What size solar panel?

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muzz67, Jan 19, 5:50am
got large caravan with decent 12v truck battery,used as a weekender,. Weekend usage is 12v lighting, 12v waterpump and 12v tv. What wattage panel would charge and maintainth battery during the week! Panel supliers all seem to suggest their most expensive units, not the adequate ones!Cheers, Muzz.
pickles26, Jan 19, 5:58am
A very large one. You will need to give more info if you want a better answer
muzz67, Jan 19, 7:28am
Thanks, I couldn't understand why I'd need a 200 watt setup, when th TV only draws 28w and may only be used for a couple of hours a week. Lights are 15w, couple of hours, and 12v waterpump may only run for half an hour a week.
mechnificent, Jan 19, 8:20am
If you take care to aim the panel towards the sun, which needs a sloping bracket, and in your case one that can be turned around, then 60 watt should be heaps.
mechnificent, Jan 19, 8:23am
If the sun hits the panel at an angle the output drops off dramatically. If the panel is turned to about thirty degrees to the sun the power can drop to about sixty percent, and at forty-five degrees the output is only about a quarter of its maximum.
pickles26, Jan 19, 8:59am
What's a descent size truck battery. N150 30 amps to charge remembering the 10 hr charge rate. This is why you would of been quoted on a large setup.
sr2, Jan 19, 9:12am
It would be a great project to build a turntable / gimble solar panel mount that was operated by two stepper motors controlled by a PLC. You could use the panels output as a reference for the stepper motors to position the panels at the most efficient angle and direction.
mechnificent, Jan 19, 9:25am
Not really Sr2, they swing towards clouds on overcast days, always using power and being off target when the clouds move again.

I have a gimbaled panel bank. It has a turn-buckle for the summer/winter change of latitude, and a five step lever operated arc for morning/afternoon. You swing the lever up to release the latch, then swing it left or right to change the angle. The weight of the lever engages the latch. The panels are gimbaled in such a way that pivot point is half way down the frame they are on, which makes for no weight on the latch and makes it light to swing. .It works really well. If it is a sunny morning you can have the batteries right up by eleven, then if the afternoon turns cloudy, no big deal.
I should mention that clouds or even just one thin shadow crossing your panels will drop the output right down too OP.
If there is one thin shadow that crosses your panel, such as a pole's shadow, it will reduce the output of those cells that it is blanketing, their resistance changes and that resistance limits the entire panel to the percentage of output those shadowed cells are operating at. You need to make sure you have full sun over the entire panel.
mechnificent, Jan 19, 9:32am
It's these sort of problems, and the fact that you can have days of overcast, that causes the experts to sell you heaps more wattage than you probably need.
If you are offline and using solar power the secret is to have enough panels to provide just a little more than you need per day, but have plenty of batteries.

We should all be using panels in summer and grid power in winter. If we had enough power from panels to supply half of the winters consumption per week or month, then in summer we would have far too much power, perhaps twice what we need. The excess can be pumped back into the grid and credited at wholesale power rates.
sr2, Jan 19, 10:11am
Interesting, with the LED lighting available these days a solar/battery powered ancillary lighting system for your average house is a real possibility.
mechnificent, Jan 19, 11:07am
Lights are small change Sr2, it's fridges and computers that seem to be the biggies in solar set-ups.
I understand that you are saying though about ancillary, or as a standby for when the power is down. (something I never have). in which case you could run the whole house off a bank of batteries and a modest solar array, assuming that the power was only off occasionally.
muzz67, Jan 19, 11:21am
How aboutmirrors on each side of a fixed panel!
mechnificent, Jan 19, 11:42am
Not recommended, they get really hot as they are.
mechnificent, Jan 19, 11:43am
In the middle of the day they would have close to three times the light on them.
mechnificent, Jan 19, 11:47am
Muzz, the easiest way to arrange the panel is to have it hinged so you can lay it flat for travelling, and then raise it to the right angle to suit the summer or winter. Then you want all of that spinning on a bearing so you can turn it towards the sun morning or evening.
mechnificent, Jan 19, 11:48am
Or just have it on a long lead and prop it up anywhere. that would be simple and adaptable.
trader_84, Jan 19, 12:09pm
Mechnificent ??
mechnificent, Jan 19, 2:11pm
Nice Trader. there are a lot of variables. So if you just want a back up supply, you need to guestimate how many hours of sun you can get a month, and allow for whether you are going to turn panels during the day or not(see my approximations of decreased efficiency with angle), how often the power goes out, and how long it is out for generally. then the rest is maths.
When you are doing the calculations you need to figure how many batteries it will take to run the system for the duration of a blackout. To calculate the panel capacity you need to supply enough to charge the batteries in the time between one black out and the next(at an absolute minimum), and preferably how much to recharge them in a day or two, which is better for the batteries of course.
The batteries generally don't like to go below about 11.5 volts, 11.7 is better. Even deep cycle batteries shit themselves if they are left for to many days, or if their voltage drops too low(11.5).

It could be that a small generator. I'd recommend an old CS lister running on waste oil.hehe, would be a better and more economic option.

If you are buying batteries, forget those expensive "deep cycle" ones they try and sell you, they are no better than off the shelf deep cycle tractor batteries such as the N70 you have there. I have had a variety of makes and types and in my opinion the yuasa batteries are absolute winners. the set I have now are years old and still seem as good as the day I bought them. Other makes i could tell they were loosing capacity after as little as four years, and eight years was a long time for them. I reckon I'm going to get double the life out of yasa batteries. and no I don't sell them.
mechnificent, Jan 19, 2:17pm
These guys sell good quality yuasa batteries. I have a suspicion that some yuasa batteries being sold for cars, are locally made and labelled as yuasa under licence. whatever the case. this guy's yuasas are winners.

http://www.batterydirect.co.nz/
matarautrader, Jan 19, 4:21pm
We have an English caravan that we have fitted a 120 watt solar panel to.
The caravan lights are all LED and we use gas for fridge, cooking and water heating. The battery is a deep cycle 75 A/hr. THe 12volts basically gets used for lighting, TV, and water pump with the lights on about 4 hrs a night as I read a lot. We find that our battery is charged after about 3 hrs sun. The best output we have seen is 8.2 amps from the panel. The caravan has not had 230 volts on it for the past year.
tmenz, Jan 19, 5:07pm
Yuasa are the suppliers of the 787 Dreamliner batteries.Just saying.
muzz67, Jan 19, 6:20pm
Thanks guys,, looks like anywhere between 6o and 120w should be heaps. Solar controller too!
mechnificent, Jan 19, 6:22pm
All you need is a regulator, controllers control power in and out(and cost heaps more). you don't need that.
mechnificent, Jan 19, 6:27pm
A decent large analogue volt gauge is the handiest thing to have to monitor your batteries. You need to be able to tell within about a quarter volt, so big gauges are good. digital is ok if they don't keep ranging all over the place with you needing to try to average the figure in your head. Analogue is nice and user friendly.you will soon spot trouble if you keep an eye on it and get a feel for how long the batteries last before being too flat.
matarautrader, Jan 19, 7:05pm
We have a Steca controller which works well for us. It has a LCD screen shows volts, amps in, amps out, % charged and lots of other things. Heaps of brands to pick from.