# Thermodynamic efficiency of an electric heater **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [December 24, 2024, 2:50am UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089 "2024-12-24T02:50:52Z")\ **Posts on this page:** 16\ **Page:** 3
**Author:** ![echoreply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/echoreply/32/3641_2.png) [@echoreply](https://boards.straightdope.com/u/echoreply)\ **Post date:** [December 27, 2024, 7:53pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/41 "2024-12-27T19:53:39Z")
> [@Pleonast](#): > > Inefficient only in comparison to a heat pump The guy who made the video is a huge heat pump advocate, [with 2 hours or so of heat pump content](https://youtube.com/playlist?list=PLv0jwu7G_DFVIot1ubOZdR-KC-LFdOVqi&si=djBMlhTLjtBMh30K). I too initially wondered why he wasn’t putting a heat pump in his garage, but these are the reasons: - He wanted to make the point that a level 2 EV charger is just a fancy switch and plug converter - He will only use the heater when he needs to work in the garage on cold days - The heater cost $85 > [@beowulff](#): > > I’m not 100% sure of the issue, but it’s some noise thing. She’s a very sensitive individual. The bathroom exhaust fan in my dad’s new apartment is extremely quiet. So quiet I initially thought it wasn’t working. There is no way you could hear it over the shower. Unhelpfully, I don’t know the brand, model, or if it moves enough air to be effective. The only point is that very quiet fans exist. But yeah, the point of a heater in the bathroom isn’t to correct an imbalance in the whole house heating, but to make one small room comfortable for someone who is wet and naked. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [December 28, 2024, 6:20pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/42 "2024-12-28T18:20:18Z")
> [@LSLGuy](#): > > Keeping a multi-car garage at a decent temperature to do car maintenance when it’s 0C outside consumes a lot of energy. Be that energy from electricity or from gas. Garages are small.If you have good insulation it isn’t too bad. The heater output is much smaller then a house furnace. My original garage is 22’ x 30’ so fairly large. It has R-11 walls and R-19 ceilings. The through-the-wall gas furnace with blower is 20,000 BTU input but I have the flame turned way down. I use it a few days nearly every winter There have been times I have used it for a week or longer. I have never seen any big spike in the gas bill. It just doesn’t run for that long of a time. The newer addition is 10’ x 22’ with R13 walls and R-30 ceiling. Also a 20,000 BTU furnace with minimal flame height. I heat it full time to 60 degrees F. No appreciable increase in the bills from before and after. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 28, 2024, 8:00pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/43 "2024-12-28T20:00:43Z")
> [@mixdenny](#): > > The through-the-wall gas furnace with blower is 20,000 BTU input but I have the flame turned way down. 20,000 BTU per what? That’s a unit of energy, not of power. Any heater at all will eventually produce 20,000 BTU, if you leave it on long enough. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 28, 2024, 8:25pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/44 "2024-12-28T20:25:18Z")
It’s always per hour. Dumb, but advertising X BTU/hr as X BTU is very common. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 29, 2024, 5:04pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/45 "2024-12-29T17:04:28Z")
Just like how tire pressures are often stated as “pounds” instead of “pounds per square inch”, I suppose. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 29, 2024, 5:19pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/46 "2024-12-29T17:19:59Z")
> [@mixdenny](#): > > > [@LSLGuy](#): > > > > Keeping a multi-car garage at a decent temperature to do car maintenance when it’s 0C outside consumes a lot of energy. Be that energy from electricity or from gas. > > Garages are small. … Yeah. My overall point was/is that a 7.5kw electric heater (the one that started this heating digression) is not some ginourmous thing suitable for heating a stadium. Like your 20K BTU/hr gas furnace, it’s appropriate for a largish space that may have little insulation while needing to maintain a 20 to 40F delta-T with the outside world. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [December 30, 2024, 1:00pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/47 "2024-12-30T13:00:58Z")
> [@LSLGuy](#): > > My overall point was/is that a 7.5kw electric heater (the one that started this heating digression) is not some ginourmous thing suitable for heating a stadium. Like your 20K BTU/hr gas furnace, it’s appropriate for a largish space that may have little insulation while needing to maintain a 20 to 40F delta-T with the outside world. **[A typical portable 120V space heater](https://www.amazon.com/s?k=space+heater&crid=3LZZHN6OHLMUR)** that you might use in a small, well-insulated bathroom is 1500 watts. So imagine five of those (totaling 7500 watts) trying to heat a large, poorly-insulated garage (which may contain several thousand pounds of heat-absorbing cars), and you start to understand that a 7.5-KW heater isn’t at all oversized for the job. > [@Dr.Strangelove](#): > > It’s always per hour. Dumb, but advertising X BTU/hr as X BTU is very common. industry shorthand isn’t dumb, it’s convenient. If you try to order a load of concrete to a job site, they’ll ask you how many yards you want, on the assumption that you know they are asking about _cubic_ yards. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 30, 2024, 1:37pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/48 "2024-12-30T13:37:44Z")
> [@Machine\_Elf](#): > > So imagine five of those (totaling 7500 watts) trying to heat a large, poorly-insulated garage (which may contain several thousand pounds of heat-absorbing cars), and you start to understand that a 7.5-KW heater isn’t at all oversized for the job. Exactly. I was pushing back at the person who thought 7.5KW was some insane amount likely to burn up the building it was in. Nope. It’s a fully reasonable amount of heat for a garage. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 30, 2024, 2:34pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/49 "2024-12-30T14:34:58Z")
> [@Chronos](#): > > Yeah, electric heat pumps routinely operate with a coefficient of performance of around 3, equivalent to 300% efficiency. When we talk about efficiency we have to specify with regard to which inputs and outputs. If you’re talking about conversion of electricity to heat, an electrical resistance space heater is the most energy-efficient device ever produced, and it’s not even a close contest. It’s 100% efficient (neglecting EM field effects already mentioned). However, the OP asked about thermodynamic efficiency, which is how much of the input energy is converted to usable work vs. being lost to heat. In that sense, a space heater has 0% efficiency. It takes all the useful input energy and converts it into heat that can’t be used to do any work. Most heaters are 0% efficient, unless you’re talking about some industrial process that finds ways to reuse waste heat, but arguably that wouldn’t really be called a “heater” since that’s not its primary purpose. Of course you might beg to differ if you were outside in freezing weather and didn’t feel like chopping wood unless your body absorbs some heat from a heater. But that’s not a conversion of energy to real work, the real work is coming from your body’s nutrients. The heater is just signaling to your body that it’s OK to spend some calories chopping wood, because the space heater will take care of your body’s homeostatic heating needs. If we’re talking about the efficiency of _converting money into a comfortably heated house_ then it’s hard to beat a heat pump. A heat pump is much less efficient than a space heater in the narrow sense of converting energy to heat, because that’s not the point of a heat pump. The heat pump is a _heat transference_ machine. Its cost efficiency comes not from converting electricity to heat, but how it uses electricity to concentrate and transfer surplus outdoor heat to heat that’s useful to the occupants of the house. So in that sense a heat pump can operate at much higher than 100% efficiency, because it’s not creating heat, it’s tapping a vast reservoir of outdoor environmental heat, which a space heater can’t access. So there you have it. Electric-resistance heaters are 0% thermodynamically efficient. But as far as converting electric energy to heat, it’s essentially 100% efficient and no other device comes close. But if you want to buy electricity to heat your house, then a heat pump is going to be most cost-effective because tapping into the atmospheric heat of the great outdoors can be done much more cheaply than converting electricity to heat. ---
**Author:** ![OldOlds](https://avatars.discourse-cdn.com/v4/letter/o/a3d4f5/32.png) [@OldOlds](https://boards.straightdope.com/u/OldOlds)\ **Post date:** [December 30, 2024, 2:50pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/50 "2024-12-30T14:50:44Z")
How about a hydronic electric baseboard? That’s what I used in the basement room I finished at the old house, years ago before heat pumps became all the rage. [https://www.homedepot.com/p/Cadet-35-in-120-volt-500-watt-SoftHeat-Hydronic-Electric-Baseboard-Heater-in-White-13411/307247894](https://www.homedepot.com/p/Cadet-35-in-120-volt-500-watt-SoftHeat-Hydronic-Electric-Baseboard-Heater-in-White-13411/307247894) Looks like the shortest HD has is 35 inches, so don’t know if you have a piece of wall large enough. I like the hydronic because it smooths out the heating. Even had a thread here about it as I was using a 220v (120 might be enough for a bathroom?) and I was going nuts because only one phase was hot, and I couldn’t figure that out. Someone here suggested a bad breaker and sure enough, that was it. Noteworthy because I would have assumed that couldn’t possibly be the issue. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [December 30, 2024, 3:26pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/51 "2024-12-30T15:26:45Z")
That would probably work. It’s basically an oil-filled space heater in a baseboard-mount form factor. I’m going to wait until I go back up there to order anything - I need to see where the outlets are, and the various wall dimensions. ---
**Author:** ![OldOlds](https://avatars.discourse-cdn.com/v4/letter/o/a3d4f5/32.png) [@OldOlds](https://boards.straightdope.com/u/OldOlds)\ **Post date:** [December 30, 2024, 3:34pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/52 "2024-12-30T15:34:57Z")
Exactly. And they can be wired into a (special) thermostat. Most of the thermostats for these have the full line voltage run through them, rather than just a signal voltage. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [December 30, 2024, 5:07pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/53 "2024-12-30T17:07:49Z")
> [@LSLGuy](#): > > Exactly. I was pushing back at the person who thought 7.5KW was some insane amount likely to burn up the building it was in. Nope. It’s a fully reasonable amount of heat for a garage. 7.5KW is about 25,000 BTU so it is indeed an appropriate sized heater in my experience. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 30, 2024, 5:57pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/54 "2024-12-30T17:57:32Z")
> [@mixdenny](#): > > 7.5KW is about 25,000 BTU so it is indeed an appropriate sized heater in my experience. It’s also about 10 horsepower, so two push/one riding mower. “Whole house” gas generators seem to be about 25 KW. I was reading some speculation about the new Chinese fighter jet and how 500 KW would be about the floor for power consumption in a Gen 6 interceptor with one million watts not unreasonable. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [December 30, 2024, 6:09pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/55 "2024-12-30T18:09:57Z")
> [@Machine\_Elf](#): > > industry shorthand isn’t dumb, it’s convenient. Industry shorthand is fine. Just don’t print it on the box or advertise it that way, since non-insiders have to interpret the info. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [December 30, 2024, 6:11pm UTC](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089/56 "2024-12-30T18:11:30Z")
> [@jnglmassiv](#): > > “Whole house” gas generators Emergency backup generators, that is. And here’s the fighter jet essay: > **[More thoughts on 6th generation projects](https://tphuang.substack.com/p/more-thoughts-on-6th-generation-projects)** > > This is a follow up on what we have seen with the recent unveiling of Chengdu and Shenyang’s 6th generation project. [Previous page](https://boards.straightdope.com/t/thermodynamic-efficiency-of-an-electric-heater/1012089.md?page=2)