GregTech Energy Systems V3.04b is one of many Written Books that may be found in dungeons to help you learn how to use GregTech 4.
This Book explains Core Functionality of GregTech Machines. You might not understand it, but it can help you in some Cases.
As you know, GregTech Machines can run on 4 different Energy Systems. Energy Units and Universal Electricity are natively compatible without Upgrades and are stored in the Electric Capacity. MJ and Steam need Upgrades and have separate Capacities.
All those 3 Capacities store their Energy in Universal Energy Units. These Energy Units are basically the converted Version of MJ, Steam and UE into the EU-System. They are stored in 3 Different Capacities (UE and EU share as they are both Electricity)
If a Machine now needs Energy, it first checks the Universal Capacity of the MJ-Units and tries to consume them. If that fails it checks the Capacity of Steam and tries to consume that. If they both failed, then it uses the Capacity for Electricity.
As Machines usually check, if they even have enough Energy, before starting to work, they always check the one capacity, which has the largest amount of Energy. That includes the Energy Meter Cover, so you measure the largest Potential of the Machine.
Almost every Cover Upgrade, Generator or similar Device produces Electricity. Electricity is the only Energy, which can be outputted by GregTech Machines. Everything else will be stuck inside the Machine until it gets consumed.
In case of Steam, the Machine will be able to output it, but only if it is inside the regular Tank (when I happen to add Steam based Solar for example). The internal Steam Energy Tank can't output Steam.