Is Electric Resistance Heat Really 100 Percent Efficient?
I’ve been making a statement for a long time that sometimes gets called out as inaccurate. The story began in the second month of this blog back in April 2010 when I wrote an article titled When Is 100% Efficient Not Good Enough? I’ve been claiming that when you turn electricity into heat (think toaster), the energy conversion is 100 percent efficient. But is it really?
Efficiency vs. COP
Sometimes this critique comes up when I contrast getting heat from electric resistance (aka, strip heat) with heat from a heat pump. I discussed this a bit in my article on the problems with electric resistance auxiliary heat in a heat pump. And I acknowledged that it drives some people crazy.
The reason talking about the efficiency of strip heat and heat pumps drives some people crazy is that it’s the wrong quantity. We really need to talk about coefficient of performance (COP), which is defined as the ratio of the amount of useful heating provided to the amount of energy used to provide that heat. So technically, I should say electric resistance has a COP of 1.00 and a heat pump has a COP of about 3.
The real problem here, though, is with heat pumps, not electric resistance heat. And yeah, that conversion does happen at pretty close to 100 percent. In an article I wrote about what happens to “used electricity” from all the electrical devices in a home, I said that:
Some of the electrical energy gets converted to heat immediately. The rest gets converted to whatever form of energy is needed for the purpose of the device or appliance. It’s mostly motion, light, or sound for the things that aren’t meant just for providing heat. Eventually…those intermediate forms of energy used in your home become heat, too.
Yeah, we can refer to that energy conversion process in terms of COP or even annual fuel utilization efficiency (AFUE), which is how it’s given on electric furnaces. But I contend that there’s nothing in the terminology to prevent us from saying electric resistance heat is 100 percent efficient.
Site energy vs. source energy
For most people, talking about energy efficiency means talking about what happens after the energy arrives at the house. But the efficiency picture is bigger than that. I covered this topic in some detail in my book. Here I’ll give a brief overview, beginning with how I defined the terms in the book.
Site energy: This is the energy used within the boundary of the site. For electricity, it’s the kilowatt-hours you get billed for. For gas, it’s the amount that flows through your gas meter. For propane, it’s the amount used from your tank. Each fuel delivered and used at the site counts toward the total site energy…
Source energy: This is the energy used onsite plus the energy that went into getting that energy onto the site. For electricity, it’s the kilowatt-hours you get billed for plus the kilowatt-hours of energy that are “consumed in the extraction, processing, and transport of primary fuels such as coal, oil, and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.”†
I think you probably see the answer now.
So is it 100 percent efficient?
As with most things in building science, the answer is…It depends! If we’re talking about site energy, the answer is yes. There’s little to no loss in the conversion of electricity to heat in your home.
But it’s different with source energy. There are significant losses of the energy that went into making the electricity delivered to your home via the electrical power lines. And that means that the answer is usually no when you consider the big picture of source energy.
Now, there’s good news here, too. Electricity is getting cleaner and more efficient every year. The less we rely on fossil fuel power plants, the more efficient our source electricity becomes. If you’re generating your own electricity from photovoltaics and using it for electric resistance heat, you can say that your strip heat is 100 percent efficient in terms of both site and source energy.
And that takes you to the edge of the next rabbit hole: embodied energy.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
† A Common Definition for Zero Energy Buildings, US Department of Energy, 2015
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Oh no, now we’ll have homeowners boasting that their electric strip heat is 100 percent efficient! And they’ll be using their solar systems to run the strips…
There’s not a lot to brag about there, but solar-powered strip heat would beat a 98% fossil gas powered furnace in terms of carbon emissions.
Yes, it would. And not just carbon emissions, but we need much swifter progress than that.
Let’s suppose a homeowner is about to finance a PV array on their 1950’s home’s roof and they’re in straight net metering arrangement. Without jumping into a spreadsheet, I doubt the project would make any sense in cash flow terms without replacing the strip heat with a heat pump.
Electric resistance heating operates with absolute precision, achieving a unity power factor of 1—every watt of electrical energy transforms directly into heat with no reactive losses, no phase shift, just pure, uncompromised efficiency. It’s the gold standard of direct energy conversion! ⚡🔥🔥⚡
AWHP heat pumps for the win!
Yeah, air-to-water heat pumps are going to get a lot more attention in coming years.
True, but we can do better.
PV’s are only about 10% efficient at converting sunlight to electricity, so your PV-driven electric-resistance furnace is only 10% efficient in terms of source energy. You could get a higher source efficiency by using the solar energy directly to heat your house.
Roy, we’re a bit further along. 10% was my uncle’s panels in the 70’s. We’re now hovering around 20% or so. But I am still confused about Allison’s foray into solar-driven electric strips…
Paul, I was underestimating the efficiency on purpose to make a point, but whether it is 10 or 20%, PV’s just aren’t “efficient”. But since the sun is free (when available), it is the cost and size of the collectors that counts for a given electrical output. I haven’t looked at the economics in terms of installed costs, but I think that if you do want to use your PV’s for space heating, you should use a heat pump instead of strip heat. Speaking of your uncle, I was involved with passive and active solar heating in houses in the 70’s before PV’s were popular. They did not work well and were not cost effective for various reasons.
Two things:
First, PV and strip heat does make a lot of sense in one application: PV plus electric resistance water heating. I covered that in the water heating chapter of my book (pp. 311-313). The equivalent upfront cost to install it is less than 60 percent of the cost of solar thermal water heating. When you pair a heat pump water heater with PV, the equivalent cost is less than 40 percent of solar thermal.
Second, passive and active solar in the ’70s lost the battle of energy conservation. By 1980 it was clear that superinsulated, airtight houses were a much better way to go. Martin Holladay wrote an article about the history of it for Green Building Advisor in 2010.
Solar Versus Superinsulation: A 30-Year-Old Debate
https://www.greenbuildingadvisor.com/article/solar-versus-superinsulation-a-30-year-old-debate
It’s behind a paywall, but you can find the info in other places as well.
Roy, yes, it took decades to “jump” from 10% to 20%. When I looked at them around 2010, the ROI was still miserable. Pending any temporary hiccups with the current executive branch, we’re in much better territory now with $/W install cost. With free source energy, we won’t argue too much about efficiency of solar->electric energy conversion, will we?
The goal for residential (both single-family and multi low-rise), for example, would be to have enough on-site renewables to make those structures net-zero, or eventually off-grid. For many sites, the current W/sq.m. capability already allows that. Any improvement in that ratio is a bonus in terms of architecture and installation.
Paul: Where did this thing about my “foray into solar-driven electric strips” come from? I don’t recall discussing that.
Allison, at the conclusion of your article: “If you’re generating your own electricity from photovoltaics and using it for electric resistance heat, you can say that your strip heat is 100 percent efficient in terms of both site and source energy.”
Also, if we agree that efficiency measured here is about conversion of solar to useful heat energy in the home, you’d need to account for inverter and other system losses (unless the inverter is in the heated space), so perhaps 95%?
Allison, that was a great, easy to understand explanation. Can’t wait for the next one on embodied energy! I really enjoy your writing.
Thanks, John!
300% heat pump label suggesting $300 a month versus an electric furnace’s $900 for typical pre-existing construction would avoid the confusion and allow the consumer to make an informed decision. Of course it would take a 3.5 COP heat pump to make the efficiency rating as electric strips would be part of the equation.
A natural gas furnace at $250 a month would be honest as would a propane furnace at $800.
Windows with a decreasing u factor hides the R2 rating. Same story.
rj: As you point out, the Energy Guide showing estimated annual costs is a much better way to compare than the efficiency of a heating system.
Back in 2003 when we were building the MN GreenStar program, I recall a similar discussion around source v site. At the time, the State’s electricity portfolio was something like 70% coal/ fossil fuel generated, and 20% renewable. Transmission loss, was estimated at 20% +/- , so 100% electric was only 80% efficient to start, and burdened with more loss of efficency in electricity generation and associated pollution. 100% COP was like bragging about driving a Dodge Dart.
Things have shifted since then. Renewable makes of over 60% of the portfolio now, and more small site production is helping with the transmission loss. Point being; Source Matters.
Michael, if I remember right source energy losses for thermal power plants is somewhere around 2/3?