An electric fluid heater includes a body having a fluid inlet and a fluid outlet and defines a fluid passage between the fluid inlet and the fluid outlet. At least two heating assemblies are disposed in the body and arranged in parallel, each heating assembly including at least two electrodes configured to heat fluid by passing alternating electric current through the fluid; wherein the at least two heating assemblies are arranged in the body so that fluid flowing through the fluid passage flows simultaneously through the at least two heating assemblies. Corresponding heating methods and heating systems employing such heaters and methods are also disclosed.