When I was in England (in the USAF), one house we were in had a system with a small (probably 10 gallon) tank-type hot water heater for the sinks and washer, and a wall mounted on-demand unit next to the tub for it and the shower. This unit was about 12"w x 18"h x 4"d. and had an on-off switch and a temperature dial. When you flipped the switch you got almost instant water at the temperature dialed in, forever. I've seen similar point-of-use heaters on Amazon, but as stated in other replies, they only advertise a temperature rise above inlet. With England's temperate climate, inlet temps didn't vary enough to really matter. With utility costs in Europe, it seemed to be an economical way to get the best of both worlds.