Does the energy of a player influences how good he plays a match?
For example does a player with 70% energy plays not as good as if he would have 100% energy?
Yes, energy count for 25% of players' performance (same for moral).
A player with 100% energy and 100% moral is at 100% of his capabilities.
A player with 0% energy and 100% moral is at 75% of his capabilities.
A player with 0% energy and 0% moral is at 50% of his capabilities.
And does the skill Stamina affects any of this?
The stamina affect the speed your player will tire during the match.
How quickly does energy regenerate?
It depends of the attribute "energy recovery speed" of the player. It can be accelerate bu upgrading the infirmary.
Can I add to this by asking........

I know a friendly does not reduce energy but does low energy actually effect how a player performs in that friendly? Or, will a team on low energy play exactly the same as one on full energy in a friendly match? Did I also read somewhere that teams play at around 50% of ability during friendly games as these are for trying out strategies and gerally experimenting.
And what about the other way around? Does 100% energy and 0% morale also result in 75% capability?
Slippery : Players' state is not changed for friendly matches. They play with their current energy and moral.

Alistair : yes
Pierrot , forgive me for clarifying but does that mean that if I play a friend one day with 50% energy I will be half as 'good' as if I play him shortly after with 100% energy? I know your answer was well meant but maybe lost a little in translation.