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Electric vs Gas Heat: Which wins?

Uploaded: Oct 6, 2019
With colder weather coming, it’s a good time to think about our heating emissions. They represent a sizable chunk of California’s overall emissions, though somewhat less in our temperate area. About 10% of California’s emissions come from fossil fuels used for space and water heating in our buildings. (1) In Palo Alto, almost half of the heating emissions are from residential use, so residents can make an impact by lowering our heating emissions. (2) The question is, how best to do that? The answer we typically get is “heat pumps”, but as one commenter asked on an earlier post: “Can you show that electrically powering a heat pump by burning gas and dealing with generation losses (modern combined cycle plants are 60% efficient), transmission losses, distribution losses, etc, is actually better than just burning the gas (modern furnaces are easily >90% efficient) for heat (and some newer designs are >95%)?”

This blog attempts to tackle the question of gas heat vs electric heat. There are two parts to that. The first is, what emissions are produced in order to get gas and electricity delivered to the home? And the second is, for each unit of electric or gas energy, how much heat can it produce? (That is, how efficient is the heater?) There is a lot to this, and I don’t have a perfect answer, but I’ll relay here what I know. This is largely based on a few presentations by Martha Brook, a policy advisor to the California Energy Commission (CEC). (3)

If you want the short summary for homes in our mid-peninsula area, I would say it is:
- A heat pump water heater is a clear win.
- A heat pump space heater is a clear win for new construction and for renovated homes that have minimal heat loss. Other homes should do more insulating first.

The analysis below has some gaps and work on this is on-going.

Gas Emissions vs Electricity Emissions

The chart below compares electricity emissions to gas emissions, as delivered to a home. (4) No appliance is using the energy yet; the chart reflects emissions just from generating and delivering the energy to the home. The chart has one square for each hour of the year. Months go across the x-axis (from January on the left to December on the right), and hours go along the y-axis (from early morning at the top to late night at the bottom). A green square indicates that electricity has lower emissions intensity than gas, measured in emissions per unit of energy, while red means electricity has higher emissions. 40% of the squares here are green and 60% are red. On average, gas emissions are lower than electricity emissions. (Yes, you read that right.) And in particular you can see that the comparison is poor for electricity during times when you might be likely to be heating a home (e.g., fall and winter mornings and evenings).

Source: June 2018 presentation at a CEC workshop

If you look to 2030, when we will have more renewables (60% compared with 31% last year), the picture is better, with 70% of the squares favorable for electricity, though it has a similar overall pattern.

Source: June 2018 presentation at a CEC workshop

It is important to note that fugitive emissions (methane leaks from transmission and distribution) are not included in the calculations behind this diagram, while the corresponding adjustments for electricity (transmission and distribution losses to the home) are included. Methane leaks can be significant (see this earlier blog post), so that is likely to make these pictures greener, at least in the near-term while those leaks are being addressed.

Heater Efficiency

Given the above, we know that heater efficiency is critical if we want to reduce emissions by switching from gas to electric. An electric heater that is “just” 95% efficient, like the best modern gas furnace, is not going to cut it. And that is the magic of heat pumps. A typical heat pump water heater produces about three times as much energy (heat) as it consumes. Instead of creating heat, it extracts heat from the outside air, using a refrigerant, and moves it into the building. This is more or less how a refrigerator operates, but in reverse. (5) A measure called UEF, or Uniform Energy Factor, is used to compare heat pump water heaters. It is a ratio of the energy output to input, but with some nuances. Per a spokesperson at the CEC, the metric is pretty reliable for our area: “UEF includes standby tank losses during the test procedure, as well as measuring the performance of the heat pump over the whole test procedure. In general we find that the UEF gets you in the ballpark of actual performance in mild climates, but for colder climates or larger households there is more backup electric resistance use and therefore the performance would be lower.”

You can see from this chart that replacement electric water heaters are required to have a UEF of nearly 3. Palo Alto and Mountain View are in climate zone 4, while Menlo Park and other San Mateo County cities are in climate zone 3. By contrast, the Tahoe area is climate zone 16.

Source: CEC Title 24 standards

Because of this high efficiency, heat pump water heaters are a clear emissions win. The chart below shows how a heat pump water heater with about 3x efficiency compares with a tankless gas water heater. This is similar to the chart above, but taking into account the water heater as well. (6)

Source: Presentation to Redwood Energy, February 2019

Space heat pumps, however, are not such a clear win. These heat pumps will run nearly constantly in the cool and relatively dirty night hours, while a gas furnace will run infrequently. That means the gas furnace has much lower emissions at nighttime, even in a well-insulated house as shown below. (7)

Source: Presentation to Redwood Energy, February 2019

When you look across the whole day, there is still a sizable benefit to using a heat pump for space heating. (Space heaters that exceed the minimum of 8.2 HSPF will do somewhat better than shown here.)

Source: Presentation to Redwood Energy, February 2019

If your house is not well insulated, though, the story is different. If you live in a home with less insulation and thinner windows, a heat pump for space heating will not reduce emissions, even assuming a cleaner 2030 grid, as shown below. Instead, the best thing is to improve insulation first.

Source: Presentation to Redwood Energy, February 2019

To summarize, if you aim to reduce your heating emissions, a heat pump water heater is a clear win, as is a heat pump for space heating if your home is built to modern codes (i.e., is well insulated). Older homes need a better “building envelope” before switching to an electric heat pump for space heating will reduce emissions. Reducing a home’s heat loss is a good idea not only for reducing emissions but also for better resilience to energy outages.

There are a number of factors besides emissions that come into play when deciding on electric vs gas heat. They include cost, resilience to outages, home impact (e.g., air quality, noise), heater placement, and safety. See the videos linked to in this earlier blog post if you want to see real-life methane leaks from gas water heaters.

Even when considering only emissions, this post neglects not only the fugitive emissions of gas to the home but also the impact of any leaking refrigerant from the heat pump. (8) So this is not a complete picture. But I hope it provides more information about how heaters are evaluated and what might make sense for your home.

Two of our local utilities are offering significant incentives for heat pump water heaters. Silicon Valley Clean Energy (SVCE) information can be found here: https://www.svcleanenergy.org/water-heating, including a link to an informative buyers guide. Palo Alto Utilities has information here: http://cityofpaloalto.org/hpwh. PCE does not currently have an incentive for heat pump water heaters. (They are running a trial, which is fully enrolled.) However, they do have terrific incentives for EVs. Check it out here: https://www.peninsulacleanenergy.com/about-evs.

If you are interested in learning more about electrifying your home, on Thursday October 10, from 2-7pm, the City of Palo Alto Utilities is hosting the Bay Area Electrification Expo in coordination with local partners to provide hands-on education and resources for building professionals and area residents wanting to further reduce their carbon footprint. Stop by to learn more about the benefits and “how to’s” of electrifying space and water heating, transportation, and cooking.

Thank you to Martha Brook and to representatives from the local utilities for reviewing and giving feedback on this post.

Notes and References

1. This data point comes from slide 4 of this presentation by Martha Brook at the CEC.

2. That is not the case for electricity. Because Palo Alto has so much commercial space, over 80% of our electric load is from commercial use(!)

3. The June 2018 presentation at the CEC (part of a larger workshop) is here, about 26 minutes in. The February 2019 presentation at Redwood Energy is here. All information for the June 2018 CEC workshop can be found here.

4. This chart and others in this post reflect the actual grid mix rather than our portfolio mix, which is much cleaner. If we were to adopt a model where we use a cleaner portion of the grid, then others would have to use a dirtier portion of the grid. And (a) they aren’t doing that, and (b) we don’t want them to do that because (c) it doesn’t match reality. We want electrification to be happening all over the state, regardless of the electric portfolio the various utilities maintain. Our own utilities’ zero-carbon contributions are to the grid as a whole, making it greener for everyone (thank you!). So I make the emission calculations in this post based on the grid mix, just as I hope others are doing in places with a less green electricity portfolio.

I should also note that this chart reflects average emissions and not marginal emissions.

5. Some videos that explain how heat pumps work can be found here and here.

6. This is for climate zone 3 (“CZ 3”) in a home built to the latest code (“2019 Std”), and it uses the 2019 emission intensity numbers shown in the earlier chart (“2019 GHG”). The “COP” or “Coefficient of Performance” of 3 is the ratio of heat energy output to electrical energy input (a somewhat less robust version of UEF).

7. That is true even when using the cleaner power of 2030, as shown below. The heater used in this case meets but does not exceed minimum federal standards, as required for this testing. (“HSPF” or “Heating Seasonal Performance Factor” is a seasonally averaged COP.) A more efficient heater would do better, but note that the red areas will stay red, given the multipliers in those squares.

Source: Presentation to Redwood Energy, February 2019

8. This slide (the right-hand side) shows how big an impact leaked refrigerant has on emissions when a home is electrified. The color-coding for these charts is: lightest blue: refrigerant leakage, green: plugs, dark blue: appliances, yellow: lights, tan: water heating, orange: cooling, medium blue: space heating. Our climate is probably somewhere between Sacramento and Los Angeles.

Source: June 2018 presentation at a CEC workshop

9. Some of you may be wondering why we don’t use gas-powered heat pumps. I should follow up on that. One thing to keep in mind is that heat pumps can air-condition as well, and I expect that when they are used for cooling, it makes the case for electric stronger. In addition, for new construction, there are some advantages to omitting the gas line entirely.

Current Climate Data (August 2019)

Global impacts, US impacts, CO2 metric, Climate dashboard (2018)

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Posted by Tom, a resident of Menlo Park,
on Oct 6, 2019 at 7:10 pm

There is a lot to unpack here, but I'll start with the parts I agree with. To reduce emissions in the Bay Area (CZ 3&4 et al), Yes Heat Pump Water Heaters replacing gas water heaters are great! I love mine and I'll never go back. Heat Pumps replacing furnaces (and air conditioners since they give you that from the same device) in new construction are great also. I also find Heat Pumps replacing existing furnaces (whether or not you have air conditioners) is also a big emissions reducer especially in the Bay Area with our cleaner electric generation procurement policies. Heat Pumps would be a big emissions reducer under regular state wide Renewable Portfolio Standards (RPS) as Martha Brook's heat map green and graphs show. Her charts are even put together biased 100% in favor of methane because she omitted the approximately 100% increases in total emissions that happens when fugitive emissions of 3% leaked methane is included. She says in her presentation that her 2020 work will reflect fugitive emissions to give the full picture that will show electric's larger environmental advantage. Leaking any volume of methane is about 30 times worse than burning that same amount of methane. Web Link (see the 20 year GWP of 86, Then adjusting it by the molecular weights of Methane / CO2 brings the warming ration to 30 times as bad on a per molecule basis. So the only assertion I disagree with, is with the well intentioned assertion that heat pumps don't reduce emissions on an older house unless you insulate the house first. I re-listened to Martha's presentation and I see she mixed her separate advocacy of insulation into her presentation about heat pumps vs. gas heat and omitted a bar from her bar chart. Her horizontal bar chart has green and blue bars showing annual emissions for a New Construction house with heat pump (New Construction green colored RPS minimum renewable electric) and with (New Construction blue colored gas that magically has no leakage). The missing bar would be a black bar for gas combustion emissions just below the grey old construction electric bar and would be twice as long out to about 1,900 kg of CO2 combustion only emissions. Then we would see that even under the looser statewide RPS and even ignoring the doubling effect of fugitive methane emissions electrifying new AND old houses saves substantial emissions.
I've learned not to require a step some homeowners won't do before allowing them to do a step that will save emissions. So for those who don't want to insulate, you can go ahead and install a heat pump. It will save emissions. It's just that you could save more emissions and be more comfortable and save more money in the long run by insulating first, and then putting in a smaller heat pump. But if insulating is off the table, you can still save emissions by switching to a heat pump compared to keeping your furnace as primary heater. You could even have both. Consider a $480 window heat pump, and "base-load" it at say 67 degrees with your old furnace set to back it up at 66 degrees if the house drops that low during heating hours. We need to get going on this transition away from fossil fuels. Let's not over complicate it.

Posted by Ken Chawkins, a resident of another community,
on Oct 7, 2019 at 1:31 pm

Of course if the state would develop a Renewable Gas Standard (RGS) the same way they have developed the Renewable Portfolio Standard (RPS), it would incent the capture and use of renewable natural gas (80% of CA's methane comes from agricultural and human waste streams...not the gas system leakage!) which would have immediate and dramatic impact on climate change AND would be far less costly and far less disruptive to people's lives.

Posted by The Business Man, a resident of Another Mountain View Neighborhood,
on Oct 7, 2019 at 6:30 pm

The Business Man is a registered user.

Simply PUT.

We live in a earthquake zone. AND Gas heat has a threat of CO poisoning. My friend suffered from a CO pisonig issue recently.


I grew up with heating oil, it is VERY efficient and allows for great TANKLESS hout water systems.


Can anyone demostrate that ELECTRIC heating is so much safer. You do not need a CO detector in a home with either heating oil or electric.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Oct 7, 2019 at 7:16 pm

Sherry Listgarten is a registered user.

Great comments, and thanks for reading and thinking about this. A few thoughts.

@Tom. Thanks for the comment. I asked the same question about methane. One thing to keep in mind is that the methane leaks apply to some degree to both gas and electric. Here is some data I found regarding where the leaks occur. Which of these differs between the gas-powered power plants and the natural gas utility?

(This is from 2016. There may be something more current.) I’m also optimistic about the big leaks getting fixed quickly. We now have much better technology for detecting leaks (e.g., drones), anyway.

FWIW, I have a few takeaways from this post. One is that we really need heat pump water heaters to be the default in our area. They need to be on plumber’s trucks, ready to go, easy to install, when a water tank fails (which they do every 8-12 years). How do we get there? Another takeaway is that refrigerants are a big deal. I knew that, but hadn’t seen it quantified in a local context. Their global warming potential (GWP) is in the thousands. I hope to do a post on them at some point, covering technology for detecting and protecting against leaks.

I will follow up on your great point about the old-house vs new-house comparison and get back.

@Ken. Are you aware of California Senate Bill 1383? That is designed to reduce agricultural methane emissions by 40% by 2030. Or are you particularly interested in displacing electric power with biogas? Can you share the resources you are looking at that are bullish on biogas? Most of the ones I’ve seen say it is too costly and/or not clean enough and/or too low in supply to make any real dent. (Here is an example, though focused more on transportation than on heating.)

@Business. Good point about carbon monoxide...

Posted by Curious , a resident of Another Mountain View Neighborhood,
on Oct 8, 2019 at 1:04 pm

Will the local monopoly on gas and electricity be cutting off gas this week?

Or just electricity?

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Oct 8, 2019 at 2:01 pm

>> FWIW, I have a few takeaways from this post. One is that we really need heat pump water heaters to be the default in our area. They need to be on plumber's trucks, ready to go, easy to install, when a water tank fails (which they do every 8-12 years). How do we get there?

Side note: many people converted to externally-mounted instant gas hot water heaters starting in the 90's to solve two problems: dangerous has hot water heaters in the wrong location (kitchen, hall, garage floor), and relative inefficiency of tank heaters, especially when not much not water is used. (Rinnai is an instant tankless brand.) . No large tank, and, now, no place to put one. A possible issue with instant gas heaters is the amount of methane release. The usage model for electric heat pumps is very different and requires space for a large tank, but, no safety problems, and, and a big win for people who use a lot of hot water (not everyone does); it is effectively a small air conditioner as well.

>> Another takeaway is that refrigerants are a big deal. I knew that, but hadn't seen it quantified in a local context.

I can't vouch for the accuracy of this table, but, the below link points to a fairly long list of refrigerants and includes both the Ozone Depletion Potential (ODP) and Global Warming Potential (GWP) of various refrigerants in a single table:

Web Link

There are other factors that affect usability. What isn't shown is the (ideal) efficiency COP or SEER/HSPF or whatever for particular applications. Also not shown are direct safety challenges: e.g. Ammonia (NH3) looks really good, except for one thing: acute toxicity/respiratory damage in case of major leak/accident. I notice that R-32 (Difluoromethane) seems most common now for water heaters, but, one wonders, from the table, why not R-152a (Difluoroethane) instead? (Many questions to mull over.) . i recommend the link for now, but, there may be a more definitive source of this info somewhere.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Oct 8, 2019 at 2:47 pm

Sherry Listgarten is a registered user.

@Curious. Just electricity.

@Anon. Where do you get your info that R-32 is most common in water heaters? I've seen in a few places R410a and R134a, which have GWPs well over 1000. The manufacturers don't seem to advertise which they use, though.

Posted by Curious, a resident of Another Mountain View Neighborhood,
on Oct 8, 2019 at 10:14 pm

So if I switched from gas heat to electric heat, and PG&E decided to cut my and 800k others' electricity, I'd just have to suffer in the cold until they turn it back on?

Posted by Ben, a resident of Greenmeadow,
on Oct 9, 2019 at 1:24 pm

Ben is a registered user.

There is at least one brand of HPWH that uses CO2 as the refrigerant. I just completed installation of a Sanden SANCO2. A refrigerant leak from this system will have relatively negligible impact on GHG emission and Sanden claims the system is more efficient because it uses CO2.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Oct 10, 2019 at 10:03 am

Sherry Listgarten is a registered user.

@Curious. I would guess that people who switch to electric heat would consider the possibility of outages. (Well, they will now, for sure!) There are some options, including dual fuel (gas as a backup) and solar and battery backups (expensive). A speaker at the local Building Expo mentioned V2B charging (vehicle to building). It's gaining traction in Japan. See here. If I lived in a stormy and cold place like Tahoe, I don’t think I’d go electric without gas as a backup. Power outages there are pretty common. Down here where it’s more temperate, I might consider electric on its own in a well-insulated house, but if the outages are more than 1-2 days, it could get uncomfortable. Hopefully the price of solar plus storage will come down over time and be accessible to more people.

@Ben. Yeah, I saw that, sounds promising. Neat that you got one. Let us know how it works!

Posted by Curmudgeon, a resident of Downtown North,
on Oct 10, 2019 at 7:35 pm

It is hard to avoid dependence on electric energy when heating a home. Unless you have an old-time gravity furnace, or gas wall heaters throughout the structure, you need electricity to power a distribution blower. No juice, no heat.

Noise is another factor to consider with heat pump anything. All systems use a buzzing whirring compressor/fan refrigeration unit, usually mounted outside the house where it can bother the neighbors. Listen before you buy.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Oct 10, 2019 at 8:45 pm

Sherry Listgarten is a registered user.

@Curm -- Of course, you are right, most gas heat needs electric power to work. Thanks for pointing that out. I guess that's why fireplaces are so common in Tahoe -- they are the backup heat!

Thanks for the very helpful comment.

Posted by From Here, a resident of Old Mountain View,
on Oct 10, 2019 at 9:22 pm

@ The Business Man. I think your speak and spell may need batteries!

Posted by Keep the gas, a resident of Old Mountain View,
on Oct 11, 2019 at 10:03 am

Simply put, all this expensive retrofit that those on the green side wish to force on us, will have zero impact on what they say will happen with the climate. That is assuming you believe them to begin with.

China and India keeps building more fossil fuel power plants, coal plants, and we are making it so expensive here to use electricity, that whatever jobs that are left here that need inexpensive and reliable power, will leave the country and go to where they use dirtier methods to generate electricity. So you are making things worse for the planet.

I vote keep natural gas for heating/cooking, and stop making everything SOooo Expensive here in California.

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Oct 11, 2019 at 3:26 pm

Posted by Curmudgeon, a resident of Downtown North,

>> It is hard to avoid dependence on electric energy when heating a home. Unless you have an old-time gravity furnace, or gas wall heaters throughout the structure, you need electricity to power a distribution blower. No juice, no heat.

I haven't seen a gravity furnace system for 50 years, although they were common when I was a youth. But, plenty of self-contained wall heaters out there. But, yes, if you have forced air, you are down with no electricity. "I thought everyone knew that."

>> Noise is another factor to consider with heat pump anything. All systems use a buzzing whirring compressor/fan refrigeration unit, usually mounted outside the house where it can bother the neighbors. Listen before you buy.

Unfortunately, some HVAC companies think 75 dB is A-OK for the neighbors, and, they don't bother to specify the outside noise level. Fortunately, at least some of the market is starting to get smarter about the effect on the neighbors of a noisy unit. A few units claim outside SPL of < 55 dB, but, those numbers are difficult to come by.

@Sherry @Anon. Where do you get your info that R-32 is most common in water heaters?

Comment withdrawn. Biased sample.

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Oct 12, 2019 at 10:00 am

Apologies for following up my previous postings, but, I wanted to throw some good information after bad.

Redwood Energy has design guide that happens to have some collected information in this regard:

Web Link

Starting at page 20 or so, you can see that almost every unit listed uses R134a for domestic-hot-water-only units, and, R410a for HVAC from small to large multifamily. I think one of the Spacepak units can also do domestic hot water (unclear). All R410a. One unit uses CO2 ("R744").

I also found another reference that might interest people who are interested in the refrigerant aspect of all this: Web Link

Finally, the Redwood Energy guide online listed above may be of general interest, as it covers a lot of details regarding the whys and hows of lower-cost higher-efficiency all-electric design and construction.

Posted by Curmudgeon, a resident of Downtown North,
on Oct 12, 2019 at 4:46 pm

"all this expensive retrofit that those on the green side wish to force on us, will have zero impact on what they say will happen with the climate."

If you are on a boat, and others on that boat are boring holes below the waterline, do you try to persuade them not to, or do you reach for a drill and insist on your share?

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Oct 13, 2019 at 2:28 pm

Sherry Listgarten is a registered user.

@KeepTheGas. Yes, it is very expensive to live here, and very frustrating when it seems like people just keep finding new ways to urge us to buy more stuff, even if it can save money in the long run.

But if we want to avoid the worst impacts of global warming, we cannot continue to use gas at this rate. We need to keep the fossil fuels in the ground.

The good news is, there are lots of low-cost or no-cost ways to do that. Voting is the biggest one. For the the purposes of this post, though, there are a variety of ways to keep your home's emissions down: using a thermostat (and setting it low), using shades to keep sun out and heat in, making sure any fireplace flue is closed, sealing cracks, using cold water for laundry, etc. Then if/when your water heater does go out, at least ask the plumber or your landlord about a heat pump.

I liked this op-ed from yesterday, which included this paragraph: "We can sit back, citizens, and watch the fires and the floods and the heat waves with a rising sense of doom. Or we can be as brave as a schoolgirl and decide that now is the time to stand up and fight." There are plenty of ways to help out at little cost and with little risk.

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