I've searched through the forums and found nothing about any sort of bug with wind mills. That may apply also to solar panels though I've not personally tested them (friend on server said he couldn't get any power for them)
Basically from what I've experienced: It SEEMS that whenever I leave the area the windmills I've created stop working (as usual, they're unloaded when i'm far enough) but when I come back they do not start back up. They work normally it seems until I leave from setting it up, charging batteries and batbox attached. This I have experienced TWICE (once on my single player game at a height of around 100-150 tops and currently on my SMP server at the 250-255 range.) I want to highlight the fact that it works normally when placed. I will do more testing on dissaembling the mills and then placing them tos ee if this "refreshes" it and kickstarts the eu trasnfer. This seems to be a really annoying bug i've discovered and my friend thinks it extends to solars as well.
Does anyone else have trouble with this? Its strange, you'd think the same bug would also plague the passive water mill but that is the only reliable source of passive power for me. I am almost postive this is not "just because" of wind being ridiuclously low / nonexistant. I have sat around the batbox for a while and not gotten a single EU from them.