A couple things can cause your battery to drain even while in standby:
- poor cell strength/signal
- apps syncing in the background (check each app's sync schedule/setting)
- app with improperly coded wakelock logic (i.e., they keep the CPU awake even when they shouldn't (i.e., normally when the screen is off))
Also, the age of your battery can make a big difference...obviously not an issue in your case, though.
Battery size would make a different--if there's a larger / extended battery available, you might consider that.
Best of luck!