How to Spot Deceptive Heating Efficiency Claims
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.
2 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 kilowatt-hour of electricity in yields one kilowatt-hour of heat out, and one kilowatt-hour 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 resistance space heater will deliver 3,412 BTU of heat for each kilowatt-hour 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 one-to-one 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 kilowatt-hour 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 three or four or five times as many BTUs/kWh as electric resistance gives you. My Mitsubishi heat pump has a rated efficiency of about four-to-one, so it can send about 14,000 BTUs of heat into my house for each kilowatt-hour of electricity I use.
(In case you’re wondering, 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.)
3 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.
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.
I came across another one recently. Although the company does 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 significantly 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. Thermal mass, control algorithms, infrared heat spectrum, controllable… 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 kilowatt-hours, 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. Let’s say you use 100 kWh of energy in a space heater over the course of a month. If space heater A used 100 kWh in 100 hours, the power rating would be 100 kWh ÷ 100 h = 1 kW = 1,000 W. If space heater B ran continuously for 720 hours (30 days x 24 hours/day), its power rating would be 100 kWh ÷ 720 h = 0.14kW, or 140 W.
By operating a heater for more hours, you can use less power for a longer time. A company can then point to the power use, not energy consumption, to claim higher efficiency. But energy efficiency doesn’t derive solely from the power rating. 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.
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.
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 or blow dryer, but for space heating, water heating, and even clothes dryers now, heat pumps are the way to go.
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 subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
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Some of the rural electric co-ops in the upper mid-west offer low off-peak electric rates for electric thermal storage heating systems. The idea is to disable the heat elements during peak hours. This not only reduces the utility’s peak load but helps smooth the off-peak load since the electric elements can be sized smaller than the peak load and thus will have much longer duty cycles.
I’m familiar with the Steffes furnace. It uses thousands of pounds of ceramic bricks as its storage medium. The same thing can be accomplished with an electric boiler that feeds a hydronic air handler or radiant distribution system. Storage capacity must be sized to handle the design heat load for the duration of the peak period. Note that this is not something that can be properly modeled with conventional load calc software since the heat load is not continuous throughout the day.
In order to be competitive with a heat pump, the ‘electric storage’ rate needs to be less than a third of the regular electric rate — and that’s before considering the high front-end cost of storage heating. OTOH, the additional first-cost of a heat pump vs. central air is typically well below a thousand dollars.
David, that is, of course, a good way to save money on heating. You could do the same thing with a heat pump, too, to get more heat into the thermal mass during the warmer daytime hours and use that heat at night when it’s colder. Then your overall coefficient of performance would increase. That may or may not save money with time-of-use rates, depending on when you’re storing heat relative to the utility’s peak hours.
Allison,
Great stuff but, what is energy?
Foster
Well, Foster, I like William Blake’s definition;
Energy is eternal delight.
“A 1,500 watt electric space heater provides 5,119 BTUs per hour (1.5 kW x 3,412 BTU/kWh”
Life would be simpler if you forgot all this British Thermal Unit stuff, we still have the Queen on our money but the BTU just confuses things.
Lloyd, I’m totally with you. SI units are so much easier to use. You may recall this article I wrote a while back since you commented on it:
The Metric System Is for Lovers—A Valentine’s Day Manifesto
Allison,
As always, good entertainment while we learn. Regarding efficiency multiplier effect, add distance from power plant to that equation.
Fortunately for GA, the portion of coal-generated energy is dwindling, and according to EIA it was at just under 9% as of October 2020. I gotta go, the microwave oven just finished warming up my tea.
Paul
Good points, Paul. I discussed how electricity keeps getting cleaner in this article:
As long as we are talking about converting electricity to heat, consider the following scenario. You are running the blower on your furnace with the furnace heating off. The blower is consuming 400 W of electricity. The blower motor is known to be 75% efficient. How much heat are you supplying to the air stream through the blower?
You and your tricky questions, Roy! I’d say you’re putting 300 W into the air stream but 400 W into the house.
So Roy, what about light? We all know that incandescent bulbs are mostly electric heaters, but assuming no light escapes the room, doesn’t the energy converted to visible light also manifest as heat?
We think of thermal radiation as somehow being different from visible light, but all wavelengths of electromagnetic energy impart kinetic energy to the surroundings, right? It’s just a matter of frequency and energy level. Energy in = energy out.
The motor is converting 300 W to shaft power which goes to the blower and the other 100 W is converted to heat which goes directly to the air stream around the motor. The 300 W of shaft power to the blower raises the pressure and temperature of the air stream. But if you look at the whole blower and motor assembly, does the temperature of the air stream go up more or less than it would for just a 400 W electric resistance heater in the air stream with no air pressure change?
David, yep, all light or other radiation emitted in a room ends up heating the space unless it passes through a window. I am not sure if I would say that it is adding to the kinetic energy of the air or surroundings, unless you are talking about at the molecular level. In terms of classical thermodynamics, it is just increasing the internal energy of the space (air or solid surfaces). Even the microwaves in the oven end up as heat.
Roy, interesting question. I will take an amateur stab at it. Since we know mechanical energy and heat to be equivalent, the amount of energy in the unit is equivalent. Temperature is a funny thing, it is the “average” heat energy and doesn’t really equate to any real measure except in a theoretical closed system of finite volume. But it is a useful mental concept. My answer would be that in the case of using a fan, more of the energy in the system would be found in mechanical energy than in a purely passive resistance heater. The theoretical temperature would be equal because mechanical energy in a turbulent fluid converts to heat at the molecular level. The measured temperature at a thermometer would not pick up the higher energy level of the moving airstream, because the energy is not equally distributed in the space and has not all converted to heat through mechanical decay. To arrive at the correct energy total, you would need a thermoanemometer to measure both temperature and mechanical energy in the airstream. (But good luck getting an accurate measurement in a turbulent flow!)
My guess is that if you had a perfectly insulated chamber and ran a 400w device in it for an hour (whether a fan or a heater), then after a sufficiently (near infinite?) long time for friction to return the air movement to near stagnant, the measured temperature would be the same.
Bobby, I think that you pretty much nailed it. For an “ideal gas”, which is a good assumption for air at our typical conditions, the internal energy or enthalpy is only a function of temperature. Thus, it doesn’t matter if you add heat or work to the air stream in the duct; the change in temperature is the same regardless of any change in pressure. Some people think that as the air passes through ductwork with a corresponding pressure drop due to friction that the temperature will go up, but that is not true. The pressure will drop, but the density will also drop such that the enthalpy is constant (assuming no heat gains or losses through the duct walls).
You did mention energy associated with the air velocity, and that is a separate consideration from air enthalpy. That is the kinetic energy of the air stream and is proportional to the velocity squared. For typical residential ductwork, that kinetic energy is negligible.
So the bottom line is that any electrical energy to the blower ends up being additional heat load to the air stream regardless of the motor or blower efficiency.
Allison, you did not mention that the Amish Fireplace does save money by limiting the sale to only 2. That’s a 50% savings over buying 4! I’m sure that was not an intentional oversight on your part.
Ah, yes. I guess that’s another kind of multiplier effect. ;~)
I was recently looking for an electric resistance heating system for our small bathroom on winter mornings. I couldn’t believe the number of different types of heaters that are made that all make various claims about “efficiency”, “savings”, or “comfort” based off of their “unique” heating system. Some were radiant, some offered natural convection, some had a fan, but all of them were 1500W. The irony is that after installing one that looks high end / mounts to wall w/ a hidden cord, etc my wife and I went back to using our tiny plug in heater that only uses 500W because we both prefer it to be on constantly at the lower output than the on / off cycling of the 1500W wall mounted system that’s obviously too large for our little space…
Ha, Allison that toaster oven looks really good right now, we had 70F weather last week and this morning it’s 22!
These marketing techniques are the same used by the foil bubble-wrap “insulation” (and even “insulation paint”) people.
If there’s a sucker born every minute, and only a sucker would buy an Amish fireplace that uses 1.5kW, but they are limited to only purchasing two, at what rate does the kWh/year increase over time? And a bonus question: if it’s 66F in my office and my hands are cold, how long will I have to walk on my treadmill at 2 MPH in order to heat myself up as much as I would if I just ran a cheap 1500 space heater from Kmart in my office to warm it up in here? Ok, thanks. Time for lunch.
The issue you haven’t mentioned is that the smallest heat pump puts out way more heat than a 1500w electric resistance heater, and many high efficiency homes have rooms where that resistance heater is more than enough. Distribution of heat is still the biggest problem in highly insulated homes, and there really is no answer for low load rooms other than ducting or electric resistance.
appropriate fans—from very small doorway corner fans to circulate room to room, to various inline fans mounted in round duct to move air longer distances, are effective.
Allison, Here’s an electric heating claim that’s a little harder to sort through. I am thinking of Electric Radiant panel heat. Whether you put it on the floor, ceiling or wall, clearly it is electric resistance heat. But the claim is that radiant heat is “perceived” differently than convective or fan-forced air heaters. There is some theoretical support for the superiority of radiant heat, which raises the Mean Radiant Temperature of the room’s enclosing surfaces, rather than the air temperature. By doing so we reduce the heat our bodies are radiating to the cooler surfaces around us, so the air temperature setpoint, the advocates say, can be lower than for simple air heated spaces with cooler surfaces, and lower space temperature means lower delta-T drive to the outdoors. Test data seems to support that. The logical conclusion is that if you must have electric resistance heat in an area, if possible use radiant. What do you think?
Mostly, I think your comments are spot on, but there are differences in how the heat is transmitted: infrared radiation vs convection. Yes, resistance electric heating is 100% efficient, but I much prefer being near an infrared heat source than a convection source.
We currently heat with wood but we now have a heat pump also (for when I get too old to harvest the 5 cords we burn). When the indoor thermometer read 65, room feels warmer with the stove than it does with the heat pump blowing warm air. I would imagine that I would rather be in a room heated by a radiant electric heater than one that was only convection.