Picking a Water Heater: Solar vs. Electric or Gas Is Just the Beginning

As we build more energy-efficient houses, particularly when we go to extremes with insulation and air tightness, as with Passive House projects, water heating becomes a larger and larger share of overall energy consumption (see Solar Thermal Hot Water, Heating, and Cooling). In fact, with some of these ultra-efficient homes, annual energy use for water heating now exceeds that for space heating—even in cold climates.

So, it makes increasing sense to focus a lot of attention on water heating. What are the options, and what makes the most sense when we’re trying to create a highly energy-efficient house?

Solar-electric vs. solar-thermal water heating

If we had built our new house three or four years ago, I suspect that solar water heating would have been included—or at least very seriously considered. But as costs of solar-electric (photovoltaic or PV) systems have dropped in recent years, more and more energy experts are recommending electric water heating, rather than solar thermal, and using PV modules to generate the electricity—so it’s still solar water heating, but not as direct.

That’s what we have done at our new place. We realized in our planning that we had a great location for PV modules on our barn roof, but we didn’t have a good rooftop location for solar panels on the house. PV panels can be located farther away from where the energy is being used than can solar-thermal panels, because electrons can be easily moved fairly long distances through electrical cables, while piping runs for solar-thermal systems have to be much shorter.

Also, PV systems also don’t have any moving parts to wear out or that require maintenance; freeze protection isn’t a concern; and pressure build-up from stagnation in full sun (if a pump fails or during a power outage) can’t occur. So PV systems are very attractive from a long-term durability standpoint.

And if we’re generating our electricity from the sun why not use some of that electricity for water heating? That’s what we decided to do: install a PV system and heat our water with electricity.

Electric water heating

So if one goes with electric water heating, what are the options? There are three primary choices:

1. Conventional storage-type electric water heater. This is an insulated tank that holds 30 to 80 gallons, typically, and includes either one or two electric-resistance heating elements. Better storage water heaters have more insulation, so less stand-by heat loss occurs.
2. Tankless water heater. A tankless, sometimes called on-demand or instantaneous, water heater heats the water as it is used. This offers the advantage of eliminating the stand-by loss that occurs with storage water heaters. Whole-house tankless water heaters are most commonly gas-fired, but electric models are also available.

   The problem with the latter is that they require a huge amounts of electricity. An electric tankless water heater large enough to supply a shower and another use at the same time will require a 60-amp or larger circuit at 220 volts. If a lot of homeowners were to switch to whole-house electric tankless water heaters, it would put a huge burden on the utility companies that have to meet peak demand—particularly in the morning when a lot of people are showering.

   There are other issues with tankless water heaters, including that they don't necessarily save energy.

3. A heat pump water heater extracts heat out of the air where the water heater is located (typically a basement) to heat the water. Because the electricity is used to move heat from one place to another instead of converting that electricity directly into heat (as with electric-resistance water heating), the energy yield per unit of electricity input is much greater.

   We measure that efficiency as the “coefficient of performance” or COP—a COP of 1.0 is, in essence, 100% efficient at converting electricity at your site into heat. Most heat-pump water heaters have COPs of 2 to 3, meaning that for every unit of energy consumed (as electricity), at least two units of energy (as heat) are generated.

   (Note that if we consider “primary” or “source” energy instead of site energy, energy losses during power generation reduce that effective COP considerably.)

   

Choosing a heat pump water heater

Heat-pump water heater.

A heat-pump water heater is what we decided on for our house. We installed a 50-gallon GE GeoSpring model and, so far, we’re very happy with it. The GeoSpring is currently available only in a 50-gallon size, though rumors suggest that a larger, 80-gallon, model could be introduced. Because water heaters operating in heat pump mode take a long time to heat water, larger tanks typically make sense. If our two daughters were still in the house, a larger heat-pump water heater would have been more important.

One of the factors that attracted us to the GeoSpring is that it’s now being made in the U.S. GE had made its first-generation GeoSpring in Mexico, but moved that production to the U.S. two years ago.

The GeoSpring doesn’t have the highest performance of any heat pump water heater on the market, but the GeoSpring costs a third as much as the most efficient model. It’s also quieter. (See our GreenSpec section on heat pump water heaters for detail on all the most efficient models available.)

Next week, I’ll say a little more about heat-pump water heaters, including some issues with placement and implications of the fact that heat pump water heaters cool off the space where they are located—depending on the season, that can be an advantage or disadvantage.

Understanding heat-pump water heaters is important, as they will soon become the standard at least for larger electric water heaters—based on efficiency standards that take effect in mid-April 2015.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.