This post was written in collaboration with Anna Brockway from the Energy & Resources Group at UC Berkeley.

Using high-efficiency electric heat pumps instead of gas for residential heating needs in California could cut greenhouse gas (GHG) emissions in half or more, according to a new NRDC analysis published in the Electricity Journal. This makes heat pumps an important tool to help achieve California’s ambitious goals to cut GHG emissions 40 percent by 2030, achieve carbon-neutrality by 2045, and improve air quality in its urban areas—which rank among the most polluted in the country.

Annual emissions from water heating technologies (without load management). Each technology name includes its efficiency or annual coefficient of performance (COP).

We found that, over a full year of use, electric heat pumps cause significantly lower emissions than either gas or electric resistance technologies. Switching to a heat pump water heaters (HPWH) could reduce emissions between 50 and 70 percent per household annually, depending on the efficiency of the gas technology they replace. Similarly, switching from a gas furnace to a high-efficiency air-source space heating heat pump (ASHP) could reduce emissions between 46 and 54 percent annually per household.

This analysis comes amid two recent reports highlighting the extremely urgent need to address climate change nationally and internationally. Buildings account for about a quarter of California's greenhouse gas emissions, and more than half of those come from the energy used for space and water heating as roughly 90 percent of furnaces and hot water heaters are fueled by gas or propane. The new analysis also follows a recent Synapse study that shows that clean space and water heating technologies such as highly efficient heat pumps have the potential to save Californians more than $1,500 upon installation in the best conditions, and hundreds of dollars on annual utility bills afterward.

A heat pump uses electricity to run a compressor that collects and concentrates heat from its surroundings. That heat is then used to warm an indoor space or hot water in a tank. Since the electricity is used to move heat around, not create it, the heat pump delivers energy that is two to four times greater than the energy required to operate it. This impressive performance dwarfs that of gas heaters (which turn only 60 to 95 percent of their fuel energy into heat) by a factor of three to five times.

It makes sense that using heating equipment that is far more efficient than conventional gas equipment, and powering it with California’s increasingly clean electricity, could dramatically reduce overall emissions. However, to get the full picture, it’s important to consider two additional factors: the timing of electricity use and how much heat pumps sometimes operate in the less-efficient resistance heating mode.

Burning gas directly for heat creates the same amount of emissions no matter when it is consumed. But emissions from electricity vary over the course of the day. They’re higher in the evening during peak demand when power also must be supplied by fossil fuel power plants, and lower midday when demand is low and solar energy is abundant.

Therefore, the GHG emissions associated with a heat pump depend on what time of the day it runs.

While heat pump operation is extremely efficient, many HPWHs are hybrids: they operate in heat pump mode most of the time but automatically switch to electric resistance heating to meet large hot water draws or when the ambient air temperature is too low. Electric resistance heating is much less efficient—it “only” turns 100 percent of electric energy into heat—and is also subject to the emissions intensity of the grid at the time of use. The most advanced HPWHs can operate purely in heat pump mode.

To assess emissions, our analysis considered both the timing of use and the times when hybrid heat pumps operate in the less efficient resistance mode.

Electricity usage by heat pumps

To better understand how heat pump emissions vary depending on when they operate, we looked at hourly electricity usage patterns for HPWHs and air-source heat pump space heaters. HPWH hourly demand data was obtained from validated modeling studies performed by Ecotope, Inc. For ASHPs, we relied on usage patterns from Pacific Gas & Electric (PG&E) published by Energy + Environmental Economics (E3). In both cases, we compared data for California climate zone 12, which includes Sacramento.

For each hour of the year, heat pump electricity consumption was multiplied by the GHG emissions of the grid at that hour. This data, prepared by E3 for the California Public Utilities Commission as part of the 2018 Avoided Cost Calculator (ACC), estimates the hourly emissions intensity of electricity generation on California’s grid. We used the ACC’s projection for 2030 emissions because policies implemented over the next few years will influence heating equipment choices throughout the 2020s, and heat pumps installed under those policies will operate for another 15 to 20 years (between 2020 and 2045). Therefore, 2030 emissions are a reasonable assumption for average lifetime emissions of equipment affected by those policies. The 2018 ACC emissions data does not include the effects of Senate Bill 100 (De Leon) signed by California Governor Jerry Brown in September 2018, which increases California’s renewables target from 50 to 60 percent by 2030.

We superimposed heat pump usage patterns and grid emissions on the same chart to help visualize how the profiles coincide. HPWHs tend to consume the most electricity during the day when solar generation is highest, allowing them to use primarily clean electricity. ASHPs, on the other hand, primarily operate during the evening peak and the morning part-peak demand periods. This suggests that load management—i.e., incentivizing residents to pre-heat their homes before 6 a.m. and 5 p.m.—could minimize operation during peak times and lead to additional space heating emissions reductions in homes that are efficient enough to retain that heat for several hours.

Combining these usage patterns and emissions profiles across all hours in a year allows us to calculate the total annual emissions of these technologies.

Average annual electricity use, by hour, of two different heat pump water heating technologies is shown in blue. System-level emissions rates for 2030 are shown in orange on the secondary axis.

We then compared the emissions of standard efficiency and advanced high-efficiency electric heat pumps to gas and conventional electric technologies for space and water heating. Over a full year of use, electric heat pumps cause significantly lower emissions than either gas or electric resistance technologies, as noted earlier.

What about other states?

This analysis is focused on California because of the Golden State’s ambitious climate and clean air goals, and available heat pump usage pattern and grid emissions data. Nationally, the results would vary by state, as heat pump efficiency depends on local climate conditions, and grid GHG emissions vary by region. But as coal power plants get replaced by gas and renewable energy like wind and solar due to policy and economics, we can expect heat pump technology to also yield significant emissions reductions across the nation.

Pathway to near-zero emissions heat

This analysis did not attempt to assess the benefits of shifting heat pump operation from high-emissions to low-emissions times of the day, but our results suggest that this kind of load management holds promise to further reduce heat pump emissions by pre-heating and pre-cooling when electricity is cheap and clean, which avoids or minimizes electricity use when it is expensive and high-emissions.

As clean energy policy such as SB 100 and market forces continue to increase the share of renewable and carbon-free generation on the electricity grid, emissions will continue to decrease. Meanwhile, the efficiency of heat pump technology is expected to continue to increase as it has over the past decade. These trends will further reduce emissions, paving the way to a near-zero emissions heat and hot water future.

California recently passed two bills, AB 3232 and SB 1477, that aim to make that promise a reality.