spell casting???, when you know when your spell has power Options

my question is simple yet Im just wondering how you guys all know how much power/energy to summon for a spell..
sometimes I feel that spells drain me quite fast..and I was wondering do you guys know when to stop sending energy into the spell and into the world..I mean do you have an internal alarm that go's off or something , how do you know when the energy has been fully sent out with the spell..???

This thread is starting not to make sense at all. Lets try and get back to the question.

To pick up on a number of posts. Energy can be gained from almost anywhere, as people have suggested: planets, elements, universal or earth energy.

I think how much it takes and knowing if something has worked comes with practice, I mean, there isn't really any way to measure it is there apart from in the results. My own concern would be with how well I could focus and put my Will into something more than the energy aspect, since it is probably going to be that the more focussed you are on what you want to achieve and the more you Will it, the more the energy you have chosen is going to grow and be directed towards what you want.