Don't agree. What if I don't use a filament? The NOZZLE has to be hot enough to melt the plastic (if that's what one is using. Once it is hot, one only needs enough power to MAINTAIN the temperature for a given mass flow through the extuder. Who cares if it takes minutes (or hours, for that matter) to get up to temperature. I am not using a filament, or cartridge heater for my extuder. Essentially, one needs to eliminate heat leakage/wast to the extuder/air/environment and just heat the plastic ... not the components of the extruder. Make the extuder a dewar.

I based my conclusion on 1 cm/sec through a nozzle with a diameter of 0.5 mm, using ABS (as a worst-case). Calculated 0.289 W will do it (with NO heat loss, so that's a boundry number). If I can keep the heat loss way down, 5-10 W should be sufficient. Best to build it and test it ... if inadequate, I'll grow the design. Thanks for comment zak...

Extruders need enough wattage to heat filament quickly; keep in mind that reducing the wattage you send to the extruder might reduce the volume of material you can produce in a given amount of time. Brian Benchoff of Hackaday provides a brief overview of the calculations in a post from a few months ago:

http://hackaday.com/2014/05/03/ask-hackaday-can-the-lix-3d-printing-pen-actually-work/

Make sure you have enough wattage for your desired extrusion rate, or you'll have to slow down your robots dramatically!