Renewable Water Heating Promises Even Bigger Savings for CA

Adding water heating to a model solar ordinance for California cities creates new opportunities for deep carbon cuts.

The Golden State is making promising headway toward a remarkable goal: making all its new residential buildings zero net energy—which requires that newly constructed homes produce as much energy from rooftop solar panels as they consume each year, and slash harmful carbon dioxide (CO2) emissions in the process. While this is an important step toward reducing carbon pollution from energy use in buildings, it does not address carbon emissions from natural gas-fired water heaters that produce hot water in most of California’s homes and offices.

But using super-efficient electric heat pump water heaters powered by clean solar power would translate to big carbon, energy, and dollar savings for California. 

To help move in the direction of zero net energy, the California Energy Commission (CEC) is developing the next version of building energy efficiency standards that will go into effect in 2020 and is proposing that every new building include solar photovoltaic (PV) panels unless it is unsuitable for that option (e.g., shaded by trees, in the shadow of a larger building, etc.). The CEC is also proposing a model solar ordinance for leading cities that want to lead on climate protection by adopting this rooftop PV requirement earlier than 2020.

Facilitating local leadership is an important step toward a zero-net energy and low-carbon future. It creates a pathway for early adopters to pave the way for the rest of the state, and eventually the nation, to follow suit.

Time for “renewable water heating”?

CEC’s proposed ordinance focuses on offsetting the electricity use of homes, but roughly half of carbon dioxide (CO2) emissions associated with energy use in California’s buildings result from burning natural gas in furnaces and water heaters. What can we do about those emissions from natural gas?

This is where “renewable water heating” comes in. NRDC is recommending an add-on water heating ordinance that will give cities more options as leaders look to cut climate pollution, reduce energy use, and save money for utility customers. We made our suggestion in comments filed with the CEC earlier this month.

Choices for buyers of new homes

NRDC’s proposal—which is supported by a broad coalition of cities, utilities, energy professionals, and clean energy advocates—would offer three options to buyers of new homes:

  1. Using a super-efficient heat pump water heater (HPWH) powered by solar electricity. HPWHs, which work like air conditioners in reverse as explained in my earlier blog, are far more efficient and less polluting than even the more efficient tankless (a.k.a. instantaneous) models. When you power the HPWHs with solar energy, the savings become more dramatic. Over a 30-year period, the average California consumer would save 13 percent on water heating energy costs, reduce energy use by 30 percent, and greenhouse gas emissions by 50 percent or more with a HPWH compared with a gas tankless heater.
  2. Using a solar thermal water heater (a collector on the roof that heats water directly from the sun and then stores it in a tank inside the building), combined with a backup gas or electric water heater to complement the thermal system when occupants need more hot water than the collector alone can produce, particularly in the winter season. The solar thermal system would have to produce at least 60 percent of the home’s annual hot water needs.
  3. Using a standard gas tankless water heater but saving energy and emissions through more efficient walls, attics, and windows instead.

This menu of options is designed to give builders and new home buyers flexibility while guaranteeing energy, carbon, and bill savings. However, we estimate the HPWH + solar option is likely to be the lowest cost because it takes advantage of cheaper solar electricity generated onsite rather than grid electricity to power the HPWH.

Only two to four additional solar panels are necessary to produce enough energy to power the HPWH depending on household size, and the equipment and installation costs are roughly the same as for the gas tankless heater it replaces. This gives cities an opportunity to provide additional savings for residents and significant cuts in emissions.

Electric water heaters have the additional advantage of being able to act like a thermal battery. When solar power production is at its peak during the day, the water heater can absorb cheap solar energy to heat the water and store it in the tank for later use, delivering hot water to consumers without incurring higher peak-demand electricity costs. All that is needed is a smart control module that schedules water heater operation (like a smart thermostat), or even better, allows it to respond to grid signals to better integrate renewable energy into the grid and help California achieve its ambitious renewable energy objectives while saving consumers money.

Cities that choose to adopt the add-on water heating ordinance can ensure that new homes get highly efficient electric heat pump water heaters that cut greenhouse gas emissions, reduce energy use, and save money on bills. By adding on a renewable water heating option to its proposed model solar ordinance, CEC can solidify California’s commitment to renewable energy, achieve significant cuts in climate pollution, and save a substantial amount of energy across the state. 

About the Authors

Pierre Delforge

Director of High Tech Sector Energy Efficiency, Energy & Transportation program

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