One project I worked on was to track indoor temperature and AC settings (via a Nest thermostat) and outdoor temperature (via weather report). I found that in the evenings I was often still running the AC even though outdoor temperature had dropped below my target. This alerted me that I should instead open my windows. I didn't have a way to track energy usage changes, but I was really happy to get so much more fresh air.

It stopped working when Google bought Nest and then changed their API authentication method. I had a newborn and just couldn't find the time to support the new API. Maybe I'll revive it this summer.