What is idle drain
In factorio, many electrical machines (processing machines, inserters and laser turrets) have a power drain, which is the power that the machine will consume whenever it is connected to an electrical network. This idle drain adds up with the functioning power, so for example, a working assembly machine will consume 90 + 3 = 93 kW.It implies idle machines will still eat energy for nothing. In some case, the same work could been made by feweer machines, with a little energy sparing and a saving a lot of room.
How much idle drain does my factory has
This drain is typically 3% of the functioning energy consumption.It may sound insignificant, but its effect can still be sensible, depending on the proportion of idle machines you have. Also, having half of your machines idle may not be a big hindrance energy-wise, but it may hint you that your factory supply and your needs are not balanced, or that some production lines are suboptimal. When you click on an electric pole, you can see electrical energy production and consumption of the whole network it links to, average over a choosen period, element by element.If you isolate, one kind of machines, their total energy consumption will be :
total_consumption = number_of_machines*drain_per_machine + number_of_active_machines*energy_consumption_per_machine
TC = N*UD + a*UC for short (Unitary drain/consumption)
In this equation, TC and N are known from the network information (click on a pole), and UD and UC are properties of the machine that can be obtained on the wiki or in game by hoovering them, may it be on a placed machine, on your inventory or in the crafting interface. There are only one unknown variable, the number of active machines, which is what we are looking for and can be computed :
na = (TC - N * UD) / UC
TC - N * UD, the total consumption minus the total drain, can be interpreted as the usefull power. The "useless" power N * UD is the total drain, so does not represent what is wasted, since drain is needed even for active machines.
The number of idle machines is simply
ni = N - na = ... = (N*(UC + UD) - C)/UC
and the drain from idle machines (the one that is wasted) is
id = (N*(UC + UD) - C) * UD/UC
Example
Let's say I have 45 basic machine assembly (N = 45), which amount for a total consumption of 300 kW.As you probably know, each basic assembly machine has a power drain of 3kw and a consumption of 90kw. The usefell power is (only!) 300 - 45 * 3 = 165 kw. The number of active is only machines is 165/90 = 1.833 ! This is not a round number, and does not have to be : ten machine working one tenth of the time will be counted as one machine working full time, so the formula does not specify the kind of waste, whether there are outright useless machines or a lot of lowly used ones. The number of active machines is 45 - 1.833 = 43.167, and the idle drain is 43.867 * 3 = 129.5kw, or 43% of the total consumption.This extreme example is taken from an early stage of my current game, where I'm aiming to the "lazy bastard" achievement, so I dropped a lot of assembly machines to craft things I would otherwise craft by hand, and as I am, well ... lazy, I left them behind. These number where also taken at a time when I did much less research that my science pack production allowed, and when I just sat up a military assembly line, which was undersupplied. Add to that that I am also going "Steam All the way", and that my starting point has not that much coal, and that kind of waste starts to really matter.
Alternative approaches
If all that bothers you is the energy wasted, and don't need to know the number of idle machine, you can look at an other number : the ratio total_drain/total_consumption, which is easily computed : N * UD/TC. Ideally, at worse, all machinees are idle, and the total consumption is equal to the total drain, this ratio is 1, at best all machines aare working : TC = N*UD + N* UC, and this ratio is UD/(UC+UD) which is about UD/UC. For Assembly machines, the best (lowest) achievable ratio is 3/93 = 0.032 (hence my "about 3 percent", this ratio is about consistent for other macines, at least never much higher). In my example, this ratio is 45 * 3 / 300 = 45%It represents the "useless" power, which can only be lowered to about 3%, depending on the machine.
What to do about it
The two main issues of having to many idle machines are power drain, and wasted space.Unless you are as bad as me, power drain is usually negligible. With a factory runing at one third of its full pace, the wasted energy only amounts for 6% of the consommation.
Wasted space, can be something to care about. For example, high idle rate can indicate bad production ratio (eg using one gear assembly/red pack science assembly. generally, gears as indgredient and science pack recipe are prone to extreme ratio).It can also indicate a failure somwhere in the supply line, a bottleneck (you still has to find where, but at least you know somethings wrong).
Finally, there are some crafts where you need idle time, because the demand fluctuates and is not very predictible, and you really don't want shortage, like military supply. If you like circuits, you can use a power switch to disconnect branhces of you r factory when they have filled a buffer above a certain threshold.
And you, what are the idle rate of your factory?