Does anyone have experience of solar power?
Looking for a decent solar panel to charge / top up a 12v 90ah leisure battery that will be running a laptop.
Laptop power supply states ... 90w max
I've used a 13W brief case style solar panel from Maplin:
http://www.maplin.co.uk/module.aspx?ModuleNo=99760&doy=11m5&j=true
They were on offer at less than half price or I wouldn't have bought one
. It came with its own controller. The 13W panel couldn't even keep the alarm on my Jeep from draining the battery in high summer.
I recommend that you find some other way of doing it. For example, look at a laptop with a methanol fuel cell, or get a generator. Twenty years ago I got a 3kVA diesel generator for 120 quid out of the small ads in the paper, it's still going strong but I probably ought to change the oil...
Solar panels are an expensive and inconvenient way to generate electricity, and unless you're in the desert they're unreliable. They're big and unwieldy for what they give you, which on a good day is no more than ten percent of the energy that reaches them. Lead acid batteries are a terrible way to store energy for something like a laptop. It typically takes many hours to charge them to full capacity. If you don't do that, you only get a small fraction of their stated capacity as usable energy before they need charging again -- if you over-use the capacity of a leisure battery you will soon damage it.
If you insist on solar you'll need to be very pragmatic with your power budget estimates. In the UK on average they will at best give you ten percent of their rated output (think night times, cloud, continually changing orientation, winter, whatever other downtime) so for a laptop (typically around 50W) then a 25W solar panel might give you an hour a day of laptop activity. Factor in conversion and storage (in)efficiencies and you're probably down to btween half an hour per day in summer and practically unusable in winter. As well as a panel, you'll need a controller to prevent boiling your battery when it's full. If it ever is.
The plate on the laptop power supply only tells you what it's capable of supplying, not what the laptop uses. What it uses can depend very strongly on what it's doing. About half the power the laptop uses will be consumed by the screen, so you can save quite a bit if you adjust the screen-saver settings to turn the screen off quickly when the laptop isn't actively being used. It's a pain though.
Consider a discless notebook machine. The power needed for a wireless connection is negligible, in the tens of milliwatts at most in the UK.
Incidentally 'leisure' batteries are usually junk, get something like deep-cycle AGM (Absorbed Glass Mat):
http://www.caravanandmotorhomebooks.com/articles/absorbed_glass_mat_ batteries_wanderer.htm
Look carefully at self-discharge rates.
From the efficiency point of view it would be a lot better to charge the laptop battery directly from the panel, but with a Lithium-ion battery without the proper protection and precautions that would be dangerous. Think fire, explosion, molten metal and stuff like napalm all over the place and you won't be far wrong.
