Your players' energy have a great impact on their performance during match. If they are tired they will run slower, pass less precise and the dribble and tackle abilities is weakened. Hence, you must understand how your players energy work during match and how your choices influence how fast energy will drop.
In your players' profiles and in physio department you can always check their current energy. These energy levels will be their initial level of energy at kick off.
Playing matches is of course exhausting. This means that your players during a match can become very tired. In the of the match it is not unusual that a player will have lost 35% points of energy or more. After the great effort players will quickly recover. Thus, the permanent loss of energy will be around 1-5% points.
The rate of how rapidly players will loose energy depends of several things:
Age: The age of the player influences the rate. Young and old players loose energy faster than players in the age 20-26.
Condition and stamina: A player's condition and stamina both have a great influence on the rate.
Opponenent's strength: If you play against a team that are stronger than yours, your players will use more energy.
Offensive mentality: The more offensive tactic instructions, the higher energy loss.
High tempo: If you've instructed your players to play at a high tempo, your players will lose energy more quickly. If your players' tempo is set to "extremely high", make sure you have set up some substitutions.
Low tempo: If your players are instructed to play at a low tempo they will save their energy. This gives them an advantage later in matches where other players have become tired.