And personally: I don't even know what benefit upgrading has on both the windmails or solar panels. I asume it will save you more money weekly. I've heard some of you saying placing 10 solar panels and 5 windmails can save you up 1/3 of the total bills every week, so maybe it will save you up 2/3 or at least 2/2 when upgraded. Who knows.
As far as I can tell, the upgrades don't help a huge amount, but they do help. In the longest running house I played that had them, their bills before the solar panels were about $6,000. When I added the solar panels/windmills it dropped to about $5,000. After upgrading the entire lot of them, it brought the bills down to a bit under $4,000.
That family also had the Renault Twizzy, and I read somewhere that driving one of those helps as well.