The Gemini series from Hotline is top selling dual-power fencer. This is a 12v battery energiser which comes with a mains adapter to allow it to work from the mains making it a very versatile power unit.
This model has an OUTPUT JOULE rating of 0.4J which will power a 4km stretch of electric fencing. Under normal line resistance levels of 500 Ohms it will still put a 3,900v pulse through the fence so it's perfect for medium length horse fences.
The mains adapter that's included is a bit like a mobile phone charger and just plugs into the side of the unit. Although it's fully weather-proof and designed for being out in all weathers it must be housed indoors when being powered by the mains.
It comes with all the required leads to attach to a battery and to an earth stake (not included) and to the electric fence line itself but as when it's mounted in an outhouse the distance to an earthstake and to the fence are variable, any additional insulated cable will need to be bought here.
Ideal for the person who powers a remote fence during part of the year and likes the flexibility of powering the fence from the mains if it's available at other times.