I want to keep track of when my furnace is on, with an eye to reducing how much energy I use to heat my house.
I could compare it with weather data (temperature, wind speed, precipitation).
I could see what it saves to turn the heat down when we're out.
I could tell my kids exactly how much it costs when they leave the front door open.
Best Answer
While this won't specifically isolate your furnace from the rest of your energy usage, I'd recommend getting a Blue Line PowerCost Monitor (or the Black & Decker branded version, which is cheaper) their WiFi Gateway, and a free Microsoft Hohm account.
Current Amazon links and prices:
That will give you a display that shows electricity used (and cost, if you enter the rates) and let you view some nice charts of usage over time (including temperature for your zip code) on the Microsoft Hohm site.
Here are some graphs from my account to show what I mean...
The 6-hour usage graph shows 'humps' where the (electric) fan for our (gas) furnace is blowing. Other 'humps' likely represent our refrigerator. The only real way to identify which is which is to turn off one or the other for a period of time and see how the graph changes. For the record, I haven't yet installed a programmable thermostat so our furnace will try to maintain ~68F all day and night.
The daily usage graph shows us returning home around 6:30 last night, that I was up until about 1:30 this morning, and that we were up and moving about this morning from 6:30-9:30.
The monthly usage graph shows our usage each day and what the local temperature was (which isn't available for the most recent couple days yet). You can see how the temperature affects our energy usage pretty clearly. Obviously the chart shows that our usage was lower both of the past two weekends (24-26th and 30th-1st), but you can't tell from the chart that this was because we were out of town those days. Had we been home, our usage would probably be higher than the normal weekday usage.
I'm sure there's plenty of energy trimming we can do, as we've just gotten started with this. But for only $150 invested (I got the Gateway for $103 in November), it's a pretty easy solution to help save in the long run.
The installation was easy for both the Monitor (on the electric meter outside our house) and the Gateway (setup my WiFi network info via USB cable, then reposition Gateway unit within range of both Monitor and our WiFi router and an electrical outlet).