text: A fluorescent tube has filaments at each end that have to heat up to vaporize the mercury necessary to start the tube.  Since they start so fast, the starting doesn't really cost that much more in electricity usage.\n\nHowever, the ballast on the twin 40 watt tube fixture uses about 20 watts by itself.  Thus, two 40 watt bulbs require 100 watts to operate.\n\nOnce the tube has started, the filaments are no longer activated and no longer consume power.  Since they burn hot, turning the bulb on and off will shorted the life of the tube.