There's no problem using the laptop solely with the mains supply.
The BIG drain on laptop's battery power is usually the screen. Reducing the brightness often makes a BIG difference to battery life. (My netbook has around 8 hours battery life with the brightness set low but only around 2 hours battery life with it really turned up high).
To adjust the brightness, look for coloured keys similar to those shown in the examples here:
http://cdn.computerhope.com/laptop-brightness-keys.jpg
Then look for a key marked 'Fn' in the same colour.
Pressing Fn and a brightness adjustment key at the same time will increase or decrease the brightness.