Quick reply...

There are many factors.
1. Battery capacity (don't drop battery onto hard surfaces! Li-ion batteries can suffer from that);
2. Connectivity (if you leave WiFi or Cellular or Bluetooth or GPS or FM or something transmitting... and if, for some reason, it has to transmit loudly - for example, cellular tower is not particularly visible indoors, or it is trying to find all the WiFi connections available around it);
3. Apps (or daemons) open, refreshing something every minute, and so on (especially widgets);
4. Memory leaks in long-open apps-or-daemons;
5. Screen-display brightness, as well as keyboard back-light.

Thank you. Best wishes.