Immersion heater running costs?

[WRCC]

I'm in much the same position as the OP; I heat the water from the oil fired heating system in winter and then go over to an immersion heater in summer. The heater is on a segmented timer, each segment setting it on for 12 minutes. With thermostat set at just below 60C, I use 4 segments ON (48mins) and think I have a pretty generous quantity of hot water for the one of me. For sure, the insulation on the tank is ultra important in keeping the costs down.