Most Efficient Tankless Water Heater

water heater
a heater and storage tank to supply heated water
Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both hot water and water heated to steam have many uses.
An appliance for supplying hot water for purposes other than space heating or pool heating.

efficient
(esp. of a system or machine) Achieving maximum productivity with minimum wasted effort or expense
effective: able to accomplish a purpose; functioning effectively; "people who will do nothing unless they get something out of it for themselves are often highly effective persons"-G.B.Shaw; "effective personnel"; "an efficient secretary"; "the efficient cause of the revolution"
being effective without wasting time or effort or expense; "an efficient production manager"; "efficient engines save gas"
(of a person) Working in a well-organized and competent way
(efficiently) with efficiency; in an efficient manner; "he functions efficiently"
Preventing the wasteful use of a particular resource

tankless
Refers to products with no storage capacity that utilize a heat exchanger or burner system to heat water as it passes through tubular heat exchanger.