So, this is new. I recently wired up my power area, or thought I did. Couldn't seem to get anyone to drop in to actually build it though. My one dupe running supplies will occasionally drop in and drop off metal, but my builders are off cleaning toilets, putting away fertilizer, and 100 other things instead of bringing the material and actually building what their priority is.
My guess is that supply for buildings is not being overlapped between the supply job/priority and the build job/priority for delivering supplies.
Lay out some batteries and plants, and some heavi-watt. High priority your builder and normal priority his supply. Watch him do everything else EXCEPT supply what he's supposed to build.