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Sizing a solar installation


Col_T

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I have a 12v cool box rated at 50 watts in operation. I reckon this will draw about 4.2 amps, and about 100Ah from a 12v battery bank over a 24 hour period. There is a thought that we might be able to run this from a solar array, so I have attempted to size the array we would need.

 

The array would need to generate 100 amps (to replace the amps used over the previous 24 hour period), plus a further 15 amps to cover assumed charging inefficiencies, so a total of 115 amps. Assuming 8 hours of useful sun-light a day at this time of year, that's 115 / 8 = about 14.5 amps. Volts x amps = watts, so I'd need a 12 x 14.5 = 174 watt panel and a 20 amp charge controller.

 

So that's how I think the calculation works - is that anywhere close to being correct, or have I missed something? All tips / pointers / guidance welcomed.

 

For clarity, this above calculations suggest the minimum size of the battery bank will be 200Ah, which is probably enough to torpedo this little dream!

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Only expect solar to provide about 50% of its rated output in the UK, and lost less in winter.

 

I think the only real answer is "as much as you can afford and physically fit onto the boat". Even then there will be times when its not enough so alternative battery charging is required.

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Get as much solar as roof space you wish to give it up for.   Don't bother with calculations on how much you will need as solar is a hit and miss affair and what you calculate you need won't be correct for the other 6-8 months of the year.

 

Edited by Robbo
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20 minutes ago, Col_T said:

I have a 12v cool box rated at 50 watts in operation. I reckon this will draw about 4.2 amps, and about 100Ah from a 12v battery bank over a 24 hour period. There is a thought that we might be able to run this from a solar array, so I have attempted to size the array we would need.

 

The array would need to generate 100 amps (to replace the amps used over the previous 24 hour period), plus a further 15 amps to cover assumed charging inefficiencies, so a total of 115 amps. Assuming 8 hours of useful sun-light a day at this time of year, that's 115 / 8 = about 14.5 amps. Volts x amps = watts, so I'd need a 12 x 14.5 = 174 watt panel and a 20 amp charge controller.

 

So that's how I think the calculation works - is that anywhere close to being correct, or have I missed something? All tips / pointers / guidance welcomed.

 

For clarity, this above calculations suggest the minimum size of the battery bank will be 200Ah, which is probably enough to torpedo this little dream!

I have a 170w 'low light' panel that apparently maximises low incidence light in higher latitudes (that's us !!)

With the panel laid flat I reckon in the Summer I average about 6 amps for 12 hours per day - clouds etc, low angle of incidence morning and night,

 

My guess would be 170 watt panel = about 72Ah per Summers day. 

 

I work on 50% efficiency compared to the theory (as per Tony) and probably 1% - 5% of theroretical in Winter

 

I think the highest I have ever seen it was 10.8 amps in June

 

 

15-6-16.jpg

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Looks like one for the WotEver Postulate

 

The WotEver Postulate (Solar Array sizing) 

"The answer to your question is as much as you can afford in both terms of roof space and wallet. It’ll be too much in the summer and not enough in the winter, however much you get." 

 

Which is deep within this thread which covers much of the ground for this question and might be worth the OP perusing. 

https://www.canalworld.net/forums/index.php?/topic/96416-solarhow-much-wattage-is-enough/&

Edited by reg
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From the figures it looks like a standard cheap 12v coolbox. These are horribly inefficient for power consumption and only drop the temperature by a certain number of degrees below ambient. A compressor coolbox or fridge, although expensive, will use a fraction of the energy and the temperature will be fully controllable. 

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I have 2 x 245w panels, one flatand one at a bit of an angle, but not much. through a Tracer 40A MPPT controller, I get about 120Ah on the sunny days of recent weeks. Which is  more than enough for a full day on board.

 

So, based on my experience, you need about 500W of solar.

 

But why base your cooling around such an inefficient fridge? You must be thinking of using it long term, so would be wise to invest in a proper 12v one.

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34 minutes ago, reg said:

Looks like one for the WotEver Postulate

 

The WotEver Postulate (Solar Array sizing) 

"The answer to your question is as much as you can afford in both terms of roof space and wallet. It’ll be too much in the summer and not enough in the winter, however much you get." 

Yup. 

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2 hours ago, WotEver said:

Yup. 

Actually your Postulate needs some fine tuning to take account of the situation on one of my boats. 

 

I have a 100W panel, decent MPPT controller and it can't quite keep up with my fridge, once LED lighting, some trivial pump use and charging the laptop is added into the mix.  I've been having to run the engine for half an hour a day too recently despite being in full sun most of the day, even this time of year!

 

So "It’ll be too much in the summer" cannot be wholly relied upon for someone with a really small roof. 

 

Granted I have space and money for a second panel so I haven't complied with your first stipulation.

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28 minutes ago, Mike the Boilerman said:

Granted I have space and money for a second panel so I haven't complied with your first stipulation.

Ahh, but you have to comply with all aspects of the postulate to ensure complete validity ;)

 

30 minutes ago, Mike the Boilerman said:

"It’ll be too much in the summer" cannot be wholly relied upon for someone with a really small roof.

Accepted :)

 

 

So the amended postulate should read...

 

"The answer to your question is as much as you can afford in both terms of roof space and wallet. Unless you have a very small roof or a really tiny wallet it’ll be too much in the summer and not enough in the winter, however much you get." 

 

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4 minutes ago, WotEver said:

Ahh, but you have to comply with all aspects of the postulate to ensure complete validity ;)

 

Accepted :)

 

 

So the amended postulate should read...

 

"The answer to your question is as much as you can afford in both terms of roof space and wallet. Unless you have a very small roof or a really tiny wallet it’ll be too much in the summer and not enough in the winter, however much you get." 

 

 

Yep. I think you've cracked it!

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Thanks to all who contributed to this, many useful opinions and a good link.

 

Yes, it is a fairly cheap, standard cool-box, rather than any kind of fridge, and the thought was to use this on a one week camping holiday, rather than as a permanent refrigeration solution on the boat - we already have a 12v fridge on that, though no solar panels -yet. Thanks to all who reminded me about the efficiency aspect e.g. to assume that 50% efficiency is all one can reasonably expect from solar panels at our latitude and with our, usual, weather! I happily accept that I over-looked that point.

 

However, and accepting that the original numbers need amending to accommodate the 50% efficiency point, were the original calculations about right, and if not, why not?

 

Why am I asking again? Well, it's a way of checking my understanding and / or bettering my education!

 

 

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23 minutes ago, Col_T said:

Why am I asking again? Well, it's a way of checking my understanding and / or bettering my education!

In principle - yes.

 

Build in the fact that your are not charging at 12v but 13v, 13.5v, 14v, 14.4v so take the worst case w = A x V (W = 14.5 x 14.5) = 210w.

Then remember that to actually achieve this you need 2x this amount (50% efficiency)

 

You need approximately 420w and a suitable MPPT controller

 

You pretty much need to duplicate Richard's set up.

 

5 hours ago, Richard10002 said:

I have 2 x 245w panels, one flat and one at a bit of an angle, but not much. through a Tracer 40A MPPT controller, I get about 120Ah on the sunny days of recent weeks. Which is  more than enough for a full day on board.

 

So, based on my experience, you need about 500W of solar.

 

But why base your cooling around such an inefficient fridge? You must be thinking of using it long term, so would be wise to invest in a proper 12v one.

 

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