i have recently put a new battery and alternator on the car but when at idle the volt guage sits just under 13v but as soon as i turn the lights or heaters on it drops down to 9v??. if i bring the revs up it will only then get to about 11v.turn lights ,heaters off and it goes back up. anyone have any ideas what the problem is

 

where is the voltmeter measuring the voltage? is it some arbitrary place in the internal loom, that may be subject to wiring volt-drops?
Check with a decent multimeter what is happening at the battery initially..