Units - by definition - are definite, representational quantities of measurement for different systems. Units can represent the measuring of electricity, amount of items, the time taken/required to perform a task or even the amount of stacks of items with differing amounts within said stacks.
Not all in-game units are simulated with full realism.
Power is defined as work being done per unit of time.
The basic unit of power is 1 watt (W), which is defined as 1 W = 1 J/s , ie. one Joule of work being done every second.
The game commonly deals with larger units, namely kilowatts (kW) and megawatts (MW).
Work is defined as a transfer of energy, or as energy being "spent".
The basic unit of work is 1 joule (J), and is equivalent to the work done (total energy transferred) by one watt applied for one second: 1 J = 1 W s.
In the real world, kilowatt hours is a much more common unit for energy, but it is not an SI derived unit so it is not used by the game.
Tick (1/60 s)
A 1/60 second in game. This is the shortest time fraction the game handles.
One second in-game. This is not guaranteed to correspond to one real second. For example, slow computers may not manage to calculate an entire tick during the corresponding real time frame of 1/60th of a second.
A day is 25000 in-game ticks or 416.67 in-game seconds (= 6.94 in-game minutes) long.
The tile is both used as a unit of distance/length and a unit of area. For example, the size of an object may be expressed as "2×2 tiles", which means the object covers an area of 4 square tiles or tiles². The unit of square tiles is often simplified into tiles. It can be assumed, that a tile has the length of 1 meter.
A chunk is a quadratic area where one side is 32 tiles long. (1024 square tiles)
Items per time, or fluid-units per time. A unit measurement is
items / game-minute
Throughput = speed × density
For comparison: A transport belt transports normally about 900 items per in-game minute. A fast transport belt up to 1800 items/min and express transport belt nearly 2700 items / min. See physic of transport belts for more information.
Throughput depends on the distance, the number of robots and their item-stacksize. Let's assume a robot can travel 1 tile per second and can transport only one item at once. It needs also to return. Then this robot can transport ½ item per second. If you use 2 you can transport 1 item per second. If you double the distance, we are again at ½ item per second.
Top speed (later referred to as S) and acceleration (later referred to as A) depend on fuel type and train weight, for a coal-powered single locomotive without wagons they are 72 tiles/s and 9.26 tiles/s/s.
After some threshold the top speed starts decreasing linearly as train mass increases; acceleration is proportional to amount of locomotives pointing towards the travel direction and inversely proportional to train mass; deceleration is proportional to amount of wagons + amount of locomotives, inversely proportional to train mass, and affected by braking force (research) (train mass is the sum of all wagon and locomotive masses; see detailed info on wagon masses on locomotive, cargo wagon, fluid wagon, and artillery wagon pages).
Warning: The following calculations assume deceleration = acceleration and do not account for red lights.
Travel time is
(2S / A) + (distance - 4 * S^2 / A) / S
if the stations are far enough for the train to achieve full speed. If they are closer than that, the time is
2 * sqrt(distance / A)
Since a train has to make a trip back to load, the total throughput is
items per train / (2 * travel time)
Basically items per transport-unit. This depends in many cases on the item-type you use. A Cargo wagon has a capacity for 2000 items for ore, or 4000 for steel- or copper-plates.
A Cargo wagon has for example 40 stacks. The capacity of the wagon is 20 stacks. But the capacity of a stack depends on, what type of item you put into, so when stacks come into play, you need to say "Capacity of 40 stack iron-ore".
Measured in items per tile.
An item, that lays on ground has the size of 0.28 tiles2. On one tile we can place 12.752041 items, which means, that we can put in the best case 12 items on one tile.
For belts this is the same: We have two lanes on a belt, 4 items per lane or 8 item on one belt.
On belts there comes also another thing into play: Compression. Good compression is, when you fill a belt so, that you come to the maximum density and so to the maximum throughput. See also physics of transport belts for more information.
On the first glance, it is simple: A chest has the size of one tile. You have X number of stack in a chest, where you can put Y numbers of items into each, so the density is simply X × Y.
The thing changes, if you use mods, that add chest-like transport boxes, which enables to pack/box items.