12 volt Electric Fence energisers are very useful and convenient but do come with one major drawback. They require a heavy 12v battery. These have a limited life and have to be recharged regularly - the frequency of which is dictated by the power drawn by each specific energiser. Essentially, the stronger the energiser the shorter the life of a battery charge.
This battery must then be removed and taken to a mains outlet to be recharged by a trickle charger over a period of time
One solution to this is to use a solar panel that will harvest the sun's rays converting them to an electrical charge suitable to recharge a battery. A solar charger converts light energy into a DC current. They obtain energy from the sun, although they can also be used in low light or cloudy applications. Portable solar chargers are used for trickle charging, meaning that low level current is fed continuously to the battery so that it maintains its charge.
Under normal sunny conditions a solar panel will easily cope with maintaining a battery but the amount of available light varies hugely throughout the year. The image shows the weekly generation of a panel over a 12 month period here in the UK. As you can see the winter period is very poor with one 7 day period reflecting NO charge at all.
You need to know that solar power is not the cure all for replacing spent energy. For example some people are trying to recharge batteries and they want it done in very short time, usually in just a few days. Assume you take a fully discharged 100 amp Hour battery and charge it with a 5 watt solar panel under ideal summer time light conditions. After a full week the battery will be just about fully charged. Conditions such as an overcast sky, shadows, improper mounting angle, or short winter days will reduce the actual solar panel output to well below the rated values.
In addition to the low output achieved in the winter period, battery output is severely affected by cold weather. The electric current generated by a battery is produced when a connection is made between its positive and negative terminals. When the terminals are connected, a chemical reaction is initiated that generates electrons to supply the current of the battery. Lowering the temperature causes chemical reactions to proceed slowly, so if a battery is used at a low temperature then less current is produced than at a higher temperature. As the batteries run down they quickly reach the point where they cannot deliver enough current to keep up with the demand. If the battery is warmed up again it will operate normally. In severe cold the liquid will freeze resulting in expansion and possible rupturing of the battery components.
Solar panels will maintain a charge in the battery for extended periods however this cannot be guaranteed in the winter months. As a result a panel of less than 10 watt output should not be considered at all. For energisers in excess of 2 Joule output then a 20 watt panel should be the minimum considered. In both cases the battery must be monitored during periods of extended cloud cover or snow deposition. Both will result in a poor generation on energy to maintain your battery.