Is there a calculator or good method to determine the correct heater size in watts for a shower? In this case I need one for a flow of 20L/min. Hafele says a heater tank is required for that much flow but I would prefer a regular demand heater if there is one that could do the job.

 

Highschool physics job

What worst case delta-T (outgoing water temp - incoming water temp) do you need?

20L / min = 0.33 kg / s

Specific Heat Capacity of water is 4,200 J / kg /C

1 Watt = 1 J / s

Then go for the next larger unit.

As Hafelle say you will be constrained by the "maximum flow rate" of your heater, but adding a mixer tap to the outlet allows you to use a higher delta-T (hotter water, less flow) and cool it down to the desired temp in the mixer.

EDIT

Let's assume you need a delta-T of 15C (20C in 35C out)

4200 * 0.33 * 15 = 20,790 = 21kW a BIG beast for Thailand.

You could get that as a 3-phase heater if your supply will take it, but I would look at LPG heaters or a big tank (which heats up slowly but won't send your meter into orbit).

EDIT2

The biggest shower heaters for UK domestic use are about 11kW (47A @ UK 230V).

 

Why do you need 20 liters per minute? You can get a good shower with 8 lpm and the right shower head.

20L/min is the spec for the shower hardware. It is normal practice to match the heater to the allowable flow. The amount of flow selected is valve controlled to personal preference. The only inline heaters over 12000 watts I have found are 3 phase which I don't have. I don't like heater tanks as much; they run can out and take up more space.

 

If you REALLY need 20L/min, don't have a suitable power supply and don't want a tank heater then the only solution is a gas heater.

 

EDIT Rinnai do several sizes of gas water heater, this one should meet your needs http://www.rinnaithailand.com/reu-vr1620wg-asn-5/

 

A gas heater like that seems to make so much more sense than electric in this case--appreciate that and it's strange none of the stores have recommended it. Would it be possible to use a single unit such as this to run hot water to everywhere needed in a house vs installing individualized units? That would make a compact solution though for further away taps extra time would be needed to bleed off the cold water in the pipes before the heated water arrives.

 

Yup ^^^, the gas heaters are multi-point, run as many taps / showers as you want off one unit.