Common joke says that heaters are 100% effient because all they make is heat. How much energy is actually lost?



Common joke says that heaters are 100% effient because all they make is heat. How much energy is actually lost?

In: Physics

No energy is “lost” in a heater.. thats not a joke lol.. because the purpose of a heater is to heat, and any energy “loss” through components is turned into heat.. so its serving the original purpose anyway.

0. They are 100% efficient. Electronics lose energy as waste through electrical resistance. As electricity passes through wires and other components, a small amount of it is converted to heat and lost to the environment.

In a heater, the whole point is to make heat. By definition, all the energy used by the heater is put towards its intended use.

The joke is equivocating the word efficiency as meaning both “how good it is at doing something” and “work done vs heat rejected”.

An electric heater is 0% efficient since it’s converting everything to waste heat. Maybe if you wanted to stretch real hard you could say that any noise it makes is “lost” since hearing it outside the place it’s heating would be energy that didn’t go into heat. That would be like not listening to the radio while driving to save gas. Technically true, but absolutely not significant.

A heater using fuel would have losses since combustion is never perfectly complete.

Typical heaters are in fact 100% efficient. But you can do better by being willing to cool it down a bit more outside, by making a heater that is effectively the opposite of an air conditioner. Rather than *creating* heat, it just *transports* heat, and you can move heat for less than the cost of making it. These heat pumps can reach effective efficiencies of 200-300%.

People are saying 100pc efficient but it depends where in the system you measure it. Sure, all the energy will eventually be heat but a glowing element seen through a window is energy heating my retina that should be heating the room.

With electricity the efficiency losses have effectively been done for you wherever the energy is generated. So it isn’t 100% efficient from the energy source, be that solar, wind, gas, coal, transmission losses etc. It will always be more efficient to use a gas boiler than use electricity generated by gas to power an electric heater.

However, it is 100% efficient in that all the energy is converted into heat, bearing in mind all wasted energy eventually becomes heat. In the sense of heating whatever you want to heat (normally you) it is inefficient in that is is merely transitting the heat to you via air conduction, radiation or whatever method. It is not directly heating you. This is why electric blankets, heated clothing etc are the most efficient heat sources in that they mainly heat you rather than the room at large.

Of course, there is also a need to heat dwellings to prevent decay, mould etc and to that extent some of the “wasted” heat is not wasted at all. Calculating the actual efficiency – e.g. heat that you want or need to have – is almost impossible, but definitely not 100%.

There are two things I would want to contribute.

First is that electrical heaters are not exactly 100% efficient, as some of the frequencies of light given off won’t be in the infrared section, so wouldn’t really contribute to any ‘heating’ desired.

Secondly, you could get a more sensible estimate for efficiency when you consider the system you require heating, (e.g. heat loss through walls and windows).

A machine is a tool designed to take energy in one form and convert it to other forms in order to do something useful with it – whether this is an electrical device or purely mechanical. A bike takes kinetic energy from your leg movements and converts it to rotational energy in a wheel. A fan takes electric energy and converts it to rotational kinetic energy for the fan blade to push air. A heater takes electric energy and converts it to heat using a heating element.

No machine is 100% efficient, because of **resistance**. When electricity passes through a wire, the wire naturally resists this somewhat, and heats up. When a machine part vibrates, it pushes against the air, experiencing air resistance and creating sound. When moving machine parts make contact with each other, **friction** (another type of resistance) causes some kinetic energy to be lost to heat and sound. Some machines emit light, either on purpose for some display indicator, or as a waste by-product of other events such as friction (eg. sparks from a collision, or metal glowing as it heats up).

So a machine will always produce waste heat, light, and sound. But, what happens to sound as it travels? It causes friction in the air, which gradually converts that energy to heat. When it hits a wall, some of it bounces off (echo) and some gets absorbed as heat. The same thing happens to light. Eventually, all forms of energy become heat.

So an electric heater, which is designed to output heat, “wastes” some of its energy into light or sound which will also eventually turn into heat, so yes it is technically 100% efficient *eventually*. That doesn’t mean all heaters are equal though, because another important factor is where that heat ends up. While it’s true that any waste sound or light will eventually become heat, that’s not useful if the waste sound is absorbed by the walls and radiated externally, or waste light literally goes out the window and heats up your back fence.

In the context of heating a building, a system is less than 100% efficient if any of the heat generated is lost to the outside world without contributing to the heating of the building. Examples of this would be heated combustion gases going up a chimney or out of a flue. (It doesn’t matter how well the building is insulated; heat lost this way never contributes to the heating of the building, so these losses can be expressed as the efficiency of the heating system.)

Direct electrical heating systems generally don’t lose heat to the outside world this way, so their efficiency in this context can be considered to be 100%.