There are two constraints in the power system;
a) How much power the generator in question can put out, and
b) a hard limit of 4 connections to a given power pole.
It is really that simple, and I don't mean to be flippant about it.
Every time you connect something, you need check the power demand on that generator. If you draw too much, you'll trip the circuit breaker just like in real like. BTW you can check the demand on the generator by clicking on it, or any pole connected to it.
A BIomass burner has the ability to put out a max of 20 megawatts (20MW). A Coal Generator can put out a max of 50MW (or two and a half times what the biomass does.
So where you can connected to an average of 3 pieces of equipment with the Biomass, you can connect to an average of 8 pieces of equipment with a coal generator.
A piece of equipment tells you how many MWs is required to power it, so it is straight up addition.
THERE IS ONE EXCEPTION SO FAR: In the game, the truck station does not tell you what it draws, and I suspect because it doesn't draw anything when its just sitting there. In fact when its sitting there, its just a "storage container". However when you pull a truck up to it and trigger the loading/unloading arm it starts drawing 20MW of power.
Now, I get that 20MW draw from the official Wiki ... but I have tried using an biomass generator to power it and I blow the circuit breaker almost every time. So I quit using biomass and just use a coal generator. A coal generator can hand two active truck stations, but I connect to like 4 because seldom if ever are 3 active.
DISCLAIMER: I haven't gotten into using oil yet, but this should be solid enough to keep you going a while. :)
Also ... as Ratte said, the ADA does tell you, "Overloading the Power System will guarantee suboptimal performance ... " I have not testing if running a generator right at the edge of max output results in the connected equipment running slower. I haven't noticed it.