Most Efficient Electric Heaters

electric heaters
(electric heater) a small electric space heater
Are most common on portable spa systems.  They are a resistive type heater that heats the water as it  flows over the element.  Water flow is critical to proper operation. Without flow, if the heater should come on, it will burn up very quickly and can even damage pipes.
(Electric Heater) A furnace is a device used for heating. The name derives from Latin fornax, oven. The earliest furnace was excavated at Balakot, a site of the Indus Valley Civilization, dating back to its mature phase (c. 2500-1900 BC).

efficient
Preventing the wasteful use of a particular resource
(efficiently) with efficiency; in an efficient manner; "he functions efficiently"
being effective without wasting time or effort or expense; "an efficient production manager"; "efficient engines save gas"
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"
(esp. of a system or machine) Achieving maximum productivity with minimum wasted effort or expense
(of a person) Working in a well-organized and competent way