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Electric Heat Pumps Can Slash Emissions in California Homes

A new analysis shows that high-efficiency heat pumps instead of gas furnaces could cut greenhouse gas emissions in half

Switching from gas-fired appliances to high-efficiency electric heat pumps could cut greenhouse gas emissions significantly in California, a new analysis finds. (Photo: Dennis Schroeder / National Renewable Energy Laboratory)

Written in collaboration with Anna Brockway from the Energy & Resources Group at the University of California 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 Natural Resources Defense Council  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% by 2030, achieve carbon-neutrality by 2045, and improve air quality in its urban areas — which rank among the most polluted in the country.

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% 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 heat pump (ASHP) could reduce emissions between 46% and 54% 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% of furnaces and hot water heaters are fueled by gas or propane.

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

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.

Impressive efficiencies

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, a 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% 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% 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% 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.


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.


Pierre Delforge is a senior scientist with the Natural Resources Defense Council who works on building decarbonization. This post was originally published at the NRDC Expert Blog and is reprinted here with permission.


  1. jackofalltrades777 | | #1

    The gas industry will not like this report/article.

    1. JC72 | | #8

      More like the investors (Pensions, Mutual Funds, etc) in gas utilities who won't be happy.

  2. exeric | | #2

    Can you hear it? I'm playing the world's smallest violin.

  3. skierpage2 | | #3

    Timing heating to when renewable energy is available would seem to reward/require having a big storage tank you can fill with piping hot water from clean energy. That makes domestic hot water and space heating the same thing, and you can use the same big air-water heat pump for both systems, saving money and space.

    This isn't just my armchair idea, the "Application Guide: Residential HVAC and Plumbing 2016," created by Energy Code Ace for the California Energy Commission, says "Combined hydronic heating and DHW systems use a central hot water source, such as a boiler, to meet both domestic hot water and space heating demand. Depending on the heating and domestic water heating loads, it may be appropriate to consider a combined system to serve both."

    Yet such a system is pretty much unobtainable in California and has to be cobbled together. The Sanden CO2 system could do it, but Sanden has a lot of provisos in its technical info.

    There is a mismatch between what California Energy Commission and well-heeled environmental customers want and what California's HVAC and plumbers provide :-(

    1. Jon_R | | #4

      You can get close with a Chiltrix Air-To-Water Heat Pump plus a heat pump water heater (for summer use) + tank. Would be interesting to see someone do it (all space heating/cooling and domestic hot water with solar/off-peak electricity) as a proof of concept.

      1. GBA Editor
        Martin Holladay | | #6

        Skier and Jon,
        Of course it's technically feasible to put a large insulated water tank in every California house -- assuming the mechanical room is big enough -- and to tie that tank to a solar thermal system or a big air-to-water heat pump connected to a timer.

        The challenge isn't technical. The problem is that the reductions in greenhouse gas emissions aren't significant enough to justify the high hardware cost ($20,000 to $30,000 per house). If our goal is reducing greenhouse gas emissions, there are better ways to spend our money.

        1. Jon_R | | #7

          I don't disagree about it being not ready for prime time, but do about the cost. Chiltrix based systems can be similar in cost to other HVAC systems and a heat pump water heater is already common. So potentially no cost premium on these parts. A 5000 gallon tank costs < $2000 (plus a little insulation). That leaves what - some pumps, controls and maybe a water-water heat exchanger? As you say, quite technically feasible. Maybe only a $5K premium.

        2. skierpage2 | | #10

          The *requirement* is to eliminate fossil fuel burning in the next decade to keep global warming below 1.5°C. Bang for the buck matters in determining the best ways to get there.

          I assume a big insulated tank is cheaper than a second heat pump. But I was thinking of something like the HTP Versa-Hydro's 119-gallon tank with internal heat exchange coils minus its gas boiler, not Jon R's 5000 gallon tank (12' x 8' ?!?).

          I would greatly appreciate recommendations for a heating consultant in the S.F. Bay Area. I thought I had found a promising one but he's completely slammed with work.

          1. Expert Member
            Dana Dorsett | | #11

            >"I assume a big insulated tank is cheaper than a second heat pump."

            Not a good assumption. Big insulated tanks aren't cheap. Heat pumps aren't all that expensive.

            >"...HTP Versa-Hydro's 119-gallon tank with internal heat exchange coils minus its gas boiler"

            Sound a lot like the 119 gallon SuperStor Ultra:


            The 119 gallon SuperStor Ultra is north of $2500. The heat exchanger is a bit small though- not good enough to use as a "reverse indirect", where the potable water is in the heat exchanger coils, and would not be able to operate efficiently with a Chilltrix pumping water through the heat exchanger.

            That's about a grand more than internet store list price of a pretty good ultra high efficiency Mitsubishi FH09 or Fujitsu 9RLS3 mini-split, either of which has enough capacity to heat many CA homes.

            A Turbomax 119 is a true "reverse indirect" thermal buffer & water heater has a much bigger heat exchanger. It can use heating system water in the tank, at a lower, more efficient temperature for heating the thermal mass with a reversable chiller. But it runs about 4 grand, comparable to list price of a 2-ton Chilltrix:



            Truly large 1000 gallon +insulated buffer tanks are only "cheap" if they run at atmospheric pressure, without potable water. They get built into wood boiler heating systems all the time, but aren't necessarily appropriate for suburban CA.

    2. Expert Member
      Dana Dorsett | | #9

      >"... use a central hot water source, such as a boiler, to meet both domestic hot water and space heating demand"


      >"Yet such a system is pretty much unobtainable in California and has to be cobbled together."


      Gas fired wall hung combi-boilers are available though the big box stores all over CA. (Not that most of them are a great fit on the space heating end- usually oversized, but not all.)

      Condensing gas water heaters are designed with side ports suitable for running hydro-air handlers, too, which are easier to size correctly than gas furnaces in most of CA. Or is doing the basic matching of an air handler from a different vendor to the load considered "...cobbled together"? (C'mon this stuff is designed to work together!) Does a boiler have to be made by the same company as the fin-tube baseboard and indirect water heater, or is that also "...cobbled together"?

      If insisting on a heat pump, doesn't Daikin market their Altherma heat/hotwater/air conditioning system in CA anymore? If not, they SHOULD be going after it, given California's carbon emissions goals. It can be a VERY good fit for many existing CA homes.

      Sanyo used to have a nice CO2-refrigerant air source combi system available in Europe & Asia, but that was never marketed in the US. Sanyo was also part of the EcoCute consortium along with Sanden, but went ahead and designed the more complete combi system rather than "leaving it as an exercise for the reader" to design a system around it. It came in a 4.5kw (~15,000 BTU/hr) and 9kwh (~30,000 BTU/hr) versions. They sold a number of systems in the UK, but had a lot of competition from other air source heat pump vendors. OK, so these aren't now and never were available in California... Perhaps Sanyo was a decade or so ahead of the market on that one, or maybe they just got clobbered by the competition (including Daikin) in Europe.

      1. Expert Member
        Dana Dorsett | | #12

        >"I would greatly appreciate recommendations for a heating consultant in the S.F. Bay Area."

        This Bay Area company is advertising Daikin Altherma radiant heating & cooling solutions (which come with hot water):

        These people aren't cheap (are any contractors in the Bay Area cheap?), but team member Larry Waters has decent looking recent ducted Fujitsu mini-split solutions in his design portfolio:

        A 1.5 ton Fujitsu system that heated/cooled this house in Berekeley was his, at an installed cost (including ducts) of about $13K:

  4. CraigRo | | #5

    In our heating dominate zone 7 & 8, I'll stick with insulation as a means to reduce consumption/GHGs. Unfortunately, even at a 70% energy conversion rate for NG to BTUs, it still beats an electric HP dollar per dollar on my utility bills. Alaska has somewhat different limitations when it comes to energy reductions.

    As another sad note, a local study by the CCHI found adding 4 inches of insulation to a 2x4 stick home would cost $12-14K in 2013 (study done in Fairbanks). The energy savings and payback is in decades, not in a typical consumer's interest or timeframe.

  5. ToddMedema | | #13

    I've been really excited about heat pumps since first reading about them on this site, but after a lot of research, it seems like they don't make as much sense for colder climates (I live in Pittsburgh, PA). Their efficiency drops as you go below freezing, and they eventually stop working (the expensive Mitsubishi units I've been looking at operate as low as -15*F; unfortunately, Pittsburgh gets colder than that about once per year) - forcing you to install a full backup heating system that offsets any potential cost savings from heat pumps' better heating efficiency.

    So - go for it in California, where they make sense! But consider your climate first if you live anywhere colder.

    1. Deleted | | #14


    2. Jon_R | | #16

      Fujitsu mini-splits are reported to keep going. On the other hand, I don't think anyone can point to a document where Fujitsu says that operating below listed minimum temperature won't cause any harm (eg, perhaps reduced life due to poor lubrication).

      For example: " ... (XLTH) Series features outdoor condensing units engineered to operate in temperatures down to -15ºF". Which implies that they aren't engineered to operate below -15ºF.

    3. Expert Member
      Dana Dorsett | | #18

      The Mitsubishi units don't stop heating at -13F= that's just the lowest temperature at which they will specify it's capacity & efficiency. At some unspecified temperature that colder than -18F most of the Mitsubishi cold climate mini-splits will automatically turn off when it's cold enough the temperature sensing thermistor could have an error sufficient to interfere with operation, then automatically re-start when it comes back in range. In experience I've yet to find any Mitsubishi owners who reported that actually happening, even when it dipped a few degress below -20F, but it's in the fine print of the submittal sheets for all FH series.

      The all time record low in Pittsburg at the airport was -22F (on January 19, 1994), but it hasn't dropped below -10F in any year since that time. 1994 was the only time it dropped below -18 F in the past 70 years. Most years never even get down to -10F. See:

      You're pretty safe to specify Mitsubishi H2i equipment in Pittsburgh without "backup".

      1. ToddMedema | | #19

        Thank you Martin, Dana, et al! Good to know - I'd thought that they would totally cut out at -15*F, so they're back on the drawing board for hooking up to my forced air system instead of the traditional AC + furnace.

        Has anyone done research on the climate cross-over point between air and geo heat pumps? Geo seems to have several advantages for colder climates (better efficiency at low temperatures because it doesn't need electric resistance backup & removes all concern about it getting too cold) and more durable... but also significantly more expensive upfront, and I'm not sure if it would save any energy in the normal temperature band. And if they aren't that much better, that money could be better spent on solar panels, insulation, etc.

  6. GBA Editor
    Martin Holladay | | #15

    Lots of people have been staying warm with ductless minisplit heat pumps -- without backup heating systems -- for years, in locations much colder than Pittsburgh -- locations like Minnesota, Vermont, Quebec, and Maine.

  7. exeric | | #17

    This is a very good article. It talks about time of use efficiencies which is important but fails to mention that preheating a home that will maintain its temperature requires a fair degree of air tightness and insulation in the structure. I guess you could call it an implicit assumption that most of us know is required. But it leads to something that is very important in California's climate.

    In California's climate peak energy use often doesn't happen in winter, but in summer. Peak energy use happens on the hottest days in summer when everyone turns on their air conditioning.
    (Enron used that as an excuse for its shananigans.) If you have already built a house that can maintain its temperature for long periods of time then you can also use that asset in summer.

    California's dry climate and cool summer nights are ideal for using a whole house fan. So you can use the same time of use efficiency to turn off the AC in summer. It would require diligence in opening windows at night and closing them in the morning but it could be incentivized financially just like other things are. It would also require a psychological adaption to not require a perfect inside temperature year around just as is advocated in this article for winter living.

    I live with a whole house fan and use ceiling fans in most all rooms to keep me comfortable. It works. I wish the people doing this study would link the thermos bottle effect of well built houses for taking advantage of that for both summer AND winter here in the west.

  8. jj1 | | #20

    Hi Pierre: heat pump efficiency continues to improve. My Rheem Platinum Performance 50 gallon heat pump hot water heater operates at a Uniform Energy Factor of 3.55. We never run out of hot water, even with many guests. During 2018, Carrier Corp. (HQ in Indianapolis USA) introduced its new Infinity 9K (cooling rated capacity) size rated BTU ductless mini split, indoor unit 40MPHAQ09XA3 and outdoor unit 38MPRAQ09AA3 featuring a cooling mode SEER of 42 and heating HPSF of 15.0. The maximum heating capacity at 5F is listed at 13K BTUs.
    The new Carrier Infinity appears to have better efficiency ratings than earlier products, plus a reasonable heating modulation down to 3100 BTU. Internet dealers report recent pricing for the pair in the $2800 range, excluding installation. Carrier also offers a larger 12K BTU rated cooling pair, albeit at lower efficiency ratings: the indoor 40MPHAQ12XA3 plus outdoor 38MPRAQ12AA3.

  9. Jon_R | | #21

    INFINITY 38MPRA; Up to 75% of Heating Capacity at -22° F (-30° C)

    While "up to" is vague and COP may be poor, it is speced to work at -22F. Looks like (exactly the same compressor case design, same -22F) a Gree Sapphire with the Infinity name.

    1. Expert Member

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