New battery discharged..

I installed a new battery on my P38, it started beautifully. Everything was fine. I drove it the next morning, my normal 20km drive to and back from the office. Everything was switched off, A/C..radio...lights..etc..This morning, I couldn't start it... the battery is discharged?? Can anyone help, what is actually draining the battery? :confused:

The Alternator. I thought my battery had crapped out because it was old. However I changed to a new Optima red top and it got discharged as well. So here I go, buying an alternator. I talked to a local LR mechanic about it and he says get an alternator with a lifetime warranty because they go out every 1-2 years regularly.