Thanks for that, great answer and easy to understand. It looks like the old Engle 12v gets resurrected for another run around.

Hi Bob,

Not sure about the analysis given so far. In fact I would think it is hard to follow because the confusion of incorrect units, of amps and amphrs and amps per hr.

So I would give this analysis for the benefit of others who are interested in the discussion.The figures have been rounded a bit.

The fridge uses 100 Watts and that translates when converted to about 8amps at 12 volts.

So the 240v inverter needs about 10 amps allowing for losses. The fridge may run for about 75% of the time

So the amp hrs used over 24hrs are about 180 amp hrs per day.

The 300W panels produce about 20amps for about 6hrs (?) so that is 180amp hrs each winter day. More in summer I think, perhaps depending on the setup.

That looks like a rough match for very average solar conditions.

The batteries are about right for overnight storage but two days would be max for safe use without charge. The inverter should be OK to run that.

Cheers Jaahn

Jaahn wrote:

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Hi Bob,

Not sure about the analysis given so far. In fact I would think it is hard to follow because the confusion of incorrect units, of amps and amphrs and amps per hr.

So I would give this analysis for the benefit of others who are interested in the discussion.The figures have been rounded a bit.

The fridge uses 100 Watts and that translates when converted to about 8amps at 12 volts.

So the 240v inverter needs about 10 amps allowing for losses. The fridge may run for about 75% of the time

So the amp hrs used over 24hrs are about 180 amp hrs per day.

The 300W panels produce about 20amps for about 6hrs (?) so that is 180amp hrs each winter day. More in summer I think, perhaps depending on the setup.

That looks like a rough match for very average solar conditions.

The batteries are about right for overnight storage but two days would be max for safe use without charge. The inverter should be OK to run that.

Cheers Jaahn

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I sometimes get a canning from someone on tech terms like amps and amphrs and amps per hr when using the KISS method to try to explain something with the aim of being easily understood, however from Bob's reply I think I achieved that, and so to Bob.... your welcome.  Note to self in future try to use the correct tech terms in the appropriate places.

In my defence of other some points above I did say "Note these calculations are very rough" that point seems to have been lost and yes you are correct in saying "The fridge may run for about 75% of the time" however I believe in running calculations at worst case scenario and not second guessing at what things might run at.

Also on the comment "The 300W panels produce about 20amps for about 6hrs (?) so that is 180amp hrs each winter day. More in summer I think, perhaps depending on the setup. That looks like a rough match for very average solar conditions." My line said "At most your panels will produce 25AH and that's really best case scenario" OK it should have said amps per hour I'm just glad Bob understood what I meant.

At the end of the day we both agree that Bob's system would be inadequate for anything longer than about 2 days. However, my very rough analysis did show that the power being used was going to be greater than the power being replaced and that I think was what the information that Bob was after.

Regards Steve. 

Bent Axle Bob wrote:

the old 12v/230v Engel upright will be installed for one last trip

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 Good move Bob, enjoy your trip we might even meet on the road somewhere???, heading that way ourselves in a few weeks.

But what's this "one last trip" just keep going til you can't!.

All the best Steve.

Bill B wrote:

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PeterD wrote:

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An interesting read for you -  A fridge test, battery power consumption comparison between a household 240 volt 140 litre bar fridge, and a 12 volt 140 litre Waeco fridge, for camping.

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Just a note of caution - This test was done in 2003.

Technology would have improved markedly since then I would imagine.

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 Yes, technology has improved, mainly in the larger fridges, less so the bar fridge size. It also has in the 12 V fridges. Please prove to me that technology in 12 V fridges has moved at a lesser pace than bar fridges.

Muddy wrote:

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A 1000w inverter will handle a 230W load.....obviously. The inverter will do the job but your set up is way under rated for the load you propose. Is your fridge rated at 1A if so this will draw up to 230W on a 230V system and not 100W so I'm choosing to ignore the 100W input power bit at this stage because it's misleading.

So if the fridge draws 1A maximum on the inverter output side which at 230V = 230W, The current in the 12V supply would be 230W /12V = 20A (rounded up). Then inverter efficiency must be taken into account so divide the amps by 0.85 and you end up with 24A (again rounded up). The inverter supply fuse rating must be slightly above the calculated max current draw OF THE DEVICE and some 15% below the rating of the cabling supplying it. So if your inverter is fully loaded at 1000w then using the same formula 1000W/12v/.85=98A then I'm guessing that to obtain a full load output from the inverter, your 12V supply leads will be rated appropriately fuse could be around 100A. Remember that if you fuse the inverter with one rated for just the fridge, you will blow the fuse each time you plug another device in so fuse your inverter to the right level. Consult with your inverters instructions or talk to the manufacturer or supplier.

The batteries capacities are measured in AH (Ampere hours) not Watts. So you have (when fully charged) 400A to discharge (could be 400A in 1 hour or 40A over 10 hours etc), divide this by 2 (because you should not discharge AGM Batteries below half charge) so this becomes 200A and so 200AH/24A = 8.4Hrs of running the fridge (flat out) before you really should turn it off to charge the batteries. Not good!

If your solar panels are optimized for direction and angle (mobile) you will generate at 25Amps max. If your fridge is running flat chat drawing 24A on a hot day you will not cope as the solar panels output reduces on hotter days by some margin. (They are most efficient on cold sunny days) Problem is, your fridge could be running continuously (all night) but the sun doesn't and so your daily input would be around 300A 6hrs at optimum generating power and assuming the panels are roof top mounted and not mobile and therefore will not be directed perfectly at the sun) and your daily draw is around 24 x 24 = 576A (assuming max current which is always best) But even if the fridge is never opened (cough, er did someone say Bar fridge) the draw over 24 hours will likely discharge your batteries to near flat each day. You will be replacing them very early and we haven't accounted for rainy days yet!!! Basically, it can be done but you have dedicated some expensive and unnecessary kit to JUST keeping your fridge going and not much else. Agree with Maboutled, buy a 12V fridge for sure. Nothing like a coldy on a hot rainy day!

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 Hi Muddy and all,

I write this to promote a good discussion not to argue. This seems a topic which people are interested in, so some correct answers would be helpful. Your analysis is good, but.....

Muddy has made an assumption that the 1A at 240V is a better assumption for power use that the 100W. I do not believe this is correct. That current is the max start current. From my experience 100 Watts is about correct for a small fridge. So basing the power usage on this is OK IMHO and having been inverted from 12v to 240 will draw about 10amps+ at the low voltage end, depending on the quality of the wiring and the state of charge of the batteries.

The inverter must be well oversized to start a fridge or it will trip out on overload or low voltage. Up to 4x or 5x the rated power is needed to "kick them away". Depends on the inverter AND the wiring too. If the wiring is a bit poor the inverter will not handle the surge needed because the voltage drops too low. Note this surge when sizing the wires and the fuse. As Muddy said above the full capacity of the inverter should be allowed for i.e. actually 1000W cont- 2000W surge.

Most bar fridges would benefit from a layer of foam insulation glued on the sides and top. Door too.They have minimal insulation. 

Cheers Jaahn

-- Edited by Jaahn on Thursday 19th of March 2015 09:06:42 AM

DeBe wrote:

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Newer bar fridges have the condencer built in to the sides of the fridge. If you stick insulation on the sides of these it wont work very well.

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 Hi DeBe

Correct! If the fridge has the condenser built into the side walls then definitely do not put insulation there. In fact the side should be ventilated.

That was an interesting post about your fitting a bar fridge with another compressor etc. I like real figures and readings.

 I wasn't recommending or not recommending a bar fridge, just trying to provide some realist and correct units to base a decision on. Incorrect units of electricity tend to confuse more than anything else on here.

Jaahn