tl;dr: Turning the Fan to On (vs. Auto) on my central A/C control panel causes high energy usage, most likely due to the backup heating elements being turned on. Any ideas as to why this might happen? Bad-wiring, flaky circuit-board, etc?
Full story:
For the last two years, in/around the month of May, I've been stuck with insanely high electric bills. The first time, I complained to utility, shut off my outside circuit, etc., but never found a cause. Given that May is the time when I least need heat or A/C, this never really made sense to me.
This time I decided to experiment, based on the one thing I could think of that I was doing differently: since I wasn't running heat or A/C (actually leaving the A/C on 80+ just in case of a heat wave), I would instead turn the fan on full-time to keep air circulating between floors. Low and behold, that was the culprit! Usage went through the roof, and dropped back to almost nothing when I left it off for a day.
Just to be sure, I turned the A/C completely off and left the fan on overnight. I had the windows open, but it never really got cool in the house. And when I went into the bathroom, it was like a sauna! And of course the energy usage was through the roof again.
The only thing I can conclude is that the auxiliary heating elements are turning on whenever I set the fan to On instead of Auto. I didn't notice it before because I always left the A/C on at 80, so the temp. never got beyond that, and I opened the windows at night.
I'm not sure how long it might have been like this, for a couple of reasons:
1) My electric meter was replaced about two years ago, and the old analog one might not have properly recorded the full magnitude of my usage (or the new one could be exaggerating it)
2) The big capacitor on my heat pump blew maybe three years ago, which could have damaged the rest of the system somehow.
I'll probably wind up calling the A/C guy, but I'd love to know what the heck it could be, especially if it's something like a simple wiring problem I could fix myself.