My new immersion heater control shows power used today, yesterday or last 7 days, and it shows just under 4 kWh, in the past always used the oil boiler, as told oil cheaper than electric, it did not keep the water as warm as the immersion heater, but did heat more of the cylinder, so the 20 kW boiler would run 4 times a week for half an hour a time, however it did not burn for full half hour more like 20 minutes, when the return water would get too warm with the C Plan, so seems likely used around 26 kWh per week with oil. So used six and a half times the energy using oil.
Now I know oil is cheaper than electric, but not that much cheaper.
I would think if using more hot water than the difference between the two would be less, but until I got solar power I never even considered using electric to heat DHW as so many people had told me gas and oil were cheaper.