there is such a breadth of knowledge on here i thought id ask if there is a cheap way to get solar power for my aahh CARAVAN im never the one in front of you
Whatever amphour the batteries are rated at you can only use half of it or the batteries will die quickly as the volts will drop too low, 12v/240v/gas fridges should never be used on 12v or batteries will die even sooner, and that inverter will hammer them as well if used for any length of time.
I can just about do 3 days on my boat with no power input (I have no solar) but I spent a lot of cash on a efficient compressor fridge a couple of years back which helps and we spend most on the evening in the pub (to save the batteries...honest), we have 3x 100Ah batteries for domestic and a separate battery for engine starting, with pubs being shut for the foreseeable future I have just bought a 12v telly (I don't have an inverter) but may need to take a spare charged battery for that (not yet allowed to actually stop overnight so keeping away for now).
I do have the advantage of if moving I have 2 alternators running and water heating from the cooling system, of course on a boat you lug a lot more weight about, a 6 ton caravan wouldn't be very useful.
Current toys: '99 XT600E, 2000 4TX, '82 Princess 30DS (where the username comes from), No longer a '03 Fazer thou.
i have been looking at the flexy ones to roof mount will be used mostly for lights which will be led and telly/music which would be through the inverter got gas for cooking heating.
only had a 1 night trial before winter/lockdown stopped us so dont want to spend a lot if its not for us.
If its a goer i will invest in a lot better van and ancillaries if we are going to use it
I'm buying some panels soon. I have a couple of very small 20W ones with a small charge/load controller. I use it to run my radio gear, where it charges a lithium pack. I want to upscale next and use some of my lithium battery mountain to run the house power in the evenings through a grid tie inverter, and I've found , interestingly , that a lot of people sell spare/old panels on Facebook Marketplace.  There was an ad last week which I hesitated on, for some almost new 'Trina' brand 260W panels at £90 each, which I missed out on.
Â
Because they're often installed as a system, and because the technology advanced quickly, when someone's solar array gets damaged in a storm their insurance often recommend rebuilding with all new panels of a higher rating (more modern) and so you can find good quality cheaper panels from an array where only 1 or 2 were damaged for sale by the homeowners who go down that route.
Â
As for batteries, deep cycle is the only way to go for lead acid. I've noticed a lot of people are selling preassembled lithium packs, called 'power wall' modules. With any lithium battery make sure there is a battery management system involved somewhere. But you can get good deep cycle batteries from computer surplus people who retrofit battery backed UPS systems, at a reasonable price, but those batteries still have a lot of life left in them
I missed the bit about the batteries you have already.
4 old car batteries, say 40Ah each, if you parallel them together (safe to do with lead acid) that's 160Ah at 12V, but without stressing the batteries too much say 120Ah
Â
Ah = Amp Hours. Or the number of Amps you can draw theoretically for 1 hour
Ah multiplied by voltage gives you Wh, or Watt hours, which is the power capacity of the system
Â
120Ah x 12V = 1440 Wh, or 1.4kWh  ( a kWh is a standard unit of electricity, same as that shown on your electricity bill.
Â
You can think of this as being the total number of Watts you can run for 1 hour. 1440 Watts for an hour, 144 Watts for 10 hours, or 14.4 Watts for 100 hours
Â
Â
That's the discharge side of things
Â
Â
For charging, assuming you deplete the battery completely overnight and need a full charge during daylight hours then you need to refill the 1440Wh in a day
Â
Allowing for 8 useful daylight hours. you need to divide the total needed by the number of hours = 1440/8 = 180 Watts an hour for 8 hours
Â
A single large 260W panel would comfortably do the job here, allowing for variations in sunlight levels (solar panel output values in Watts are given for the condition of the panel being in 'full sun' illumination, in the UK we can probably assume 50% - 70%.....I haven't looked but there will be some good 'daylight assumptions' available on a website somewhere.
Â
Â
Â
So you need to know how low you will discharge your batteries first, then that tells you the size oft he panel needed (making allowances for lower light levels than the panels are specified for
I've fitted about 4 solar setups to campers over the years, currently have 2 80W panels on my camper which is providing a great supply of power during these sunny days and will keep both the vehicle and leisure battery fully charged over winter no problem.
Â
Off grid I can probably go a week no problem in summer without issue and that's just with a single 110Ah leisure battery with this setup.
Â
I would recommend Photonic Universe for their solar panels / charge controllers. I've bought the German solar panels which are more energy efficient for their size, and so you can usually get more power on the roof if size is tight.
Â
Try to fit rigid panels if you can. Felxi are good, but need bonding down and the worse thing for solar is the panel getting hot (an irony I know when they need direct sunlight), so the problem with fixing directly to the roof is the roof absorbs all that heat and stops the panel cooling. A rigid panel can sit in a frame and then gets an air gap between it and the roof, which allows an air flow to keep it slightly cooler. If a flexi is the only way due to shape it's no big deal, just may not be quite as efficient.
Â
Multiple panels can be wired either in series or parallel (I have 2 x 80W in parallel), if you can't fit on large panel on the roof but have room to two smaller of course.
Â
Wiring together old batteries isn't great to be honest. When you set up a bank of batteries in parallel, they need to be matched to each other to get the benefit, so all need to be the same Ah rating ideally. With mismatched batteries, the lowest capacity one pulls all the others down to it's level, so if you have 3 x 100Ah and 1 x 90 Ah, you effectively end up with a total of 360Ah instead of your expected 390Ah.
Â
Also the problem with 4 old car batteries is you could have 3 that are great and one on it's way out. All that will happen is those 3 will keep sending charge to the rubbish one until all are as low as the weakest. I know you want to do this on the cheap, but you could be looking at a false economy. I'd test each battery individually first to see what the capacity is like - https://www.instructables.com/id/AmpHourTest/
Â
Also, there's no point in having masses of Ah's in the batteries if you only have a small solar setup that can only put a few Amps back in, so matching the solar panel to your usage is key, although if you only intend to go for a weekend and simply want a large bank of power and the solar just to 'top up' then you might get away with it.
Â
I'd say for a weekend off grid you want a minimum of 100W panel and make sure all your electrics are efficient - i.e. all lighting changed to LED, if you have a 3 way fridge keep it on gas etc etc. but with 4 batteries I'd probably go higher like Fixitsan says. I'd also be wary of those cheap panels though, they never kick out the wattage they say they do and they are most likely cheap polycrystalline panels that are less efficient and more susceptible to heat. All that setup can be had for dirt cheap, but won't be nearly as effective for the size and effort. Just depends how much you're willing to put into the initial outlay.
The link isn't working, but assuming it has the same output from the controller as that needed for your batteries it should be good. Some controllers also have a power output terminal which you hook your electrical load up to, which prevents the battery from over-discharging by cutting it off when the output drops too low
a link just a short description of a panel for sale on facebook
It's amazing what comes up on Facebook which doesn't go on ebay. Sellers fees and postage costs limit ebay possibilities, but i think also by using Facebook you are more likely to maximise exposure in your local areas
