Here is one of those travel tips videos you should watch and probably think is not important. This video shows both how an on demand hot water systems works on most of the planet and a novel idea for hotels where they can put both the sink and the shower on the same water line.
Two types, there is the shower head, very common and this more expensive type shown in the video. The shower head type cost about 15-20 dollars and this one cost about 50-100 dollars. The gas ones I seldom find, my friend in the Platypus Bogota in Colombia has a gas one, I do not know how much they cost, I do not see them.
90 percent of the planet uses 220, so easy outside..
Normally they do not make enough hot water, you have to take great care and adjust, they do not hide them in the under-developed world. 99 percent are in the shower with me, this one is unusual because it is better, bigger and does the sink also.
I would say extremely cheap in most countries to have this type, just not the USA.
They do not hide under sinks, they are not hidden, I know exactly where my hot water come from, I am an expert on this. More expensive hotel will sometime hide, or worst yet try to use one for two or three room, this is truly a jerk move. These things need to be seen by the person taking the shower to make sure they are:
1. Turned on.
2. The reset button is reset
3. The light comes on.
4. The water entering the device is turned on.
5. The breaker switches or fuses before the thing are turned on.