Seeing the old trailer vs panniers thread come up AGAIN, I thought I would post this link to a calculator that can give you some real numbers for your riding set-up. This calculator can help you make decisions about equipment, packing methods and riding styles. Tell us what you find??
The final number given is watts of energy needed to maintain the speed you set under different variables. Those variables being;
Frontal area, (trailer vs panniers comparisons)
altitude (air density)
weight of rider
rolling resistance (trailer vs panniers comparisons, if resistance for 2 wheels is .004, then 3 should be .006, half again as much
slope (hills vs flat ground)
and more that for our purposes can just be left on the default settings.
So plug-in your assumptions and see what you get.