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Lime Scale reduces the efficiency of electric water heaters ?

At first sight it seems reasonable. Limescale clogs up machinery and covers heating elements and other things, so you would expect it to reduce efficiency. Don't get me wrong; I know that scale CAN reduce the efficiency of most types of equipment, so if you are concerned about that and would like to save money, see Scale Watcher. With gas powered water heating, the efficiency is reduced by scale because the scale forms a barrier around the heat exchanger element and so the heat doesn't all get to where you want it, in the water. Crucially it's a heat exchanger, so energy which was going to heat the water doesn't get there and continues round the loop. Although it's tempting to think the same thing happens with electric water heating elements, there is something else to consider: the First Law of Thermodynamics. Or, to put it another way, energy can't be destroyed, so where does the supposedly wasted energy go? With any of these energy-wasting matters, the answer to where the waste energy goes is: HEAT. But that's what you were trying to create in the first place. The electricity turns to heat, and so even if the scale covers the electric heating element, it doesn't reduce efficiency! Although, to be fair, it might shorten the life of the element.

The same is true of electric kettles. Even if the heating element is covered with scale (or "fur" as it's sometimes known), the kettle will still boil the water at the same efficiency, because electricity converting into heat is incredibly close to 100% efficient. There is nowhere else for the energy to go!

Also see relevant items at the electrical shops where you can get kettles, washing machines, and other devices which as part of their functionality heat up water.