How much solar power needed?

[simon_sluggett]

I need some good advice on what rating/size of solar power panel and regulator is needed. My requirements are to top up two wet leisure batteries, 110 AH each. The boat is in the solent on a non swinging tidal mooring with no shore power and is used mainly weekends. The batteries have always tended to be only 75% recharged after a days sailing and couple of hours motoring so just need topping up between uses. Size of panel would have to be no more than 0.5 Metre Sq. The panels Ive seen this size are 30 W. A couple of smaller panels wired together would be also be OK. Thanks all.

 

[fdmarsden]

The usual recommendation (for 12v batteries) is to have a solar panel wattage rating about 10% of the battery amp/hour rating. i.e. a 110 a/h battery will require a panel of about 10 watts .

Also panel suppliers say that a charge controller is required for any panels of greater than 15/18 watts

 

[vyv_cox]

With similar setup to yours, and the boat in Holland, I had good service from 38 Watts. Now in the Med and running a fridge full time I have double the capacity but lots more sunshine.

 

[LadyInBed]

The Basics of Solar Power for Producing Electricity is a usefull link.
The 2nd page considers sizeing.

 

[Gin]

I'm no expert but I have the same set up as you ( 2 x 110 AH BATTS.) and the same sort of useage and idle periods.

My semi flexible 30 W panel feeds my main battery and then overspills via a regulator to the secondary battery and I never have less than 12. 8 volts in either when I return

 

[Bodach na mara]

Er, 10% of rating of two batteries of 110 Ah at 12 volts would be 2 x 110 x 12 / 10 which comes to 264 watts does it not? Actually, this would deliver 22 amps and in theory recharge the batteries completely in about 10 hours, which I assume is not what is required.

However various articles suggest it does take a lot more solar panel capacity than most people think. You do not get the rated power in anything other than full sun and that has to be at "normal incidence". Having looked into it, I found that solar panels are unlikely to make up for the drain of domestic use on the batteries in Scotland without being impossibly large, but could provide trickle charging to make up the losses of a weekend of drainage during the week of non-use following.

 

[William_H]

Basically the solar panell you choose is a function of cost and available space. I reckon 10 watts with 2 diodes one to each battery with no regulator will give you some usefull recharge and maintenance.
If you went to much more than 20 watts then you should fit a charge regulator.
Without a regulator (smaller panel) you can potentially get a full charge more easily as the full panel current will go in all the time regardless of charge %.
Any calculations of amps expected will be a bit misleading as already mentioned however the current will be usefull. good luck olewill

 

[VicS]

[ QUOTE ]
Er, 10% of rating of two batteries of 110 Ah at 12 volts would be 2 x 110 x 12 / 10 which comes to 264 watts does it not

[/ QUOTE ] What he means is that the power of the solar panel in watts should be numerically 10% of the battery capacity in ampere hours.

Thus a 110Ah battery requires a panel of 11watts. It is only a rule of thumb and as far as i can see gives you the minimum size of panel that will recharge a partly discharged battery in a reasonable time. Like by next weekend!

 

[simon_sluggett]

With that amount of solar wattage per battery would a regulator still be needed?

 

[VicS]

This suggests 10w per 100Ah is the threshold above which you need a reulator but I am sure I have seen a higher figure elsewhere.
There is also a lot of info on the rest of that site.
Try looking at some of the manufacturers/ suppliers websites

There is a lot of technical stuff on LVm's technical web site

If you are intending to charge both batteries from the same panel then you will have to look into the methods of doing that. The electronics boffins will make their own diode splitter no doubt but you may find that a combined regulator and splitter is the way to go.

I think if you need to recharge two batteries by 25 Ah each in the course of a week you will be looking at rather more than 10 watts of power for each.

Sorry rather a disjointed reply!