Exactly how efficient they are is determined by how much hot water is used but here is a break down.

Homes that use 41 gallons or less of hot water daily, they can be 24%–34% more energy efficient than conventional storage tank water heaters.

Around 86 gallons per day, they can be 8%–14% more energy efficient.

Even greater energy savings of 27%–50% can be achieved if you install a water heater at each hot water outlet.