Electric Vs. Gas: Water Heating By the Numbers

Innovation, the growing menace from Climate Change, and data support conversion from gas to electric technology for domestic hot water.

With colder weather coming, it’s a good time to think about reducing our heating emissions. When we burn gas and oil to heat the air and water in our buildings, we generate about 10 percent of our emissions in California and an even larger share in less-temperate states (for example, 25 percent in New York).

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How can we reduce our heating emissions? One good approach is to use electric heat pumps rather than gas-based heaters. In fact, beginning in 2022 the updated California building code specifies these as the default option for space or water heating in each of its 16 climate zones.

Many building owners and contractors are skeptical about this. After all, they reason, modern gas furnaces are 90 percent or even 95 percent efficient, while even the best combined-cycle gas turbines used to generate electricity are about 60 percent efficient. On top of that, the grid suffers from transmission and distribution losses. While the grid does have some clean energy, solar power is least available when we most need the heat (e.g., nights and winters). Given all of that, can it really be true that electric heat has lower emissions?

Let me tackle that question by focusing on California, which relies heavily on solar energy. There are two parts to the question. The first is, how much pollution is produced in order to get energy for heating (gas or electricity) delivered to the building? And the second is, for each unit of electric or gas energy that gets to the building, how much heat can it produce? (That is, how efficient is the heater?) 

The answers, based on what is currently known, are largely inspired by a few presentations by Martha Brook, a policy advisor to the California Energy Commission (CEC).

If you want the short summary for California:

  • The extraordinary efficiency of electric heat pumps trumps the inefficiency of electricity production and transmission. As the grid gets cleaner, the case for heat pumps grows even stronger.
  • Hot water is used year-round, including when the grid is cleanest in California, which makes heat pump water heaters an easy win.
  • Heat pump space heaters are a win in well-insulated homes, and even more so when the heat is turned down for a few hours when the grid is dirtiest (e.g., evenings). For other homes, it may be best to improve the building envelope first.

The California analysis has some gaps, and work on this is ongoing. The analysis will be quite different for states with different grid mixes and heating needs. For example, a state that relies more on wind energy and less on solar would have relatively more clean energy available during colder times, further strengthening the case for heat pumps.

Getting Energy to the Building: Gas Vs. Electricity Emissions

How much pollution is generated, just to transfer energy to a building? The chart below compares electricity emissions to gas emissions, as delivered to the building. No appliance is using the energy yet; the chart reflects emissions just from generating and delivering the energy. 

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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.

You can see that 40 percent of the squares here are green, and 60 percent 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 are likely to be heating a building the most (e.g., fall and winter mornings and evenings).

If you look to 2030 in the image above, when we will have more renewables (60 percent compared to 31 percent in 2019), the picture is better, with 70 percent of the squares favorable for electricity, though it has a similar overall pattern.

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, so incorporating those leaks would make these pictures greener.

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 percent efficient, like the best modern gas furnace, is not going to cut it. And that is the magic of heat pumps. 

A modern 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.

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 temperate areas: 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, 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.

The 2019 California building code requires that replacement electric water heaters for consumers have a UEF of at least 2 when more than 55 gallons. In practice, it is easy to find heat pump water heaters with much higher efficiency (i.e., UEF 3 or more) for residential buildings. These water heaters are a clear emissions win. The chart below shows how such a heat pump water heater compares with a tankless gas water heater, which is already much more efficient than a gas tank water heater. This chart is similar to the 2019 chart, but it takes into account the efficiency of the water heater. 

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Space heat pumps have a more difficult job in some ways, because they tend to run more when it is cooler. These heat pumps will run nearly constantly in the 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, shown below.

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This 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 expressed as BTU’s of heat delivered per Watt-hour of electricity consumed.) 

A more-efficient heater would do better, but note that the red areas will largely stay red, given the multipliers in those squares. But when you look across the whole day, there is still a benefit to using a heat pump for space heating. The 2019 GHG chart was made using the federal minimum efficiency of 8.2 HSPF. A typical heat pump today would be at least 20 percent more efficient (HSPF of 10), while a high quality heat pump would be 70 percent more efficient (HSPF of 14).

As California’s grid gets cleaner, the advantage grows. You can see the overall advantage of using heat pumps for space and water heating, assuming California’s projected 2030 grid.

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As always, keep in mind that insulation will save energy and can simplify installations. Heaters in well-insulated homes can time-shift to avoid the dirtiest and most expensive electricity. Good building envelopes also reduce the amount of conditioned air that needs to flow through any duct system. 

In summary, even though modern gas furnaces are very efficient, heat pumps save on emissions, because they use much less energy to extract and deliver the same amount of heat. Furthermore, heat pump emissions will only go down as our grids get cleaner. As an important added bonus in our warming climates, heat pumps can provide cooling as well.


Reprinted with permission of Palo Alto Online, which hosts the author’s blog, “A New Shade of Green.”

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