Martha's post reminded me of a couple of propane water heaters i just saw 
in a solar power/off grid electrical catalogue. They are on demand 
propane or natural gas fired, come in three sizes, here are the stats on 
the 2 smallest units...

Paloma brand

model: PH-6
rated input: 43,800 BTU
hot water 38 C rise: 0.56 gal
min. volume 38 C rise: 0.56 gal
size(HxWxD): 9.6"x10.2"x10"
weight: 20 lbs
flue diameter: 4"
application: kitchen/bathroom sink, water saving shower, power and sailboats.

Bosch brand

model: WR-125
rated input: 43,800 BTU
hot water 38 C rise: 0.5 gal
min. vol. 38 C rise: 0.5 gal
size (HxWxD): 25.4"x10.6"x9"
weight: 25lbs
flue diameter: 4"
applications: water saving shower, bathroom/kitchen sink.

Both units have piezoelectric ignition and safety shut-offs.
I think the hot water rise data means that the units will raise the 
incoming water 38 C (at a flow rate of 0.5/0.56 gal/min).

sorry no prices.

alistair