There are those 10l tanks with an element in which can feed 2 outlets, say a kitchen sink and a bathroom sink. Washers are cold fill now anyway and in a small flat you might just fit a cubicle with an electric shower. Cheap air to air heat pumps for heating. I considered this for a flat conversion project I've just finshed. Wish I'd done it now. Would have saved thousands compared to putting gas supplies on and wet plumbed heating systems.

>Has anybody come across a good electric instant water heater that is suitable for the small home?

I've allways wondered why it's thought a smaller house needs less instant hot water? Is the shower head any smaller? Taps any slower? :-) What's meant by "good"?

The amount of power needed to heat up a gram of substance by one degree is called it's "specific Heat". It's a constant. A good heater will do that about as well as a bad heater if they are the same power. Perhaps not as reliably but thats another matter. It's not hard to work out how much power you need to provide a particular flow rate at a particular temperature rise. That used to be an O-Level physics question in my day at school....

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html

"The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance"

1L = 1000g
1 Joule per second = 1 Watt
so
1L/sec = 4186 Watts per degree C rise
or
1L/Min = 70 Watts per degree C rise

So if the water comes into the house at 5C in winter and you need to heat it to 40C (perhaps too hot?) then that's a rise of 35C

So..

1L/min = 70 x 35 = 2.4KW
or
10L/min = 24KW

To put this in context.. some countries are trying to steer consumers towards shower fittings that only use 7L/min to save water. If we took that as a target it means a heater of around...

16KW for a 40C rise
12KW for 30C rise
8KW for a 20C rise.

So the answer to the question is... How much water and how hot do you want it? Try measuring the flow rate and water temperature from a tap using a bucket and stop watch. Plug in the numbers and you get an answer in KW for your heater.

Say you have a tank of water that is heated by solar and a back boiler on a wood stove, is it better to have an electric immersion within the tank for back-up (say if didn't get round to lighting the stove in winter) or is it better to feed the water from the tank through an instant electric water heater to bring it up to temperature if necessary?

Chris W - Depends what you mean by better? Assuming you mean better for the environment.... I think it's more eco friendly not to store electrically heated water. (obviously storing solar heated water is OK). I think you could start with an immersion heater then if it turns out you need to use it a high percentage of the time, switch to an instant water heater. It should be possible to design the plumbing of a new house so that a instant heater can added later. Need to ensure there is just one hot pipe coming off the tank and that it goes to where the heater would be mounted before branching. Plus an electric supply.