I used a 3kw to start with and looked for a 16amp option as 13 amp plugs would scorch and extension leads had to be unwound.
I changed to 2kw, made an extension lead the 'right length' and use a timer to turn the power off for half an hour every 2 hours.
I don't want very hot water - no more than 40 degrees in the tank as I use cold water brushes hoses and connectors.
I have fitted to four different tanks and always use a flat tank and a 27" fitted within arm's reach of the filling hole as low down as possible at least 100mm from any tank side or baffle.
Personally I did an immersion after a sharp winter in 2010/11 in Bristol for frost protection and supple hoses.
If you want hot water for first cleans, occasional cleans etc., I would stump up for oil fired.