The first check is Alternator voltage output. You should be seeing around 13.5 volts at idle. The output should ideally be between 13.75v and 14.5v.
If it is around the 12 - 12.5v region it's not enough to top up the battery to keep in line with the engine/car requirements. As you are turning more things on, more amps are required and the alternator just can't supply it.
With a multimeter, set to 20v, clip the +[red] terminal to the + terminal of the battery and the - [black] to the neg of the battery. Check the battery voltage with the engine off. It should read @ 12.5 v. Start the engine and read at idle, it should be around the 13.5 -13.75v region. Increase the engine speed to @ 2,000 r.p.m. and it should go up to around 14.5v.
THIS is a cheap multimeter if you don't have one, there are cheaper ones around and can be bought at local motor factors for a few pounds.
If the above readings are not uniform with your tests, it is probable that you have a faulty alternator.
If the above test confirms that your charging circuit is working fine then i would look at the idle control valve although this should throw a check engine light.
There are other possibilities but the above checks should be first.