This one is decided by a single question: how far is your fence from a power point?
If power is close: mains
A mains energiser is the default for a reason. It is the cheapest option for the power you get, it costs a few dollars a year to run, and it never has a flat battery or a shaded panel. If there is a power point in a shed or on the house within reach of your fence line, buy mains and spend the savings on a better earth kit. The connection between the power point and the fence is made with insulated lead-out cable, so the energiser can live indoors, out of the weather.
If power is far: solar
A solar energiser is a panel, battery and energiser in one sealed unit. Mount it on a post facing north, connect the fence and the earth, and it looks after itself, charging by day and pulsing off the battery around the clock. Modern units carry days of reserve for cloudy weather. You pay more upfront for the same joules, which is the entire trade-off: money in exchange for not needing a power point.
The third option: battery
Battery-only units run off a rechargeable battery you top up yourself. They suit temporary fencing, strip grazing and short seasonal jobs, where the fence moves too often for a permanent power arrangement. As a permanent boundary solution they are the worst of both worlds, because the recharging routine is yours forever.
The rule of thumb
Mains if you can, solar if you cannot, battery only when the fence itself is temporary. Browse mains and battery energisers or solar energisers, and if you are genuinely torn, email us a photo of the paddock and where the nearest power point is.