Increasing tire air pressure in conjunction with fastidious adjustment of the bearing cones and lubrication on both the trailer and bicycle could concievably win another 20 watts. Reflect, that batteries, like liquid fuels, represent consumibles. Their upkeep requires constant looking after. This burning up of personal and natural resources holds true whether or not artifical machinery is added to the bicycle after-market. Tires and brake pads wear out as does any moving part in proportion to useage.
Gasoline blantly burns at a clearly definable monetary rate.
The same principles of financial and time drag apply to all forms of transport, the muscle-powered vehicle merely holds inevitable depletion rates to a sustainable level. Social drag refers to operating costs per mile compared the amount of income directly or indirectly generated from a job commute. That is, instead of spending lifetime pedaling an HPV, we chance giving away a greater percentage of wage-earning lifetime directed towards purchasing transportation. It's somewhat of a dicey question to be able to answer properly for each individual's situation. See this page of me getting started last January one last time before I change it.