As jaytor pointed out you need to know how many watts the amp draws at idle. An amp that draws 450 watts at idle will put out the same amount of heat as 4 100 watt light bulbs and one 50 watter while idling.
Class D amps put out the least amount of heat, then Class A/B amps which switch over to Class B at low watts, say one watt. The more watts the amp puts out before switching to Class B the more heat it will produce. Some A/B amps switch at 10 watts, some 25, etc. Class A amps run at full power all the time and most of the electricity they use is turned into heat. A 25 or 30 watt Class A amp will not cause heat problems for most people, though. Finally, if you really want some heat, get a tube amp. This is an overly simplified explanation, but maybe it will be some help.
Class D amps put out the least amount of heat, then Class A/B amps which switch over to Class B at low watts, say one watt. The more watts the amp puts out before switching to Class B the more heat it will produce. Some A/B amps switch at 10 watts, some 25, etc. Class A amps run at full power all the time and most of the electricity they use is turned into heat. A 25 or 30 watt Class A amp will not cause heat problems for most people, though. Finally, if you really want some heat, get a tube amp. This is an overly simplified explanation, but maybe it will be some help.