Calculate savings from/cost-effectiveness of installing a tankless hot water heater?
You get the bast cost saving when you have to replace your tank or boiler anyway for another reason.   Remember that for some of the year, the lost heat from the tank may be of value to you depending on how the tank is sited and how cold and long your winters are.

Also are you having to use more air con due to the tank, if so the air con savings may be well worth while.