Today, I installed a solar panel on the shed. It's rated 100 W but will generate but a fraction of that because of next-door's tree.
My initial idea was that I ought to be able to get, say, 10% of the quoted 100 W on a dull day, possibly for a third of the 24h period during winter, giving an average of 10 W x (8 h / 24 h) = 3 W, give or take. As long as my panel pumps out 10 W during daylight hours and my Pi (plus all the weather gubbins) uses 3 W or less on average, I'll be fine!
According to this, https://www.pidramble.com/wiki/benchmarks/power-consumption, 2.5 W sounds about right - plus whatever my sensors consume. Actually, thinking about it, my charge controller must consume a fair bit by itself. Hmmm...
So anyway, I've attached the solar panel (as you can see above) and wired it up to the aforementioned, rather fancy charging controller I'd bought for a previous project:
And to an old, knackered (possibly) car battery I rescued from the garage:
All the wiring's a bit temporary for now. I just wanted to get it hooked up to see if the panel and charger would work. And they do! The panel showed 20 V across its terminals, open circuit. When connected up, that dropped to about 13 V. As you can see, the panel is supplying a tidy charging current of about an Amp here:
That's about 12 W. It's a bit sunny, but the panel's not in direct sunlight. And I'm not sure if the charging circuit is throttling things back a bit for my old, neglected and no-doubt shagged battery.
Time will tell if this will be good for keeping the Pi going through winter!
Okay, now it's about 15:44, it's cloudy and starting to get dark (sun sets in half an hour.) The charging controller is reporting barely a tenth of an Amp now. About one Watt. Dunno if the battery having maybe charged up (I doubt it - but did I mention it's probably shagged) is having a bearing on that but that sounds a bit thin. We'll see.
I need to test how much the Pi will draw and maybe run a test load in the shed that uses a similar or greater amount of power (e.g. a 12 V lamp or something) for a week or so to see if it works. Or I could just install the Pi and see if it crashes...
Actually, sod that - the first thing I need to do first is disconnect this setup becuase, like an idiot, I haven't used any fuses!!! I'm pretty confident that my wiring is as sound as it could be - but still, the potential for a battery that can supply 100s of Amps to burn down a wooden shed should not be underestimated! I think I also need to add a fuse for the solar panel - just in case.
It's a couple of days since putting the panel up (and starting to write this post) and the average power capability of my solar panel has been weighing on my mind. Since my over-specced charge controller includes wind-turbine capability, I've now ordered a little wind turbine! It's coming from China so I'm not holding my breath.