I had just seen a humorous Tik-Tok video regarding the thermodynamics of various electric devices, and how none can achieve 100% efficiency: there will always be some energy lost as waste heat.
So…what is the thermodynamic efficiency limit of an electric heater?
(I’ll try to find the video 
)
Electric heaters are 100% efficient at point of use as they convert every watt of electricity taken from the wall into usable heat, meaning there’s zero waste. In this sense, all electric heaters do in fact have the same efficiency. Sounds perfect right? Well, it’s not quite as cut and dry as that. We’ve also got to look at other factors that contribute to this. For example, generating electricity through a renewable energy source such as wind or solar power is much more efficient than generating it through the burning of fossil fuels, meaning any heater that uses this power will be more efficient too. This is just one way efficiency can be measured, but there are other aspects for us to take a look at as well. - SOURCE
I assume there are some losses in the electronics of the heater but that is heat too so still 100% efficient.
Where it might not be efficient is how to best get that heat energy into the room you want heated. While every watt may be turned into heat it could be inefficient in getting your room warm with the least use of energy (e.g. using your stove is not a good way to heat your house).
The problem with electric heat is that you’re converting something with very high exergy (electricity) and converting it to low-level heat. It’s more efficient to do it with a low-exergy energy source.
If you used that electricity to power a heat pump, you could do even better.
Yeah, electric heat pumps routinely operate with a coefficient of performance of around 3, equivalent to 300% efficiency. There are still fundamental thermodynamic limits, though, depending on the temperatures outside and inside (the greater the temperature difference, the harder it gets), such that for extreme enough temperature differences, it’s more practical (note: “practical” is related to but not the same as “efficient”) to just run a conventional resistive heater.
Nitpick: electric heaters are not 100% efficient.
Even though 100% of the power they consume ends up as heat, it doesn’t all end up in the room that is being heated - some of ends up in the wires feeding the heater, and some of that heat is not useful.
Doesn’t that ultimately end up in the room? Where else would it go?
Well, at least some of the heat ends up leaking out of the house and just warming up the exterior.
In fact, eventually, it all does that. Which means that, first, we should be looking at the efficiency of the house as an entire system, and second, that we should be looking at some metric other than energy benefitted from divided by energy consumed as a measure of efficiency.
In the walls, or outside.
I mean - an electric space heater is probably 99.5% efficient at heating a room, but it’s not 100%, if you are measuring usable heat.
(FWIW, I just bought a highly-efficient heat pump for our summer house. My wife wants to add a space heater for the attached bathroom, which irks me to no end, since a through-wall fan would do exactly what we need, and consume 1/2 as much energy. But, there’s no fighting City Hall).
Especially if you have a large house with few occupants, you can also do things like keep most of the house cool, and use small radiant heaters at the places where people spend most of their time. This accomplishes the same ultimate goal of keeping the occupants comfortable, while consuming less energy overall (and is hence more efficient, if efficiency is measured in comfort per energy consumed).
Since this thread starts with a statement that just begs for close scrutiny, I think we have to note that a tiny proportion of the electrical energy winds up creating an alternating magnetic field around the heater, and some (much?) of that will radiate outward through the walls defining our space. Being even more fastidious, we have to acknowledge there’d be an electrostatic field leaking out, also. And, my electric heater sitting next to me has a fan, which creates sound and mechanical vibrations that will do likewise.
I mean, jeez, throwing down the 100% efficiency gauntlet, what can you expect?
For all practical purposes we can assume all the heat goes into the room (and then leaks out of the room). Arguing about the 0.001% (or whatever) of the heat that immediately escapes the room is being a bit pedantic, IMO.
Having said that, it’s true that not all heat is created equal. As an example, a 100 watt electric heater w/ fan is much more effective at heating a room vs. a 100 watt incandescent light bulb. Even though, in both cases, we are injecting 100 watts of heat into the room.
Some of my heat warms up the ducts that are in the attic which is well insulated from the rest of the house.
It depends where you draw your system boundary. There are transmission and distribution losses all the way from the power station to the point of use, and indeed there are losses within the power station, and in the power station’s fuel supply chain. These are normally excluded from the input side of an efficiency calculation for a device. By the same token, I would expect the losses in the wiring within the house to be excluded, although of course you can choose to draw the envelope to include them if you wish.
When it comes to measuring COP of heat pumps, the international standard (I forget the number) allows a choice between a number of envelopes, including or excluding various pumping loads and losses. There are times (contractual performance guarantees) when it is very important to be clear about which one is being measured.
I always wondered about this. If all of the electrical energy going into the element is converted to heat. Why do I see current flowing out of the element? The resistance to movement of electrons creates the heat. I know the math works out. And the device has to have a path from positive to negative. But to be truly 100% efficient, wouldn’t all the current need to be somehow converted to heat, with no exit of electrons? Somehow the element itself impossibly being the negative. The total voltage is dropped over the element. But the current is not stopped. I know that if another element is placed after the initial element, the total resistance goes up and the voltage drop is now split up. So both are functioning at 100% at their respective now lesser voltages. I feel that the 100% term is more that it is mathematically 100% but not in a purely energy converted to heat way.
Your question is a little confusing. An electron can transfer energy without being annihilated. So can water flowing downhill.
First of all, in electrical circuits, current must flow in a loop. The operative word there is must. This is true regardless of the type of load.
Furthermore, your assumption that electrons flow “in and out of” the heater isn’t correct. The vast majority of heaters are AC, which means the electrons just wiggle back and forth - they don’t really “go” anywhere.
Lastly, the energy isn’t carried by the electrons themselves. The energy is in the electric and magnetic fields that are outside the wires.
I have a friend who is a huge heat pump fan, and recently wrote about his experiences in a family cabin heated with a pump. It didn’t do nearly enough, and the air near the bed was 45F. Finally, he tried turning on the fan, which he associated with cooling, not heating. That did the trick, and he got the air up to 65F near the bed. The bathroom was still way too cold.
A thru-wall fan would have helped a lot. But there’s a reason or took him hours to think to turn the fan on. Drafts are cooling. Because we are heat sources, still air feels warmer than moving air. At 65F, there’s a pretty big difference between the two.
Anyway, a low-speed thru-wall fan will help, but why not try both? The space heater will give her some radiant heat that feels nice, especially when you are wet from the shower. It’s nice to have the bathroom warmer than the bedroom, even if you aren’t wet. And if the thru-wall fan does a lot of the work, the space heater will be used less.
Think of electricity in a circuit like a bunch of marbles inside a hose. If you push a marble into one end of the hose, all of the marbles in the hose are pushed, and the last marble pops out of the other end of the hose. So the “motion”: of the marbles goes from one end of the hose to the other very quickly, but the individual marbles only moved a fraction of an inch (the width of a marble).
Simple enough as a concept, right? Electricity always flows in circles though, so imagine instead of a hose that your marbles are inside of a hula hoop. If you push one marble an inch to the left, all of the marbles all the way around the hoop move, and this happens almost instantly. But even though the motion goes all the way around the hoop, each marble only moved an inch. And even with all of this motion, none of the marbles was consumed. You still have the same number of marbles inside the hula hoop.
Now imagine that you have something on one side of the hula hoop that kinda grabs on to the marbles and makes it more difficult for them to move. If you push the marbles from the other side of the hula hoop, it takes more force to push them, since that force ends up going into the thing on the other side of the hoop that is resisting the flow of the marbles. So what you are effectively doing is transferring force from the side of the hula hoop where you are pushing, over to the thing on the other side of the hula hoop that is resisting the flow of the marbles. There is your energy in (you pushing the marbles from one side of the hula hoop) and your energy out (the energy going into the thing that is resisting the flow of the marbles), so it’s one big energy transfer across the hula hoop.
That’s how electricity works. It’s a bit more complicated than that, so don’t take the analogy too literally, but that explains the concept. You aren’t pushing marbles through a hose from the generator to the heater. You’re moving marbles back and forth to transfer energy from the generator to the heater. At the end of the day, you still have all of the marbles in your hula hoop.
In AC power, like what you have in your home, the marbles don’t even constantly move in one direction. First they move in one direction, then they reverse and move in the other direction, back and forth and back and forth, 60 times per second (in the U.S., the frequency varies elsewhere).
To take the analogy a step further, the voltage is how many marbles you have, and the current is how fast they are moving, i.e. how hard you are pushing them through the hula hoop.
In the real world, as soon as you flip a light switch, the electrical “motion” moves at about a foot per nanosecond. So if your light bulb is 10 feet away from your light switch, it takes about 10 nanoseconds for the electricity flow to go from the switch to the bulb. But in AC house wiring, each electron only moves back and forth a fraction of a millimeter. The electrons don’t flow all the way from the switch to your light bulb. They just move back and forth, like marbles in a hula hoop.
Electrical energy isn’t embodied in the electrons. It is embodied in the flow of electrons. No flow, no energy transfer. Electrons flow in response to an electrical field - in terms of what we are discussing this is the voltage across the ends of the heater. That voltage will make charge flow in the wire. You need both ends of the wire to have a potential difference, and charge flows in one side and out of the other. (The actual flow of physical electrons is ridiculously slow. Known as the drift velocity it is less than walking pace for most applications. That it can be so low underlies just how many electrons there are in a wire.)
Wires are not perfect conductors. The charge carriers bump into stuff, and that impedes their flow. This bumping also transfers kinetic energy from the charge carrier into the wire, making the wire heat up. This habit wires have of impeding flow is funnily enough generally termed impedance, but in terms of what we are worried about here, the simpler term - resistance to flow, aka resistance is what we care about.
So the higher the electrical field potential we apply, the more impetus there is for charge carriers to flow in the wire, and move against the resistance of the wire. So we get more flow, more bumping about, and more heat. In the end we arrive at Ohm’s Law. Electrical flow (I) equals potential difference (V) divided by resistance to flow (R).
I = V/R
What we also may discover is that the power dissipated in the wire (P) is equal to the potential times the current.
P = IV
In units, power in Watts equals potential as Volts, and current in Amperes.
which also leads us to find that
P = I^2R and P = V^2/R
Turn the applied voltage up, and the power dissipated in the same device goes up as the square of the voltage.
The smell of I^2R is also generally something associated with the magic smoke escaping from devices.