Solar panel efficiency in the UK (aka 100w panel vs the 47w fridge - round 2!) (1 Viewer)

[Vanman]

I have 1x100w solar panel and 1x47w Electric only fridge. I also have 2x 95aH leisure batteries.

I am trying to decide the best way to make my electric beer fridge last x3 days circa 0900 Day1 to 2300 Day3 off EHU.

What I don't know is how efficient a 100w panel is.

In order to do a calculation I am accepting the manufacturers statement that the fridge uses an average of 47w. Of course that will depend on many things like how often you open it to get beer out and if you need to put new, warm beer, back in. It can also help to have some frozen bacon and sausages in there, but they seem to disappear around breakfast time.

I'm guessing that the panel is only really 100w when the van is parked in the Sahara dessert with the sun overhead? 0900 on a dull Manchester morning heading for the M62 is unlikely to be producing 100w ... if any? Can a panel tell the difference between travelling and standing still?

So can anyone give me a steer as to what percentage of the 100w is likely to be generated on an AVERAGE UK day and how that would be spread through the day? Presumably, as the panel is flat to the roof it will be more efficient when the sun is high and next to useless early/late in the day?

By my rough calculation, if I had no solar then my fridge would use half of my batteries (95mah) in 24 hours. I have read this is as far as you should deplete them to keep them healthy and rechargeable. With 100w Solar panel on a recent weekend away my batteries lasted about 32 hours before I had to switch off the beer fridge - night was falling anyway so there would have been no more going in for 12 hours. As it happens the 3rd day was really overcast and might not have helped either.

So, I await your advice, facts, speculations and rumours - please. Only proviso is that you don't mention Gas or travelling to another country as none of these are options at the moment. Damn, I've just mentioned them - see you in the naughty corner

 

[vwalan]

fit more solar . another two panels would be ideal.
possibly another battery as well.
its hard to know when or if the solar are going to work. a few cloudy days not good . remember they are possibly only working ideal power for perhaps 6-8 hrs aday on a good day.
but if its sunny you can power other things as well.
plus an inverter . then washing machines etc make life much nicer.

 

[DBK]

I think you might struggle over 3 days unless the sun is shining strongly but there are many variables.
We had a 90W panel and when running a Waeco CDF 26 coolbox (compressor type) in France in a cloudy September the battery voltage was getting low (11.9v) by the third day. This was with two 90Ah batteries plus led lights and occasional water pump and TV use. However, we do have diesel heating and that takes a lot of power at times.
The cool box take 2.5A when running, which equates to about 30W, but it only ran for 5 minutes every 20 minutes as the ambient temperature wasn't too high. We have subsequently added a second panel of 100W and all seems good now.
I can only suggest give your setup a try and see how you get on but my gut feeling is get a second panel. My brother did three weeks in Spain recently off grid with a compressor fridge and 200W of panels.

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[Neckender]

We have managed easily 4 days with 2 95AH AGM batteries, and 200 watt of solar, with compressor fridge, lighting some tv and some heating.
This was in April. And also the same in May.

John.

 

[Richard n Ann]

I agree with the above, you definitely need more solar. And it would be a big help if you could tilt the panels up towards the sun when parked.

 

[Minxy]

Definitely need another solar panel ... you can add another battery at a later stage if you want but if you haven't got anything actually generating the power in the first place storing it isn't an issue!

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[Camping Gaza]

Sell your solar panel and put it toward a Lister diesel generator set and a trailer, the bigger the better, and you will be able to enjoy your bacon, sausages and your beer 24/7 regardless of the solar peak. That is how I solved my energy supply problems whilst camping.

 

[mitzimad]

my son has 120w panel and a 110 battery and did chester long weekend without any problems using a waeco fridge

 

[SuperMike]

On a brightly lit sunny day, at noon, and with the 100w panel pointing directly at the sun, you will get 8A out of it. Anything less than that is downhill. We have 400w and get these sort of numbers on a good day, because the panels are flat on the roof and not angled at the sun. Irrespective of the location, or time of year, or the weather, the solar always keeps up with our needs, together with a big battery bank. So, you need to fit more panels

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[Vanman]

Thanks for all the replies so far. My MH is small and both roof space and weight need to be considered. Also my pockets are not deep so I can't just throw money at it, hoping it will work well enough on a dull day ... I was trying to be a bit more scientific than that.

@SuperMike that Solar level indicator looks useful for seeing what I am getting from my existing panel. Is it easy to fit? I think my panel has a Sargent unit under the passengers seat. Also, do you have any other figures for dull days and early/late in the day? Thanks.

It's also making me wonder though if there's a limit to what the batteries will take in when charging, presumably the rest is just wasted if you can't use it as fast as it's being generated?

 

[SuperMike]

Yes it's easy to fit. But it is the same make as the controller, so just plugs in.

 

[Lenny HB]

On a brightly lit sunny day, at noon, and with the 100w panel pointing directly at the sun, you will get 8A out of it. with a big battery bank.

More like 6 amps, a solar regulator needs a higher voltage going into it than the output of the regulator for it to work and the output needs to be higher than the battery volage for the battery to take a charge. You need to do the calculations using 16v not 12v to get a true picture of the charging current.

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[Lenny HB]

If you are using the built in Sargent regulator it would worth while fitting a decent MPPT solar regulator and checking cable sizes any cables under 4mm sq need replacing. The MPPT regulator will help on cloudy days when light levels are low.

 

[ukbill]

I have 3x120 solar 3x 110 batteries,I always have the fridge on gaz,but here in spain I tired the system on fridge,and it ran without a problem for 2 days max,that was in april now I'm not sure as the sun is hot

 

[Viennese]

Would it be cheaper/easier to fit a 3 way fridge so you can use gas when no EHU???

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[Lenny HB]

I have 3x120 solar 3x 110 batteries,I always have the fridge on gaz,but here in spain I tired the system on fridge,and it ran without a problem for 2 days max,that was in april now I'm not sure as the sun is hot

The OP has a compressor fridge (electric only) they tend to be a lot more efficient than an absorption fridge running on electric.

 

[Richard n Ann]

Thanks for all the replies so far. My MH is small and both roof space and weight need to be considered. Also my pockets are not deep so I can't just throw money at it, hoping it will work well enough on a dull day ... I was trying to be a bit more scientific than that.

@SuperMike that Solar level indicator looks useful for seeing what I am getting from my existing panel. Is it easy to fit? I think my panel has a Sargent unit under the passengers seat. Also, do you have any other figures for dull days and early/late in the day? Thanks.

It's also making me wonder though if there's a limit to what the batteries will take in when charging, presumably the rest is just wasted if you can't use it as fast as it's being generated?

I fitted a NASA BM1 marine battery monitor. It displays all current in and out, a bar graph with estimated charge left, time until fully charged and starter battery voltage. Best bit of kit I've fitted:

http://www.allgadgets.co.uk/marine/pc/NASA-Clipper-Battery-Monitor-BM-1-12V-p127.htm

 

[EuroTrotters]

The best way to make an efficient 12v fridge is this....

Buy the most Energy efficient fridge you can. A++
Replace the compressor with a Danfoss 12v compressor.

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[Richard n Ann]

The best way to make an efficient 12v fridge is this....

Buy the most Energy efficient fridge you can. A++
Replace the compressor with a Danfoss 12v compressor.

How much are danfoss 12v compressors?

 

[SuperMike]

More like 6 amps, a solar regulator needs a higher voltage going into it than the output of the regulator for it to work and the output needs to be higher than the battery volage for the battery to take a charge. You need to do the calculations using 16v not 12v to get a true picture of the charging current.

Picky, picky, sorry that my numbers were a bit general. Note to self, try harder when trying to be helpful.

 

[Viennese]

The OP has a compressor fridge (electric only) they tend to be a lot more efficient than an absorption fridge running on electric.

Not if they drain the batteries like that, how limiting. I'd rather have the choice!!

(PS and I would be no good at working out all the watts, amps, etc)

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[Lenny HB]

Not if they drain the batteries like that, how limiting. I'd rather have the choice!!

He stated 47 Watts, small 3 way fridges take around 100 watts larger ones 180 watts, and would require running for much longer periods to obtain the same temperature reduction.
I would rather have the choice too but the OP is trying to make the best of what he has got.

 

[Vanman]

Would it be cheaper/easier to fit a 3 way fridge so you can use gas when no EHU???

None of the manufacturers seem to offer them in the 5.4m PVCs so I'm guessing it's technically difficult and/or expensive

 

[DBK]

None of the manufacturers seem to offer them in the 5.4m PVCs so I'm guessing it's technically difficult and/or expensive

The compressor fridges tend not to need external vents so easier to fit and no chance of draughts. And of course much more tolerent of slope and high temperatures.

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[Minxy]

For the cost of the display panels etc being recommended @Vanman could fit another 100w panel for just a little bit more money which WILL give him more juice which is what he needs whereas the panel will just tell him what he's getting but won't provide any more ... ergo ... get another flipping panel!

 

[Richard n Ann]

Your right there Minxy, but I'd fit both so I can feel smug watching the extra amps flowing in

 

[two]

I suggest you do an experiment: Fill the fridge with the beer you expect to keep cool. Remove EHU and let the batteries settle. Measure the voltage of your habitation batteries first thing each the morning, ideally before the sun has risen. The voltage ought to start at around 12.7V and may fall each day if more power consumed than was gained from the panel. Luckily, there will tend to be more sun at the times that you need most cooling but I’m not sure that the survival time experienced on a sunny day would match that when overcast. I’d try to emulate ‘normal’ use such as putting other devices on for a while and replacing contents with uncooled products in the fridge, to make the test more representative. So long as the voltage does not drop below 12.2V you are fine (the battery is more than half charged) but I’d prefer to avoid a discharge as low as that.


It won’t matter if you are moving. It should provide a closer to average performance because orientation and degree of shading will be changing all the time. In a camping situation you may tend to seek more shade and not do quite as well.


I think panels are only around 22% efficient at converting the enegy available from the sun’s rays into electrical power, but I don’t think that’s what you were really seeking to know.

I’m pretty sure that a 100W panel will achieve that only under optimum conditions, but it may be that ‘maxing-out’ occurs before 90° incidence is achieved. We will never have 90° here unless a panel is tilted, but you would’t need to go as far as the Sahara. Different panels are claimed to be less affected by the incidence of the light but I suspect that may only be by a small margin.

I’ve not found any results to demonstrate the effect of the sun not being directly towards the panel. Even if there are some, they would only provide guidance as to the behavior of a type of panel. There are many other factors in the equation that would also need to be applied, hence my suggestion to run some trials with your actual set-up.


If you run short of capacity within the required time you should either get used to warm beer or consider more panels, but I reckon you might just get away with it. The tests should enlighten you about your power consumption and the availability of ‘free’ power from the sun. Of course, no single day will be typical but it’ll be difficult to draw a any useful conclusion without some form of measurement. To obtain more meaningful measurement of power, consider the installation of a battery monitor (Amp-hour counter). These provide a view of the capacity remaining in your battery bank at any time (for around £100) and would help you to understand your needs and manage consumption before splashing out on additional energy sources.

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[gerry mcg]

The reason smaller vans and some 6m vans have compressor fridges rather than 3way ones is the location of the fridge / galley in relation to the sliding door. If the fridge is overlapping the door, one cannot fit a 3 way as you need vents on the external wall to vent gas exhaust fumes

 

[Minxy]

I suggest you do an experiment: Fill the fridge with the beer you expect to keep cool. Remove EHU and let the batteries settle. Measure the voltage of your habitation batteries first thing each the morning, ideally before the sun has risen. The voltage ought to start at around 12.7V and may fall each day if more power consumed than was gained from the panel. Luckily, there will tend to be more sun at the times that you need most cooling but I’m not sure that the survival time experienced on a sunny day would match that when overcast. I’d try to emulate ‘normal’ use such as putting other devices on for a while and replacing contents with uncooled products in the fridge, to make the test more representative.

I can see ONE major problem with what you suggest ... as he removes cool beer from the fridge to replace it with uncooled stuff ... I suspect he'll then drink the cool beer and get slowly sozzled and won't be in any fit state to check the battery levels!

 

[Vanman]

Geronimo! I just found some 'facts and figures'. According to the chart 1x100w Solar Panel will (on average) produce 600-700Wh over 24 hours in British Summertime. The same panel will produce 80Wh over 24 hours in Wintertime.

By my back of fag packet calculations I would need 1x150w panels during summertime to run my fridge and 15x100w in the Winter.

I suppose I could leave my beer outside in the winter - no one would nick it at a rally/bike racing ... would they?

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