Originally Posted by cyccommute

There shouldn't be any heat problems but your run time is going to be cut down. The 20W bulb will draw 3.3 A while the 15W bulb will draw 2.5 A. If you have a 3.3 Ah battery, you'll lose about a half an hour with the 20 W bulb. In other words, you'll lose about 25% of your capacity.