Hot Showers and Cool Rides: Wind, Sun, and the Duck Curve


As we look out over the power sector transformation that is unmistakably upon us, there are those who warn of the sacrifices, the risks, and the costs that could come with more and more intermittent electricity generation—wind and solar energy, for example—on the grid. They show us the “duck curve” that suggests that balancing supply and demand is more challenging with more stop-and-go power.

However, in our recent work Teaching the Duck to Fly, we point out that the net load curve can be flattened, with simple strategies and with positive results.

Let’s look at just a couple of strategies on the customer side, starting in America’s basements.

Across the country, there are about 45 million electric water heaters. With active control systems that are available today, these heaters can heat water when electricity is plentiful and cheap (maybe from 10 am to 3 pm when the sun is high, or from midnight to 6 am when demand is low) and hold it for hours, so it’s ready to supply a hot shower after an afternoon soccer practice—during the electric system peak. Then, late at night, the heater can charge again when there is surplus electricity on the grid. Maybe it’s wind, or nuclear—but there’s plenty of it at 2 am.

Operating on schedules like this, the water heaters can be controlled to reduce peak demand and to augment demand at slack times. If we controlled all of them, we would reduce peak demand by about 22,000 MW. (With the average new coal plant at 500 MW, that’s about 44 coal plants worth).  And we could find a ready market for off-peak power. Companies such as Steffes Corporation, Sequentric, and Power Over Time are already deploying this technology today on water heaters in dozens of utility service territories, but this only scratches the surface of the full potential.

But wait—there’s more.  We can convert many of these water heaters to high-efficiency heat pump models that use about one-third as much electricity. This won’t work everywhere—heat pump water heaters are generally not a good choice for apartments or mobile homes—but probably half of the electric water heater fleet across the United States are candidates for this conversion. That’s 22 million water heaters.

Because a conventional electric water heater uses about 4,000 kWh/year and a heat pump unit about 1,500 kWh/year, each of these more efficient hot water heaters could free up about 2,500 kWh per year of electricity per household. That’s essentially “found” electricity. So what do we do with this new energy from 22 million converted water heaters?

Let’s move to the garage.

Guess how much the average electric vehicle uses in a year? That’s right, about 2,500 kWh, the same we save by improving each hot water heater. If we did half of the water heaters, that would free up enough electricity to supply 22 million cars. But there is even more value here.

Electric cars (like all cars) are normally just parked most of the time. Like the hot water heaters, they can be controlled to pull electricity when power is plentiful (often in the middle of the night, and increasingly in the middle of the solar day).  The necessary controls are already installed in all new electric vehicles, making them perfectly suited to take advantage of low demand times. That means they, too, can help contend with the duck curve, staying off the grid during peak periods and soaking up power when the sun is shining, the wind is blowing, or people are sleeping.

So, first we reduce peak demand by controlling water heaters. Then we add a market for off-peak power. Then we conserve a big part of the water heating load, and use that power to displace gasoline. And we control the electric car charging, so that it also is done at low-cost hours.

What all of this illustrates is that households can effectively cash in wasteful hot water heating for cleaner, cheaper mobility. And what this also means for power sector transformation generally is that these particular fixtures of life in America—hot water heaters and automobiles—may be among the most important tools in managing demand on the grid during the course of the day, to adapt to more and more intermittent power. They help flatten the “duck curve.”

So, what big-picture, power sector conventional wisdom does this all challenge?

  • It challenges the idea that the system can’t handle large amounts of variable energy and that the system is less reliable with more renewables. (We just need to direct that power to suitable tasks.)
  • It challenges the notion that the grid needs massive investment in new distribution to serve solar and to serve electric vehicles. (We can do that with the grid we have today if we manage it carefully.)
  • And it challenges the belief that to add renewable energy, we need to also add gas peaking plants to back up the wind and solar. (Remember the “found” electricity?)

So even in an era of change, we can have hot showers and cool rides. In fact, modern hot water heaters and electric cars may be a key part of a less costly, less risky, more reliable, and cleaner grid.