Kilowatts, amps and other hard stuff

[Crazyhorse 47]

The weaco has a "fridge" part where the cooling plate doesn't reach so not everything gets frozen. Of course depends on temp setting! I like icecream...guess the rest. The peltier fridge we have works very well, doesn't run all the time and probably uses about the same power but of course less than half the size but 240ah of battery, 120amp alternator, 40amp wind turbine and 140w of SPs keeps it all running. We load up both fridges before leaving then migrate to just the one as the food disappears running the little fridge at times to keep eggs from going off over summer. Seems to work well.

[Steve Pope 243]

The weaco has a "fridge" part where the cooling plate doesn't reach so not everything gets frozen. Of course depends on temp setting! I like icecream...guess the rest. The peltier fridge we have works very well, doesn't run all the time and probably uses about the same power but of course less than half the size but 240ah of battery, 120amp alternator, 40amp wind turbine and 140w of SPs keeps it all running. We load up both fridges before leaving then migrate to just the one as the food disappears running the little fridge at times to keep eggs from going off over summer. Seems to work well.

Eggs will keep for 6 weeks to 2 months out of a fridge, if you jump through a couple of simple hoops. 1st buy direct from an egg farm, ensure that eggs are of the latest crop. mustn't be washed, as they have a natural seal making them non porous, end for end them once every 2 or so days, this keeps the yolks from settling and sticking to the shell. Keep in the bilge or somewhere coolish. I made up one of those squareish plastic cubes, a 30 egg tray fits just nicely. You transfer as required to your daily use locker. We took a cube full to Tonga and on to Fiji where they were all used up. towards the end they suited omelettes and baking rather than fried.

[Guest 000]

Ok, back on the fridge thing again, and looking for clarification.

If a fridge or any other appliance for that matter running on 240 volts uses 100kwh per year,do I,

 

Multiply 100 kWh x 100 to get watts 100,000 watts

Divide by 8760 (hours in a year) 11.41 watts/hour

Divide by 12v to get amps .951 amps

Add 1.5amp to power inverter 1.5 amps

Total 2.45 amps

 

Once down to temperature I imagine the fridge will have a duty cycle of perhaps 40 minutes on, 20 minutes off per hour in which case the battery monitor should show a current draw of about 3.2 amps. Is this more or less how you calculate this?

[Crazyhorse 47]

I think you are over calculating all this. There are way too many variables like what temperature setting will it be at? How full will it be? How often opened? Will it be well ventilated? In the sun at any time? Used in summer/ winter? How long and what gauge wiring are you using? Voltage constant and or above 13v? Its crazy they should say so many KW per year but my thinking is allow for 4ah on average and you should be good to go. Less than that is very optimistic.

[Island Time 1,232]

Agreed, but lets get something straight first. While KW is a RATE of consumption (which the fridge certainly does not have!) KWhrs is the AMOUNT of power used. Their testing will be in ideal conditions, in a lab. Fixed temps, fixed mass of "product" in the unit, and minimal opening/closing.

 

Next, calculation. Watts to amps at a known voltage is easy Amps=Watts/Volts

BUT, your using an inverter, right? It will have conversion losses,  and take power to run it as well. Efficiency usually depends on loads (less with smaller loads, better when its being used near its capacity) but it varies with make and model. Good units, with good battery voltage, are better than 90% efficient.

 

Now, lets look at your calcs;

 

Multiply 100 kWh x 100 to get watts 100,000 watts 100 Kwhrs is 100,000 Watt hrs
Divide by 8760 (hours in a year) 11.41 watts/hour Yep
Divide by 12v to get amps .951 amps yep, without allowing any losses, allow 10%, so 1.05 amps draw
Add 1.5amp to power inverter 1.5 amps
Total 2.45 amps I'd say min 2.5 amps

 

So, 2.5 amps x 24 = 60 amp hours per day from your power sources or batts.... AT LEAST. As you'll likely not run the unit in perfect conditions.

[Guest 000]

Thanks for the replies.

I'm trying to get my head around this stuff because firstly my electrical know-how is best described as limited and secondly because I am comiserating with my no.1 son who is forking out megabucks to replace all his 2 year old batteries that he has sucked the life out of with his oñ board fridge.

I imagine I can live with 60 - 80 amps per 24 hours.

 

All electrical equipment on board is from Victron Energy.

Battery capacity is 495 amps hour and new this year.

Solar panels are adjustable to the sun and total 240 watts.

Solar regulator is MPPT.

The engine has an 85 amp alternator charging through a smart regulator.

We motor on average four hours per day.

 

The fridge purchase is on the go again because we have done the rounds of the appliance stores here in Nancy and if you stay away from the big name brands you can get a nice little fridge/freezer combo of around 85 litres, made in Europe and with a hopefully honest good energy rating for under €150.

Joke is tempted. Me not so much.

[Island Time 1,232]

One thing about batts. They very rarely die, they are murdered!

Charge them properly, is per the manufacturers spec for the model they are. 10ths of volts are important! Get the PDS for your batts. Use that, and only that info, no matter what anyone tells you.

Use a good quality battery meter, correctly programmed. Don’t discharge below the recommended amount for the type of batts you have.

Looked after, quality batts will give you more than 10 years of service.

Also, lead acid batts can take 10 hours to charge the last 10% of capacity, regardless of their charge source, it’s their nature. Many vessels rarely get their batts back to 100% for this reason.

Lots of boats charge to 80%, as it can be done fast (er).

Look after your batts, and they will give you great service.