Factories continue to drain power even when not in use. The only way to prevent this is to not supply them with power. As of the release of 0.13 a new entity called the power switch was introduced, which made this task simpler.
To use the power switch to turn off power, you must connect one side of the power switch to an electric network via copper cable, and the the other side to a different electric network. Say for example you always connect the left side to the electric network which contains your power generation facilities, and the right side to the network which demands power. The two networks must be different networks, unconnected via copper cable, in order for the power switch to have any effect.
Now you can toggle the power through the power switch, by interacting with the power switch and setting its state to off/on. You can also connect the power switch via green/red wire, and select a condition which must be true to enable the power switch
Connecting circuit conditions to the power switch is a good way to automate the power in your factory. You may decide on which condition must be true for the power to flow, based on your own criteria.
Automatic furnace power
In this scenario, we will assume your furnaces are outputting onto a belt, and that you wish to turn the power to the furnaces off when the items on the belt are not moving, which indicates that the furnaces are not needed. To do this, you can connect a red/green wire from the power switch to a belt backlog detector. A simple belt backlog detector consists of the input belt, a splitter, two belts connected to each other via red/green wire and set to read contents and hold, then another splitter, and the output belt.
Now the power switch can be set to enable only when the condition of *anything* < 12 is true