Electric shower - what rating?

What shower rating gives optimal balance between efficiency and running cost? My current shower is an 8.5kW unit and while output is moderate it performs reasonably well. For my new house I am thinking of fitting bigger rated unit, 9.5 or 10.5kW. It says the bigger the wattage the greater the flow, so in theory you cut down on the length of time the unit is on. But surely it will use more electricity per shower? And will it not use more water because of bigger flow rate (water at my new property is metered). I'm getting confused - can't work out the equation between electricity cost, water charge and comfort/efficiency. Can anyone point me in the right direction?
Alec
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However even a 10.5 has problems raising bitter cold winter water to a nice temperature at a decent flow rate- it's the price you pay for this sort of shower.
The power rating makes no difference, it's just that higher power can get more water hotter, but it's how much flow and how hot you choose that dictates cost, but a smaller shower will only get cold water up to a decent temp at a dribble.
You will always wish you have more power in winter
Mike R
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Hi Alec,
A higher Wattage will give you an increase in water flow rate, due to the fact that the water heats more quickly in the higher rated shower, so you will be able to turn the flow rate up a bit to increase the output at the shower head.
If you can imagine, for example, a normal kettle with a 1.5kW heating element. The water in the kettle will take, lets say, 5 minutes to boils. Now put a 9.5kW heating element in the kettle and it will boil in, say, 1 minute. Well the same principle is happening inside the heating tank in a shower, therefore, less time to heat means increased flow rate at the outlet.
Using the above example, you'll see that the increase in water flow rate will give a more powerful shower, and, if the same principle could be applied, it should rinse you down quicker. :-)) No, I don't think that's what does happen either.
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