The on-demand heater wastes no more water than a tank water heater if you get the correct model. You can get a tankless heater which has a "warm water" mode that keeps the water inside the heater at 105F. When the water flows the heater automatically turns on and heats the water going through it to the full hot water temperature. In use, you stop to set up camp, turn the water heater "on" in the 105F mode, and within a couple of minutes there is hot water. The only water that is "wasted" is the ambient temperature water being flushed out of the line that precedes the heated water - which is exactly how a tank water heater operates.