From the theoretical side, with some VERY quick-and-dirty math:
My car uses
110w = 2x 55w headlight
75w = 3x 25w taillight, including high-mounted
185w extra load
Alternator: 60% efficient (
Alternator - Wikipedia, the free encyclopedia)
Lights on = 308w extra load on the engine
308w/ 745 watts in hp = 0.41 hp extra load
Using the
calculator, my car needs ~10hp to roll at 55 mph.
0.41 hp is a 4.1% loss of efficiency at that speed.
11% loss at 35 mph
2% loss at 70 mph