This article describes how to assemble a simple solar power installation that can power LED lighting and a car radio/CD player for entertainment in a shed or outbuilding. Another use for this sort of installation is to power a 12V electric fence energizer from the sun, reducing the need for maintenance and regular battery charging.
12V Solar Power Safety First
12V is not a shock hazard like regular AC power because the voltage is much lower than the 110/230V of AC mains systems. However, the current capacity of a battery is very high, which can heat cables and cause a serious risk of fire. So it is important to install the system fuses where shown, so that in the event of a short the fuse breaks the circuit rather than the power burning out the cables causing damage and fire.
For the same reason of high currents, remove all metal jewellery like rings, watches, and necklaces before working on a battery system, otherwise if these cause a short and heat up there is a serious risk of injury.
Finally, car batteries should never be used indoors because of the explosive gases vented on charging - in a car the engine compartment is well ventilated by the headwind when the battery is being charged.
Leisure batteries can be used in some locations if the manufacturer's instructions are followed. Usually there is a vent kit to vent the gases to the outside. Or, as used here, a sealed lead acid battery is used, but this must be fitted with a charge controller to prevent overcharging. At all times be sure to follow the battery manufacturer's instructions for safe installation.
How to Select the Right Battery
You first need to gauge the anticipated load. For an electric fence drawing say 0.2A all the time, each day the load will be using 0.2A * 12V * 24h = 57.6 watt-hours. If you will be using a shed light of about 2W this will also draw about 0.2A but you will probably only need to run it for a couple of hours, so you need 4 watt-hours.
A lead-acid battery should only be discharged to about 50% of capacity for good service life, and you probably want to be able to store about three days' worth of power to smooth out a run of gloomy days. The battery will probably be quoted in amp-hours, which for the fence will be 0.2A * 24 * 3 = 14Ah. This is the capacity of a moderately sized gel cell, and easily within the capacity of a leisure battery (typically 60Ah or more), so any leisure battery will do.
How to Size the Solar Panel to the Load
This is a bit more difficult. If you will be keeping an eye on the battery and are prepared to charge it up at home if it falls low, then you can allow for this. If you need the system to run all the time, then you will need to estimate how much sun you will get on the shortest day of the year, which is usually associated with the gloomiest weather. Solar panels are usually quoted at a power rating assuming a light level of 1000 watts per square meter, which is bright sunlight. Someone in Britain would be wise to derate the panel by 10 times for the British weather, whereas someone in California would probably realise the rated power for a few hours around noon.
Assuming sunlight for four hours, for the lamp application you will only need 4 watt-hours, so allowing for four hours of operation you would need only a 1W solar panel in California. In Britain you would probably need a 10W panel at least. For the electric fence application you would need a 14W panel in California, and approximately a 140W size in Britain. The economics improve greatly if you are prepared to swap batteries to help the system out in the gloom of midwinter; alternatively, consider adding a boat-style wind generator to the system as the wind tends to a maximum in winter while solar tends to a maximum in summer.
How to Wire Up a 12V Solar System
The diagram shows how to wire up the system - a charge controller is used between the solar panel and the battery to prevent the battery being overcharged in summer. The charge controller monitors battery voltage and disconnects the solar panel when the voltage indicates the battery is fully charged.
A fuse is wired at the battery end between the battery and any loads - using a multi-way fuse panel allows any load falling short to be isolated from the battery without taking all loads down.
Twelve volt LED lamps are a good solution for shed lighting or reading lights. Alternatively, 12V CFL energy saving light bulbs are available, but these do not tend to appreciate damp, humid, cold conditions that might be encountered in unheated outbuildings.
12V Solar Power is a Good Alternative to AC Power for Isolated Sites or Outbuildings
The cost of running armoured cable to outbuildings can be considerable, so if the need for power is just for lighting or for lighting and a car radio/CD then a 12V solar power standalone system can be a cost-effective alternative. Similarly a system based on a larger solar panel can keep an electric fence powered on an isolated site, or alternatively extend the time between battery changes.
Richard Mudhar - Originally from London, now enjoying a less hectic pace of life in Suffolk, Richard Mudhar is a keen wildlife sound recordist and ...
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