I've seen them used /installed on "this old house" the pbs show.

But not personal knowledge of them.
Did you google /search for prices and comparisons of brands/efficiency
that kind of stuff.

Is your water heated by the oil heater/furnace??
most are gas or electric usually.

is everything insulated - pipes tank etc.

on most electric water tanks is is recommended to drain the sediment out of the bottom of the tank every so often you will see if it is nasty or not.