Scale decreases electric water heater efficiency?

[Semipro]

I worked on my electrical resistance hot water heater last weekend in a constant battle with hard water. As I inspected the mineral-crusted heating element I recalled that its widely held that scale reduces the efficiency of electric water heaters.

My question is how? Electrical resistance heating is about 100% efficient and any inefficiencies that do occur would do so as heat (which is the end goal) so how does the coating of minerals on the element decrease efficiency?

Getting a little deeper into the weeds I can see how the efficiency of my GeoSpring heat pump water heater might decrease with mineral buildup on the inside of the tank since the HP condenser coils wrap around the outside of the tank. The mineral build up would act as insulation and force more of the heat out through the outer tank insulation.