While there are a number of aftermarket suppliers of 'built for the purpose electric water pumps for Chevalays, there are none that i know of for the A series Datsun, so i assume that we are talking about an electricly driven original type pump. That means an electric motor driving the pump via a deive belt, [V type or Gilmer], or perhaps even direct.

If the electric motor draws, say 6 amps as previously mentioned, then that current has to come from somewhere. The battery? Good answer, but since the battery is only a storage device, it needs to be kept charged by something,... & that's the altenator. It takes power to drive the altenator when it is producing current, & that power comes from whatever is driving it. In a car, this is usually the engine.

When you have an electrical load, like lights, aircon, radio, whatever, the load is buffered by the battery, but is carried by the altenator, which passes it on to the engine. Horsepressure saved,... i don't think so, but the electricly driven pump will flow more coolant at lower engine speeds, & if this is what you need, then go for it, but forget the horsepower saving theory, you get nothing for nothing.