs a 190 ltr fridge and a 190 ltr freezer with a single Ozefridge compressor driven, water cooled unit. It draws 5.5 amps, and runs about 30% of the time. That’s 5.5a x 8 hours = 44 a/h per day. At 12v that’s 528 Watt hours. Divide that by 24 hours gives an average of 22 watts.

So it can be done.

Just remember inverters are not 100% efficient, and will cause additional power use, some 20% or even more.

To be 100% up front, that’s what my boat used to do. The cabinet is old, and the insulation is not what it was. It’s closer to a 40% run time now, and it’s on my work list to rip out the insulation and re do it. After all, it’s 30 years old next month....

This might be of interest to you Matt? Dunno how you vaccume clean your fridge though!??

shagfigure.jpg

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Another interesting (and confusing) point.

The fridge I wish to replace is and old 3-way,12v,240v and/or gas Electrolux job of unknown vintage, probably at least 20 years old. It is not currently set up to run on gas. It does not have a compressor and uses ammonia as a coolant and runs continuously when powered on.

I have a battery monitoring system on board and from it I can see that the total draw including the inverter when operating on 240v is 12 amps. If I switch it over to 12v and shut down the inverter then the total draw is close to 15 amps. Either way its a lot of amps for a fridge that does very little in terms of cooling. I have heard before that when it comes to cooling it is often more efficient to run a 240v/inverter system than a 12v system. True or not I dont know.

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Those 3 way fridges were never good to run on batteries, ok on gas , 240 or while motoring like driving a camper van, but very inefficient on batteries. They have a rating of how much propane weight they will use per hour, but our use was 9 kg per month while living aboard both before and after getting the fridge which we ran constant on gas. They also need to be kept level so not good on keelers when sailing. The 12 v compressor fridges have never given us any trouble and seem to be good value for money.

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Waeco are good. Got a small eutechtic fridge and a 65lt compressor freezer. Had to take the fridge back as the cooling fan stopped working. Found that if you run it at its full setting, the plate freezes solid and food gets warm. The thermostat gets ignored so a setting lower and chills down nicely but they use a bit of power compared to the compressor freezer. The pic shows the freezer power usage when the mains went off so no battery charger from 0630 to 0930. Not bad!Screenshot_20180826-230045.jpg

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So, as it turns out we cannot buy the AEG fridge because we can't get it to fit in the space. So we will settle for a Waeco/Dometic compressor fridge freezer unit of 70 litres capacity. This is small but will be ok for us as we shop,if not every day then at least every other day so dont need a big capacity. Down side is the AEG fridge was €349 and the Waeco unit is €700 (about 1100NZD). Interested to know how that compares with NZ?

Again, spending megabucks to buy quality I don't really need

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Got ours through All Marine, $1700(NZ) I remember. Had it over a year now and its run continous since we got it, a 65ltr version. As a trucky years ago in Oz, I had an old Engle that just refused to die, incredible fridge but unfortunately they changed compressors, and for the price the Weaco seemed the way to go but they had (have?) issues with the led display winking out. Ours hasn't...so far. You will be happy with it Chris.1535312791721120482391.jpg

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Thanks for all the good info - we will go ahead and buy the Waeco job, but not just yet.

Got out of my bunk just now to the realization that in one calendar month we are on the airplane on the way home. Where did the last 6 months go? So the fridge purchase deferred to next April when we are back on board again.

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Thanks for all the good info - we will go ahead and buy the Waeco job, but not just yet.

Got out of my bunk just now to the realization that in one calendar month we are on the airplane on the way home. Where did the last 6 months go? So the fridge purchase deferred to next April when we are back on board again.

Some folks back up the waeco / engel with a Chilly Bin, they freeze down bottles (square if you can find them) with salt added for lower temp, and use the CB as a fridge, re-freezing the bottles as required. That leaves the freezer to be used as one, rather than as a fridge. Of course you need somewhere handy to the galley to tuck the CB. Keep the updates coming, really enjoying them.

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