On Sunday, 28 April 2019 13:36:27 UTC+1, David WE Roberts (Google) wrote:
(1) The maximum power output of a UK electric shower is around 10 kW. The
output of a small(ish) combi boiler is around 28 kW. So the combi should
be able to apply 2.8 times the heat to incoming cold water and so (I
assume) heat 2.8 times the amount of incoming water to an acceptable
temperature compared to the electric shower.

Yes.

(2) Electricity costs around 3 times gas (per unit of energy) which kind
of balances out. You can either have about 3 times the hot water in your
shower for the same price or save money each time you shower at lower flow
rates.

Yes.

I think most people would go for the more satisfying shower which does
reduce/eliminate the cost saving from using a combi boiler.

Possibly, but the saving is only eliminated if you are using 3x the heat. You might in winter, but for the rest of the year only use 2x the heat. That would be a better shower than electric, and still a bit cheaper.


I realise there are all sorts of minor (hah!) constraints but if you had a
100 amp incomer then is there any reason that you couldn't (with some very
thick cabling) take 60 amps for a shower giving you 15 kW of toasty
goodness?

(a) they don't make 15 kW showers [1]
(b) you'd have to get permission from (or at least notify) from your DNO if you got one

I do note that taking all the output from a 100 amp incomer still gives
you less heat input than a combi boiler.
3 phase electric shower, anyone?

3-phase common in European countries, but max demand is usually more limited than here.

[1] But you can get 3-phase instantaneous water heaters eg

https://www.aquahot.co.uk/stiebel-el...er-heater-27kw

https://www.stiebel-eltron.co.uk/en/...r-heating.html

"45% of new build properties in the UK are city centre flats and apartments.. For most of these electricity is the preferred fuel. And for many years the default solution for hot water for each new appartment has been an unvented cylinder heated by 2 x 3kW electric immersion heaters. By its very nature this solution takes up a lot of space.
... 3 phase electric supply, which has not traditionally been run to individual domestic units in the UK.
But market conditions in the UK are now changing. Space is more than ever at a premium. Instantaneous water heaters on a 3 phase supply give the kind of powerful, limitless hot water supply that young city dwellers look for.
New build apartments have 400V provided to each floor in the riser. Currently this is then split down into a phase per apartment. By taking the 400V to each apartment the instantaneous solution becomes possible."

(Would need 3-phase metering though.)

Owain