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Building Science

How to Spot Misleading Electric Heating Efficiency Claims

Understanding the two primary types of electric heating

Electric resistance heating is 100% efficient. But you can do better! [Photo by Energy Vanguard]

Sometimes companies that make products stretch the truth, embellish features, or rely on their potential customers’ ignorance of how things work. Shocking, I know. The world of electric heating has such companies. But if you’re looking for a device that will provide electric space heating for your house, it’s actually pretty simple to figure out the truth.

Two methods to convert electricity to heat

Electricity is a marvelously versatile way to move energy from one place to another. One of the most important things to understand about it, though, is that it’s not an energy source or fuel, like natural gas. It’s an energy carrier, whose source is the heat of burning coal, the motion of water rushing through a hydroelectric turbine, the movement of electrons and holes in semiconductor photovoltaics, or some other energy conversion process. At the other end of the electrical system, electricity gets converted again, this time into heat, light, or some other form of energy in our homes and workplaces.

We have two primary methods to use electricity to provide space heating: electric resistance or heat pumps. Let’s look at them separately.

Electric resistance heating. Also called Joule heating or ohmic heating, it simply wrings the energy out of the electricity and turns it into heat directly. For every one unit of electricity, you get one unit of heat. Starting with Count Rumford’s experiments showing that mechanical energy and heat are equivalent, scientists have shown that all forms of energy can be converted from one to another. (Rumford, despite the title, was an American who deserted his country and his wife to fight on the losing side of the American Revolution.)

And that’s where the law of conservation of energy comes in. To convert energy from one form to another, you have to end up with the same amount of energy you started with. Looking at the conversion factor for the units of electrical energy (in kilowatt-hours, kWh), we see that the amount of electrical energy that can be converted to heat (in British Thermal Units, BTU) is:

3,412 BTU per kWh.

So, with electric resistance heating, we’re simply converting the energy in the electricity to heat, and the number above represents the ultimate reality of the conversion. It’s a one-to-one energy transformation. One kWh of electricity in yields 1 kWh of heat out, and 1 kWh of electricity equals 3,412 BTU. You can’t get 5,000 BTU/kWh with electric resistance heating because that would violate the law of conservation f energy. Another way to say this is that electric resistance heating is 100% efficient, which makes it sound pretty good . . . even though it’s not.

An electric furnace and a toaster oven do the same thing: convert electricity to heat at 100% efficiency. [Photo by Energy Vanguard]
An electric furnace and a toaster oven do the same thing: convert electricity to heat at 100% efficiency. [Photo by Energy Vanguard]
An electric resistance space heater will deliver 3412 BTU of heat for each kWh of electricity used. The same is true of an electric furnace, a toaster oven (photo above), and a standard electric water heater. But there is a way to convert electricity to heat at better than a 1:1 ratio.

Heat pump. If you take that same electricity and put it into a heat pump, you can deliver more than 3,412 BTUs for each kWh of electricity you use. The reason is that you’re not doing a simple conversion from one form to another. You’re using the electricity to do work. It runs a compressor that moves refrigerant through a system that transfers heat from a cooler place to a warmer place. And it can move 3−5 as many BTUs/kWh as electric resistance gives you. My Mitsubishi heat pump has a rated efficiency of about 4:1, so it can send about 14,000 BTUs of heat into my house for each kWh of electricity I use.

(In case you’re wondering, there is another method of heating deserves mention here, too: the induction cooktop. It’s not electric resistance and not a heat pump, but it does essentially the same thing as electric resistance. It uses the energy in electricity to create heat in the cookware, so, like resistance heat, it’s 100% efficient at converting electricity to heat. It’s just better at putting the heat exactly where it’s needed.)

Three ways companies can confuse customers

A 1,500-watt electric space heater provides 5,119 BTUs per hour (1.5 kW x 3,412 BTU/kWh). But that doesn’t stop companies from putting them in fancy packaging and using confusing language to make people believe they’re getting something “miraculous.” Take the Amish fireplace, for example. It’s just a 1,500-watt space heater with an Amish-made mantel.

There’s nothing miraculous about an \"Amish fireplace\" [Photo by Energy Vanguard]
There’s nothing miraculous about an “Amish fireplace” [Photo by Energy Vanguard]
If you need something bigger, there’s the electric furnace. Common in places like Florida that don’t have much of a winter, these devices again use electric resistance heating. It’s 100% efficient, as is the simple space heater and Amish fireplace.

Here are some things to look out for when evaluating efficiency claims for electric heating systems.

1. Misleading comparisons. Newspaper ads for the Amish fireplace compared it to a coffee maker. That’s just another form of electric resistance heating, with the same 100% efficiency. Interestingly, they said it uses less energy than a coffeemaker, but the current specs for a Mr. Coffee rate it at 900 watts, significantly less than the 1,500 watts of the Amish fireplace.

One that I came across recently is the Cocoon Thermasi electric furnace. Although they do a good job of camouflaging it, the source of the heat seems to be electric resistance. That makes it 100% efficient in converting electricity to heat, just like any other furnace. They claim, however, that it “uses up to 41% less energy than a traditional electric furnace.” More on that in a bit. The most important thing to note, though, is that they’re comparing their product to electric furnaces, not heat pumps.

Another way to mislead by comparison is to use the old apples-and-oranges trick. An electric furnace is 100% efficient, whereas a gas furnace may be only 80% efficient. But you can’t compare the direct burning of a fuel onsite to heating with electricity brought in from a power plant. Coal-burning power plants, which still provide much of our electricity in North America, are only about 35% efficient. So there’s an efficiency multiplier effect here that makes a 100% efficient electric furnace only about 35% efficient when you consider the source.

2. Using technical terms without much explanation. The Cocoon Thermasi electric furnace does this well. Thermal mass, control algorithms, controllable infrared heat spectrum . . . It’s easy to use this type of language to fool people who don’t have science or engineering backgrounds. It sounds impressive, right? Once you understand that there are really only two ways to heat a house—electric resistance and heat pumps—you can cut through the nonsense. Which category does it belong in? In this case, it’s electric resistance: 3,412 BTU/kWh. It’s not a heat pump, which can be 10,000 BTU/kWh or greater.

3. Muddying the waters between total energy use and rate of energy use. Energy and power have led to a lot of confusion. Energy, measured in kWh, is what you pay for when you get your electric bill. Power, measured in watts or kilowatts, is the rate at which you use that energy. A 1,500-watt space heater might well use less energy than a 15 watt LED light bulb over the course of a month. It depends on how much time each operates. If you use the space heater for one hour and the light bulb for 200 hours, yes, the space heater uses less energy: 1,500 watt-hours vs. 3,000 watt-hours.  (This is a doubly clever trick because it also uses method #1 by throwing in a misleading comparison.)

Alternatively, a product could use more energy and claim to be more efficient by using less power. All you need is to find an appliance that cycles on and off when it’s heating. By reducing the amount of time a product is off, you can use less power for a longer time, and then point to the power use, not energy consumption, to claim higher efficiency.

Although the Cocoon Thermasi electric furnace descriptions are spare on details, I believe this is what that product does. Thermal mass, once it’s heated up, allows you to use less power because you can draw out some of the stored heat when there’s a call for heat. The UL page “verifying” the manufacturer’s claim, also spare on details, indicates that their test was based on power: “The power usage results for equal air flow and heat rise were compared among the three units.”

Understand the fundamentals

Once you understand that all-electric heating uses one of the two methods above—electric resistance or heat pump—it’s a lot easier to sort through efficiency claims. If it’s electric resistance, you’re not going to get more than 3,412 BTU/kWh. Period. If it’s a heat pump, the same electricity can move 10,000 or more BTU/kWh. That’s why Georgia, my home state, banned electric furnaces used as primary heat sources a decade ago.

A 100% efficient electric furnace sometimes comes with an Energy Guide label showing its efficiency as 100 AFUE (Annual Fuel Utilization Efficiency) [Photo by Energy Vanguard]
A 100% efficient electric furnace sometimes comes with an Energy Guide label showing its efficiency as 100 AFUE (Annual Fuel Utilization Efficiency) [Photo by Energy Vanguard]
As I mentioned at the outset, electricity is a great energy carrier. It keeps getting cleaner every year, and it’s the key to getting away from fossil fuels. It’s also the best way to do zero-energy homes, which is my goal for the home I live in and the reason I got rid of my gas meter.

But we have to be smart about how we use electricity, which means using electric resistance heat only in places where it makes sense. We’re never going to have a heat pump toaster oven, but for space heating, water heating, and even clothes dryers now, heat pumps are the way to go.

_________________________________________________________________________

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

24 Comments

  1. Randy Williams | | #1

    Good article Allison, and I agree with you that heat pumps are the future, but there are limits. As I'm writing this, the outdoor temperature is -24°F here in Minnesota, we haven't been above zero for almost a week and there's still another 4-5 days to go in this current polar vortex. In a recent project, I had an cold climate ASHP installed along with an optional electric strip heater. That's what's heating the home this morning, and probably most of the past week. The homeowners are thrilled with the performance of the heat pump and understand the limitations. Sometimes, electric resistance heating is a good choice, as long as it's limited.

    1. William Hullsiek | | #2

      Randy which heat pump did you use. Down here in the cities we are a balmy -13f.

      1. Randy Williams | | #15

        Hi William,
        We used a Mitsubishi PVA air handler. We were looking more for a central ducted system with the performance similar to the wall hung systems. Very happy with the system's performance though we are having to deal with the condensate from the outdoor unit. -13, anything below 0 is just cold. I saw a low of -30F on my truck thermometer on the way to an energy audit yesterday. A little uncomfortable.

    2. GBA Editor
      Allison A. Bailes III, PhD | | #3

      Randy, yes, the key is using electric resistance for heat only where you really have to - a toaster, a blow dryer, or, as in your case, supplemental heat on the really cold days in Minnesota. Gary Nelson in Minneapolis has a Fujitsu heat pump that did just fine a couple of years ago. It held the 72° F setpoint when the outdoor temperature got all the way down to -17° F. And he has no strip heat on his system. The indoor temperature dropped to 58° F when it was -27° F outdoors, but they were out of town at the time. He said if they'd been home, their body heat plus baking a batch of cookies would have made the house comfortable.

      Of course, he's mister air leakage/blower door guy, so his house is really airtight and has great insulation and windows. The more extreme the climate, the more important it is to pair an excellent building enclosure with heat pumps.

      1. Randy Williams | | #16

        Being from Minnesota and owning a Minneapolis Blower door for over 10 years, I know who Gary Nelson is. I got to meet Gary once, at a trade conference somewhere. I've enjoyed listening to a few podcasts I've found him on, he was recently on the Unbuildit pod cast with Jake and Peter, no Steve on that one. I get the feeling he would be really fun guy to go have a coffee or beer with and just talk BS. Thanks for the response Allison!

        1. GBA Editor
          Allison A. Bailes III, PhD | | #18

          Yes, he's a great person to hang out with, especially if you're interested in the history of blower doors and duct leakage or the technical details associated with air & duct leakage. Here's a photo of him in his basement with his heat pump.

    3. Doug McEvers | | #6

      Randy,

      How many panel amps are dedicated to run the electric resistance backup heat? At what outdoor temperature does the resistance heat kick in? Some good old fashioned Minnesota winter going on right now.

      1. Randy Williams | | #17

        Hi Doug, yep, I was starting to wonder if winter would show up this year, should have known better to doubt mother nature.

        I don't recall the size of the electric plenum heater, though I'm sure it's sized large enough to handle the heating load of the home, which is a little over 20,000 BTU's, so around 7 Kw. The homeowners think it kicks in between -15° and -20°F. The thermostat must have a slight lag before the electric heater starts, the homeowners have notice a temp drop of 2 degrees, from 68 to 66 during very cold temps. The inside temp does move back up to 68 after a short period. It's interesting learning how these systems work together. We did install a Leviton electrical panel in this home with a smart breaker on both the ASHP and plenum heater but I've been having an issue getting the breakers tied to the home's WIFI system. Hopefully have electrical consumption data for the heating system soon.

        1. Doug McEvers | | #19

          It is temperature runs like we are having in MN that really showcase the value of energy efficient design and construction. Run of the mill building practices will slide by in a month like January where we did not get below zero in MSP, but not now. The cold creeps in everywhere in leaky, under insulated and poorly detailed buildings. So much full sun these very cold days highlight proper glass placement and performance. Hard to overvalue comfort in cold climate housing. I like to call it inheritance quality real estate.

        2. William Hullsiek | | #20

          Interested how that Leviton panel works out. I have Sense monitor but it can’t detect the ECM pumps for the hydronic panels. D

  2. Trevor Lambert | | #4

    The short answer is if they make any claim at all, it's going to be misleading.

    I see now you included heat pumps as a form of electric heating. I don't usually think of them that way, as the electricity is moving heat rather than generating heat. But in the sense that electricity is the only power source, you're right of course.

    You've got a typo in section 3, where it says that the electric heater will use "1,00-watt hours", where it should be 1500. I might as well get pedantic and point out that the hyphen is in the wrong place. It should be between "watt" and "hours", since it's a single unit. There's no reason to have a hyphen between the number and "watt".

    1. GBA Editor
      Allison A. Bailes III, PhD | | #8

      Trevor, thanks for pointing out the typos. They weren't in the article I uploaded so the editor made some changes there. And I agree with your hyphen location, which is where I had it. I've fixed it now.

  3. Charlie Sullivan | | #5

    I think the "Amish fireplace" shown in the picture actually has a COP of about 1.05. It works by attracting a 75 W dog to sit in front of it, adding 5% to the total output.

    1. Expert Member
      Malcolm Taylor | | #7

      Ah, but you aren't factoring the energy input in the form of dog food necessary to all0w the dog to emit heat.

      Almost all the houses I've done here in the PNW have relied on wood-stoves as the primary heat, and electric resistance as a secondary supplement. Recently I've done a couple where the primary heat is a mini-split, still relying on electric resistance for backup. In our fairly mild climate I find both combinations work well

  4. Robert Opaluch | | #9

    Agree on the main points of the article. In converting electricity to BTUs, resistance electric is 100% efficient and mini-splits do better. (How much better can depend upon the human operator behavior, outdoor temp, ducting, ...)

    Saying that electric heating is 35% efficient when you consider the source of coal-fired plants is cheating. Coal is going away. Some use solar power, wind power, ... to generate electricity. And gas doesn't magically show up at your home. Consider all that goes into getting that gas from the ground, processing, transport, and of course the significant percent that gets lost into the atmosphere. Not to mention the costs of global warming from fossil fuel addiction.

    However, another plus for electric resistance heat is that its cheap to install, and it can be placed in more spots in the building. So you only have to heat the area needed by occupants at the moment. (A mini split or electric central heat furnace is set up to heat a larger space or number of rooms.) And there's no duct loss if you use spot resistance heaters. You may lose some heat energy if the minipsplit is ducted. Even if it stays inside the theoretical envelope, it may be heating areas that don't add anything to human comfort.

    Also...consider radiant heat somewhat different than resistance heating? Its more comfortable, and people say they can use a slightly lower thermostat setting to feel comfortable. Minisplits heat air and move the air around, they are not radiant. Just like induction cooking puts the energy where you need it, and thereby is more "efficient" at the task at hand, so radiant heating units might be more "effiicient" in that less electricity may be used to complete the task of human comfort. (Just ask the dog in the photo, or the cat lying in the sunshine to feel that radiant heat from the sun.) Neither resistance nor radiant is more efficient in terms of converting electricity to BTUs, but some end up more efficient because less electricity is used to reach the same comfort level.

    An electric central heating unit might be the worst of the bunch, with duct losses and heating the immediate area of the furnace, where people are not gaining any comfort. Not to mention the installation cost including furnace, ducts etc.

    Of course the best radiant heating source is the sun. No electric bill. Too bad we don't exploit that more.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #10

      All good points, Robert. I didn't want to get into a full-blown discussion of site-vs-source energy generation and use, so I threw out the 35% number for coal just to point out that the 100% efficiency of electric resistance heat isn't as good as it sounds when you consider the energy that went into generating the electricity. And I did point out toward the end of the article that electricity does keep getting cleaner and cleaner as the amount of coal-fired electricity drops and renewably-generated electricity increases.

      Radiant heat produced by electricity is no different than electric resistance heat in terms of energy use. It's just converting electrical energy to heat that's distributed mainly via radiation rather than convection.

      [Edited to add:]

      Oh, and pretty much all the electricity you use in a house turns into heat. The exceptions would be a little bit of light that escapes through glazing or vibrations (sound) that leaves the building.

      1. user-6998645 | | #21

        With regards to "consider radiant heat somewhat different than resistance heating? Its more comfortable, and people say they can use a slightly lower thermostat setting to feel comfortable." Canada Mortgage and Housing Corporation did a study about 15 years ago and found that radiant floor heating was not actually more efficient and that homeowners with radiant heating still kept there thermostats near the standard setting of 72 degrees F (21 degrees C). I believe Don Fugler may have been the research project officer overseeing this study.

        Cheers,
        Conrad

    2. Expert Member
      Malcolm Taylor | | #12

      Robert,

      I agree. Electric resistance heaters allow the house to be zoned better than any other heat source I can think of. The benefits in being able to heat each room when and to what extent you want is huge.

  5. Tyler Keniston | | #11

    I'm really glad you brought up the holes, Allison.
    Lots of people are concerned about over-mining certain earth minerals like cobalt for batteries, but not many people are aware of how large a supply of holes we need for implementing solar at scale.
    And how does one go about recycling them? Environmental disaster waiting to happen.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #13

      I have good news for you, Tyler. We have as many holes as there are electrons moving across those pn junctions. And we can always get more by moving more electrons. Recycling holes is likewise not the problem it was once thought to be as they disappear as soon as an electron moves into their place, reappearing behind the electron. ;~)

      1. Charlie Sullivan | | #14

        Additionally, based extensive blower door testing, I can report that most buildings are well supplied with additional holes, if they are needed. In fact, I think that my own house may have a semiconducting envelope, based on the number of holes it has.

  6. WillKlatt | | #22

    It seems like trickery is at work: the efficiency of a heat pump decreases as the temperature outside drops, therefore providing the least benefit when it is needed most and causing huge fluctuations in the overall demand for electricity at cold temperatures. Might as well just install a resistance heater; the grid load would be a bit more linear and infrastructure could be better planned for, or install a natural gas heater which converts 80-90% of the btus into heat, where you want it at your house, instead of the 60% efficient conversion of natural gas into electricity where the other 40% of the heat goes up the stack and isn't being used for heating.

  7. masincalgary | | #23

    I enjoyed reading the article and all the comments. Mid Nov my central gas furnace died and I had to learn from scrap (with not even high school physics) what a heat pump was and what I'd need, and what would happen to my bills. For good reasons, I'm still living without central heat, and just coming out of a 10 day snap with many -30 C temps. Due to the excellent building envelope work done, I've managed really well with some space heaters and in fact enjoyed finding out how my house responds. I now feel really safe and not worried about what would happen in a power outage or on a string of super cold days.
    I pay a few pennies more to a utility that sources clean electricity as much as possible and that will increase of course. Most houses in Calgary have sufficient space for solar on their garages alone, so with some incentives we could certainly and easily have a lot of net zero homes.
    I agree that targeting heat in a well insulated area is important, but unless you have a new build, as I understand it, having each room with its own control is not easy. I think it's easier to be more economical during a really cold time to turn down the central heat (which would be pumping out resistance heat at -30 C) and add space heaters judiciously.
    I've looked at the two companies, and Mitsubishi seems to have won some awards for transparency in ESG (environ/sustain/goals) whereas Daikin sells warheads. Both have been around for a long time. I'm looking forward to the day when I have a heat pump ducted into my central system, and I hope the electric upgrade won't be too onerous.

  8. John F Cross | | #24

    Before I retired, I used 2' X 4' Radiant Heat Ceiling Panels in my Water Access location, for when I was away at work, to maintain above freezing temperature - even though I drained the water (includeing Seisco, Inline Water Heater) as iwhen the power went out (frequently), being Water Access, it was last to be restored. I would arrive on a Friday night - the temperature was typically in the 40's. I would crank up the wood heater, take off my boots - and mind the fire, gradually removing my outside garments, as it warmed up. My feet were cozy on the rug floor, as the radiant heat kept it warmer than the air,

    Before I retired and moved in year round, I would use the radiant heat (and a sweater) and have the air temperature in the low 60's and be very comfortable - when it made no sense to use wood heat. Upon retirement, I installed a 'cold temp' mini-split, for year round use. When it is [email protected]# cold, outside, II still use the heating panels (and sweater) when the mini-split, seems to be 'always' on defrost. I am comfortable!

    Other features of my dwelling include windows that are 90' opening, double casements, hinges at the centre post, to catch the breeze from 3 directions - an entry door slab, to close off the upstairs when iy is not needed, with Windward !! Ceiling Fans above/below the stair opening to pull/push air up the (center of the room) open riser stairs. My water system is a drain back, 63 watt maximum Heatline, only at the water level with a submersible pump, as I am at leat 45' above the water level.

    I am 'lazy cheap' do it right and do it once!

    Works for me.

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