The only thing you will need to know is how many watts your particular energiser draws and the price/kilowatt charge from your particular electric utility company. This is assuming that you have a standard mains unit that plugs into a standard 220V outlet.
The basic equation is: watts x time / 1000 = kWh
Watts = The watts per hour consumed by the electric fence energiser during operation. This is probably not printed on the cover of the charger, but is usually on the box or the instruction manual that should come with it. If you don’t have the box or manual, then call the manufacturer and they should be able to tell you.
Time = The amount of time the energiser is operated. This should be calculated into hours per day and then days per month. Assume you will be running your energiser on a continuous basis.
1000 = Dividing by the number 1000 places the total into kilowatt-hours, which is what most utility companies use as the rate of consumption.
The power consumption of most energisers for agricultural / livestock control purposes will range from 1 watt up to 25+ watts. Operating a 4.5 watt energiser (a medium sized EquiStop M3) continuously for 24 hours per day for 30 days at a utility company charge of 10 pence per kilowatt-hour will equal:
4.5 watts x 24 hours per day x 30 days / 1000 = 3.2 kilowatt-hours (kWh)
3.2 kWh x 10 pence = 32 pence/30days
So £0.32 is the cost of operating the 4.5 watt energiser for 30 days at this rate, or a cost of £3.84 per year. Now you know how much your electric fencing system is costing you to run.