Water heating accounts for about 10% of the energy consumption of the average American household - a significant amount that can be reduced by using a few basic low-energy principles.
But, what system to choose for a new home?
A gas-based system, or an electric system?
A condensing gas system?
A solar thermal system, backed up by a small gas or electric system?
A heat pump water heater?
An on-demand or a tank-style gas-fired system?
A heating boiler system?
A solar electric PV (photovoltaic) system?
Solar water heaters
Solar water heaters are a well-tested technology. 90% of the Israeli homes have them in their rooftops, and there are between 30 and 40 million solar water heaters installed in Chinese homes.
The EESI (Environment and Energy Study Institute) estimates that there are about 1.5 million solar water heaters in use in the US and at least 29 million American homes homes with good solar resources.
Solar water heating is now a competitive option in moderate and hot climates, even without state incentives. The payback period is estimated at around 5-10 years for these climates.
Location and orientation to the sun, the tilt angle, the workmanship, the insulation, the type of system and some installation and design issues are very important for the efficiency of solar water heaters.
Though there are now sophisticated systems especially designed for colder climates, their performance and cost-effectiveness vary significantly, depending on factors like solar irradiance and local prices.
Paybacks periods are often too long in colder climates. See, in this issue: Solar water heater prices and paybacks.
Solar electric water heaters
Solar electric water heaters systems – involving common electric water heaters or new heat pump water heaters, powered by photovoltaic systems (PV) – are becoming an interesting choice in some cold-sunny climates.
The recent decline in PV prices are making this option increasingly appealing in regions where traditional thermal solar heaters are expensive.
High-Performance (Condensing) gas water heaters
High-performance condensing gas water heaters may consume half of the power of traditional gas units. So, if building a new home in an area or site with poor solar resources, they can be a good alternative... as long as you have a large water heating load.
Otherwise they are too expensive.
Air and ground heat pumps
Likewise with American-style high-performance gas heaters, heat pump water heaters make more sense (or only make sense) in houses with a large water heating load.
They consume a fraction of the electricity of electric-resistance water heaters, but the best units are expensive and are also more prone to failure than traditional electric water heaters.
Besides, since heat pumps are designed to extract heat from the air, they also cool the rooms where they are installed, which is a negative feature in the winter (Japanese models may overcome this problem: their compressors are installed in an outdoor unit).
Geothermal heat pumps (also called ground-source heat pumps, because of their loop buried in the ground) are also a way of generating hot water at low prices. They can be equipped with a device (a desuperheater) to produce hot water. But they are very expensive (they cost a few tens of thousands of dollars, typically more than $40,000) and do not make sense in energy-efficient houses, properly designed.