The majority of homes have a storage tank for hot water 50 gallons or more that's regularly reheated and ready to use 24 hours a day. Tankless Water heaters are the alternative, often billed as the more efficient European system,a tankless, or on-demand water heater is a suitcase-size unit that heats water as it flows through a coil of pipes and makes hot water only as you use it.
These on-demand heaters are wall-mounted and smaller than tank-storage heaters, but space is rarely an issue. Either type can be fueled by gas or electricity, but the fuel source isn't key. Both use conventional burners and heating elements, so there is no new technology that makes onesystem better than another.
I was referred to Green Planet through a friend when I needed to move a washing machine - and all the hookups. I was quoted a variety of prices from other plumbers. Green Planet came through with a quote that was toward the bottom of the spectrum for price, but the service was top notch. The service was performed in the time quoted, and they left the place clean when they were done. I liked the experience so much that used them again and have referred several friends that way!