I was reading this question. A person was asking what is the efficiency of an electric heater, and to sum up, people were answering "All resistive heating, can be considered to have 100% efficiency" and that this is true also for things that are not built to heat, but that produce heat, such has computers, TV etc.
Now my question then is, why different ovens have different levels of "energy efficiency"? If all the ovens are just made by a resistance, shouldn't them all have a 100% efficiency?
If all the ovens are just made by a resistance, shouldn't them all have a 100% efficiency?
An oven is not the same thing as a space heater. The purpose of the space heater is to deliver heat to the space that surrounds it. The purpose of the oven is to maintain a certain, elevated temperature inside a closed chamber.
I'm not sure how one would define "efficiency" for an electric oven. Efficiencies usually are expressed as some ratio—most often, as a ratio of some actual value to some "ideal" value.
An ideal oven would have perfect insulation. It would not allow any heat to escape. Once it achieved the desired temperature, it would not require any energy at all to maintain that temperature. Compared to that, every practical oven would have to be called 0% efficient, regardless of how well it actually was insulated.
Now my question then is, why different ovens have different levels of "energy efficiency"? If all the ovens are just made by a resistance, shouldn't them all have a 100% efficiency?
I didn't see ovens listed in the examples in the "energy efficiency" link, so I don't know what criteria are used to determine their "efficiency". But in general terms I would think "efficiency" would be the ratio of the energy that produces the desired output or outcome divided by the energy required to produce that outcome (energy input).
In that context, the desired output of the oven is to cook food. Similarly the desired output of the clothes dryer is to dry clothes. Only heat that elevates the temperature of the contents of the oven (or dryer) can produce the desired outcome. Heat lost to the environment of the oven (the kitchen) is not available to produce the desired output. An example would be a poorly thermally insulated oven. In the winter that may be a plus since it might save on heating bills if the home is heated by electricity. But in the summer it increases your air conditioning bill. A refrigerator cools its contents but in the process transfers heat to its surroundings (the air in the kitchen). Again, in the winter that may be desirable, but in the summer it wouldn't.
I have put "efficiency" in quotes because different terms are used to describe the ratio discussed above. For example, the "efficiency" of a refrigerator or heat pump is called its Coefficient of Performance (COP), which is the ratio of the desired heat transfer to the work required to make the transfer.
It has been said that the energy input of all electrical products eventually winds up as "heat". While that may be true, it alone provides no information on the efficiency of the product as described above, with the possible exception of electric room fan type air or radiant heaters.
Hope this helps.
