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KERS. How exactly it works?

How exactly KERS works? How is KERS effect measured and in what measurement units? Is it %s of speed or seconds?
Is it depends on fuel load at the moment it is used?
Is it depends on the tyres used at the moment?
Is it depends on the moment tyre wear percentage?
Is it depends on the current setting of push level?
Is it depends on the exact moment speed of a car when you press the KERS button?
Is it depends on basic lap time counted for a clear track (no cars around and low push level)?

You always get a same amount of boost, no matter if you run low or high fuel if I am correct, but ofcourse it will work different in corners to when you use it on a straight, you can use it the best on exit of corners, because there is the most time to win.

The same amount of boost measured in what units? How do you compare different boost levels? Stronger/weaker is not the way. We need numbers.

Its just an amount of extra power and depends of all above in my opinion. (about the result not the power)

Cannot give you numbers for lower levels and how its scaling but i can say with full kers use (20Level) you can do more less 7-9 sec faster lap.
Compare my last 2 laps