AC fridge

[Happy Bachelor]

I am thinking of installing 240 watt solar panel connected to three 120 amp batteries that I run a 600 watt inverter from. This in mind the question is would this be okay to run a small under counter domestic fridge with freezer box rated approx 50 watt. I am thinking the fridge only cycles periodically so doesn't continually draw power.

I am a CC and cruise at least two hours a day sometimes more.this is currently plenty without solar to keep the batteries around 12.75v.

[Chalky]

We have a similar arrangement. We have a standard under the counter fridge with a 65W motor. This runs off an 800W inverter connected to a 400Ah battery bank. We have a pair of 75A alternators to charge it with. We turn the fridge on when we get up in the morning and off at about 9:00pm. When the engine is off the thermostat is set to minimum and max when the engine's running. We need about 2 to 3 hours a day of engine running to maintain load balance (depending on what else we've used.) If you're cruising at least 2 hours a day you'll probably be ok for the fridge, however it does depend on a lot of other things. What does your powr audit suggest?

[Happy Bachelor]

Hi Chalky thanks for the prompt reply. You have lost me at the power audit sentence am I assuming average amp consumption per 24 hours?

Gary

[Tom Richmond]

I have 330W of solar, three x 100AH batteries and a 12v under the counter fridge. During the warmer six months of the year fridge is on 24/7, the solar is enough to get the batteries full or near to everyday. Voltage will generally have dropped to 12.4 or 12.2 overnight. (I estimate this to be something like 60% - 70% SOC).

 

During spring and autumn we run engine or genny for 1-2 hours every day or two.

 

In winter we turn fridge off. Solar keeps batteries full with pumps, lighting, phones, laptops...

 

Fitting solar was best thing I have done since buying the boat.

[Tom Richmond]

Power audit is a look at all of your usage, a bit like a budget sheet for your on board power. It will contain everything you have on the boat, how often it gets used, and what consumption it has, in some sort of table or spreadsheet.

 

I have never done one. Just like I have never done a weekly or monthly domestic budget.

 

I try to keep my batteries above 50% SOC (roughly 12v) just like I try to keep my bank balance above zero. I have an overdraft with the bank, and my batteries still work below 12v.

[Alan de Enfield]

A fridge will use approximately 45 amp/hours (per day), but you also need to remember that the inverter will 'use' some power just to operate - say it is an efficient version using 0.5 amps - that is another 12 amp/hours.

 

As has been said - a power audit is a look at your total electrical usage and an estimate of how much you are using, add it all together and see if you are actually using more than you are putting back into the batteries :

 

Fridge = 45

Inverter = 12

Computer = 22.5

TV =

Water pump(s) =

Phone charger =

Lights =

Toaster =

Deep Fat Fryer =

Washing machine =

Curling Tongues =

Hair dryer =

 

To work out what you are using look at the label on the appliance and look for 'watts', for mains powered appliances divide this figure by 10 and it will give you amps taken from your battery via the inverter,

 

Example - my laptop is 45 watts, that equals 4.5 amps (in an hour), so if I use it for 5 hours then that is 22.5 amp/hours.

 

For 12v appliances (example lights) then take the wattage and divide by 12 - so if you have 2 x 12 volt lights with 6 watt bulbs that is 2 x 6 divided by 12 = 1 therefore you are using 1 amp (per hour) use those two lights for 5 hours and you have used 5 amp/hours.

 

Once you have tabulated everything add it all together to get a total - lets say 100amps.

You therefore need to put back in at least 100 amps.

 

During the summer your 200 watt solar + 2 hours engine running should just about keep up with that demand.

 

In the winter you will not only have greater demands (more lights for longer etc) but will have minimum output from the solar - you may well have to run the engine 4 hours+ per day

Edited March 1, 2015 by Alan de Enfield

[Pen n Ink]

For an electrical numpty like me, can you just tell me why "divide by 10" gives the battery amps used? I don't understand that bit.

[Alan de Enfield]

For an electrical numpty like me, can you just tell me why "divide by 10" gives the battery amps used? I don't understand that bit.

 

There is a bit of 'rounding' and taking into account inefficiencies (you dont get nowt for nowt), and its easer to mentally divide by 10. It also errs on the side of 'safey' (ie assumes you are using more than you actually are).

 

Electrical theory :-

 

W = A x V

Therefore A = W divided by V

 

At 220 volts a 1000 watt appliance is using 4.54 amps

 

At 12 volts a 1000 watt appliance is using 83 amps so the inverter is drawing 83 amps (plus its inefficiencies of maybe 10% ie 8 amps) so a total of 91 amps.

 

Now the 'maths making it easy' part

 

Divide 1000w by 10 gives you 100 amps (but actually it is 91 amps)

 

I find it far easier to just divide watst by 10 than go all thru' the formulae.

 

If I am using my mains powered 900w deep fat fryer I work on 90amps, I run it for half an hour, it has therefore used 45 amp/hours.

My alternator is 50 amp, therefore I need to run the engine for 1 hour (approximately) to replace the amps used in half-an-hour by the fryer.

 

All 'roundings' and approximations- but it makes sure my batteries never drop below 50% SOC

Edited March 1, 2015 by Alan de Enfield

[Pen n Ink]

Perfect - thank you.

 

I can now go back to my homework and hopefully come up with some reasonable figures. I suspect that laser engraving machines, lathes and bandsaws aren't on the usual list of onboard appliances...

[OldGoat]

We have a similar arrangement. We have a standard under the counter fridge with a 65W motor. This runs off an 800W inverter connected to a 400Ah battery bank. We have a pair of 75A alternators to charge it with. We turn the fridge on when we get up in the morning and off at about 9:00pm. When the engine is off the thermostat is set to minimum and max when the engine's running. We need about 2 to 3 hours a day of engine running to maintain load balance (depending on what else we've used.) If you're cruising at least 2 hours a day you'll probably be ok for the fridge, however it does depend on a lot of other things. What does your powr audit suggest?

 

As a matter of interest -

is your 800W inverter dedicated to the fridge and,

is it a pure sine wave jobby?

 

(FWIW, some years ago Danfoss (who make the best compressors) produced a small inverter which fitted into 'the compressor bay' - so it had to be small and in its smallness was probably low powered as well - trouble is I can't find any specific details)

 

To my mind part from the cost I can find no 12/24v fridge freezer to replace my aged Batts unit and of the same capacity. The unit works well but the case.... ('twas a Lec unit)

 

Apologies for Fred Drift.

[Chalky]

The inverter is a sterling pseudo sine and runs the fridge and the 240V sockets. Since we inly use the 240V sockets to charge phones etc then it is in effect for the fridge only. The inverter was sized to be able to cope with starting the fridge on a hot summers day (which is worst case).

 

When you do your power audit there are several things to consider. (vastly simplified)

 

• Your alternator specification is lying. It might say 75A however it'll probably only produce 45A when cruising and 35A on tick over. Making it spin faster / adding charge controllers etc may help however it's the battery that will limit what it can take. (charge acceptance)
• Converting power from one form to another is inefficient.
  • The battery is not a bucket of volts. When you fill / charge it not all of the power will go into the charge. Also when you pull power out of the battery not all of the power stored in the battery will come back out. (google charge acceptance and pukerts equations).
  • Changing voltage levels wastes power. An inverter will require some power to work its internal electronics (usually between 0.5 and 1A). It will also waste some power in the conversion process. The amount depends on the quality of the inverter and its type but recon on 5 to 10%.

[OldGoat]

Chalky,

I assume your post is in reply to mine (just in case it was a general observation).

I've avoided any mention of Peukert's equation as I've been attacked by The Smartguage Mob in the past... Similarly I've not gone into to the alternator capacity vs: actual power output - both of which are responded by a deafening silence!

 

Since my last post I had a look at Sterling's site and his inverters are all quoted as having a standing current of .89 amps (that's a lot for doing the square root of nothing). Victron are a bit more informative quoting a range that depends on the power output and the input voltage - around .60 amps.

 

Oh for a magic (?) arrangement that could switch a dedicated invertor on (rather than coming out of standby) when the fridge's system needed to start up. Then the overhead would be more acceptable....

[Chalky]

Yes it was in response.

 

The problem is that Peukert's equation models how a battery works. Gibbo will have included it in his software somehow. My suspicion is that Smart Gauge runs a battery prediction then compares the voltages with what the prediction says using an enhanced Kalman filter. This is then presented to the user in a more simplified way. (Unless Gibbo's found a very different method)

 

 

Oh for a magic (?) arrangement that could switch a dedicated invertor on (rather than coming out of standby) when the fridge's system needed to start up. Then the overhead would be more acceptable....

 

It could probably be done. If you re-wired the fridge so that the thermostat turned on the standby switch in the inverter that might do it. It would however mean playing round inside the inverter and the fridge electrics and could potentially be quite dangerous (unintended pun).

[Jambo]

Do you definitely need a freezer box? In an under the counter fridge you don't get much usable freezer space and it adds quite a bit to the load. For example the most efficient under the counter larder fridge available in the UK uses 64kWh/yr but the most efficient fridge with a freezer box uses 146kWh/yr. That's a big difference.

[Happy Bachelor]

That's a very good point about the freezer box. Problem is I can sometimes be miles away from shops and keeping meat fresh would prove difficult.

[Tom Richmond]

That's a very good point about the freezer box. Problem is I can sometimes be miles away from shops and keeping meat fresh would prove difficult.

 

Sometimes boating is difficult...

Get a gun and shoot y'r meat fresh? Better yet, fashion yourself a bow and arrow.