I was on my world and then suddenly I see an electric furnace!
What is this and how do I use it?
An electric furnace is far superior to an iron furnace, due to its ability to use energy efficiently.
Imagine you need 10 energy to complete a task. Coal uses 20 energy in this example because it will be used up entirely. However say you use a battery it will take 10/20 of its energy and you will be able to use it again producing twice as much for the same amount of energy.
This means that instead of wasting energy regardless of what you are smelting you are using only the required amount.
2 Redstone Dust, 1 Electronic Circuit, 1 Iron Furnace.
The following sources both show how to use it.