Time for a shower and look what I found. Never seen anything like it. It doesn't take much to figure out it's an on-demand water heater, but every demand system I've ever seen uses gas and takes up about three cubic feet. This thing is built in to the shower head and runs on the 220 mains.
I turn on the water, flip the switch on the wall, and two seconds later have a perfect shower. Well, the water jets are needle-fine and there isn't much water, but it's enough and I know I'm not going to run out. I'm impressed.