John Butterworth checks out recent developments in solar panels as he provides solar-powered lighting for a tractor shed
I wrote about using a solar panel to power a hen-house light in a previous post, so now it’s time to see how things have moved on, and to get up to date with a newer, bigger installation, for a newer, bigger tractor shed!
The main difference is affordability, though they are not yet what I’d call ‘cheap’ – £100 bought you a feeble 25-watt panel when I originally wrote that article in 2008, and now it buys you a 100-watt panel, but that’s peak power at midday in midsummer. The rest of the time it will produce much less, so you still couldn’t run energy-hungry items such as a 12-volt fridge using a single panel – or at least not for long.
The second difference between now and 2008 is the cost and easy availability of LED lighting, which has improved to the point that a rechargeable 10-watt LED lamp for around £25 is now very bright – around the same brightness as an old 60-watt incandescent light bulb.
Thirdly, the ‘solar controller’ that you’ll need has improved since 2008. The original controllers used high-power semiconductors, which had a fairly short life, and the first two I got only had half the circuitry they really needed – they stopped the battery over-discharging, but the circuitry to prevent it over-charging had been left out! Presumably the sellers wanted to keep the price down to a nice round £25. For the same £25 or less now, you get a much cleverer ‘Pulse Width Modulation’ controller, which stops the battery both over-discharging and over-charging using much cheaper circuitry. Here’s the little schematic that came with the new one.
It’s a simple task to wire up solar power in a shed or outbuilding – all you need is a solar panel, a car battery and, in my case, a lamp. In the diagram it’s called the ‘DC load’ for good reason, as it could be anything we want that is powered by 12V DC. The controller sits in the middle, and they come in different power ratings: typically 10–30 amps – the one I used is 30 amp. To calculate the maximum power you could supply to your ‘load’, multiply volts (12 volts) by amps (30 amps), which gives 360 watts, but I wouldn’t risk going up to that theoretical rated maximum – you can bet your life it’s optimistic, given the controller only cost £25. Still, 250–300 watts would be quite useful.
The solar panel is cabled to the controller, which regulates voltage coming from the panel – it doesn’t allow it to go above about 13.5 volts. If it did, pretty soon the battery would be ruined, as it would overheat and the electrolyte inside – dilute sulphuric acid in the case of a ‘lead–acid’ battery – would boil away.
Likewise, the ‘load’ (in my case a worklight), if left switched on when the battery isn’t charging, would eventually wreck the battery by over-discharging it, so that, too, is connected via the controller. When the battery gets down to about 10.5 volts, the controller protects it from discharging further by the simple expedient of switching the light off!
The going rate on eBay for a 100-watt panel is £85–£90. On a shed, a ‘rigid’ panel is perfectly acceptable, so don’t pay extra for ‘semi-flexible’ panels, which are aimed at the motorhome and narrowboat market.
You’ll need a mounting frame to fasten it to your roof. For flat roofs, the cheapest option is four plastic corner brackets costing about £20, but my shed roof is corrugated, so I had to use aluminium brackets. There are loads of aluminium mounting kits for sale, at about £30. The panels themselves have an alloy frame with four holes underneath the panel, and with the mounting kit that I got, I had to bolt four L-shaped brackets to the panel, and then bolt these to aluminium channelling. I know this is the right mounting kit for large panels, because I scrounged it from the fitters who put the 250-watt panels on our barn roof, so it’s a bit oversized for a titchy little 100-watt panel, but it was free, which makes all the difference.
There are some clever fastenings to attach the L-shaped brackets to the channelling, which I’ve shown in profile, as they’re very commonly used – there’s a rubber bung in the end; you push the fastener into the slot in the channel, then turn it through 90 degrees, at which point the bung acts like a little spring and holds the fastener in place – a nice design.
Not so clever was the fact that the holes in the panel frame were too small for the bolts that I had, so I used a ‘cone cutter’ to enlarge them, rather than buying more bolts. Cone cutters are exceedingly useful. It’s nearly impossible to enlarge an existing hole with a drill, as it wobbles off-centre, even using a pillar drill. A cone cutter has a gently stepped profile so always sits dead centre. The drawback is the cost ‒ around £20 and upwards ‒ but nothing else does the job so well. Be careful not to push right through and bust the panel!
Once the holes were drilled out, I bolted the four L-shaped brackets on, then, to get the position on the roof just right, I used the packing that the panel came in rather than the fragile glass panel itself. The brackets have slots rather than holes, to give a bit of movement. This allows you to position a piece of channelling on the tops of two corrugations and then slide the brackets in or out to suit the width of the panel. Why the tops of the corrugations, which means using longer screws? This is to stop water leaking in through your roof. I used 150mm ‘Structural Timber Screws’ to screw through the channel into the wooden joist below, and I also put a ring of sealant round the hole and a rubber washer under the screw head.
Fasten them down using a spanner, or socket driver (easier!). When both are in place, the panel can be fastened to them by sliding the fixings into the right position to fit the L-shaped brackets on the panel.
WIRING THE PANEL
These solar panels produce a low DC voltage (about 20 volts) at quite a high current (power = volts × amps, so for a panel to produce 100 watts, there must be 5 amps flowing in the wires). High current needs thick cables! Some panels come with cables attached that are long enough to get from your roof to inside the building, but some don’t, so buyer beware! If you have one that needs cables attaching, as this one did, either buy extension cables with connectors already fitted (easiest), or fit your own (sometimes cheapest). The connectors need to be special, waterproof, high-current jobs called ‘MC4 connectors’, so don’t be tempted to bodge, especially as MC4s are cheap and easy to fit. They come in ‘male’ and ‘female’ variants – one plugs into the other. The pin should be crimped using a crimping tool or pliers, and soldered as shown – easier than it looks. We’ve looked at soldering previously, and a soldering iron should really be part of any home farmer’s toolkit. If you don’t fancy doing it yourself, buy cables with connectors already attached.
Plug them into the panel and run the cables from the roof to wherever you intend to fit the controller and battery, being careful not to let any bare ends short out by taping them with insulation tape first. A wiring tip – when you run wires into a building, often through a hole in a wall (though, in this case under one of the roof corrugations), make a loop with them first. This gives a bit of spare length, should a connector corrode and need re-making at a future date, and it stops rain from creeping down the wires and into the building – it drips off the loop.
I made a little box to shelter the battery, controller and connector, as my shed is ‘Yorkshire Boarding’, so the rain blows in on very windy days (which is most of them!). I used plastic sheet for the back and the top, and plywood for the sides, as I’d run out of plastic sheet! The base is made from thicker strips of timber to take the weight of the heavy battery; the timber was jointed using a biscuit jointer – a great way of making good use of odd offcuts of timber.
Once the box was constructed, I first screwed the controller, which says ‘mount vertically’ on the instructions, onto a little steel panel, then screwed that into the box – controllers have metal heat sinks on the back, and screwing something that gets hot straight onto a plastic panel seemed like asking for trouble. You need to be able to see the display on the controller, and to leave room to poke six cables into the connectors on the bottom of it.
The rechargeable LED lamp I got for the shed has a 12-volt car cigarette lighter plug, so I got a matching 12V socket (£2 on eBay) to mount on the battery box. I made a little plastic panel for the socket and screwed that to the front of the battery box – formerly part of a bath panel, hence the tasteful avocado colour.
The battery is an old tractor battery I thought I’d destroyed by leaving the sidelights switched on for several days. Miraculously, it came back to life after a long charge on the battery charger, so this set-up should give it a new lease of life.
To make connectors for the battery, a crimping tool is best (£2 on eBay), then you can use these DC connectors. There are spade, bullet and ring terminals, and it’s the latter that’s best for a semi-permanent connection to a battery. Crimp a ring terminal onto a piece of thick wire (I used some solar panel cable offcuts), drill a small hole in the centre of each battery terminal, then screw the connector to the battery for a good connection. Wipe a smear of Vaseline or grease on it to stop it corroding.
It’s a good safety precaution to put an in-line fuse in the battery power supply, as shown. It’s a ’10 Amp 20mm glass fuse and holder’; these cost pence on eBay. Use a higher-rated fuse, up to the rated power of your controller (in my case 30 Amp), if you intend to run more powerful 12V equipment than my LED lamp.
It also lets you disconnect the battery without having to find a screwdriver – simply unscrew the fuse holder. There’s already a fuse in the 12V ‘car’ socket.
Now all that remains is to wire the leads from the solar panel, the battery, and the socket, to the controller. Do it in the order the controller instructions recommend, being very careful not to short the battery or the panel connectors together. The best way to strip insulation from electrical cable is to cut round it with a sharp knife, but not quite through, then bend the end back to ‘crack’ the insulation. That way you don’t cut through the strands of copper wire.
Here’s the wiring into my controller. The panel is the left two connectors, the battery the centre two, and the socket for the ‘load’ (to plug my lamp into) is the pair on the right. This controller allows you to alter the over-charge and under-charge voltages, but the factory settings should be OK.
COMPLETING THE INSTALLATION
I varnished the timber on the box to waterproof it, and rather than sit all that weight on one of the shed’s structural timbers, I made legs with brackets on them for the box to stand on. Now I can just leave it to its own devices, but if I need to go into the shed in the dark, I know I have a charged-up worklight, and as an extra bonus – a spare charged-up tractor battery for the tractor!