Is my system undersized?

What are some key indicators that my HVAC system is undersized? My utility bill averages $600/month and I’m convinced that my HVAC system is the issue. I installed a smart thermostat a few months ago as a means of saving money and viewing heating/cooling trends, but I’m not techy enough to understand all of it. Is anyone able to recommend or identify issues from my Temp Profile? Any tips appreciated!

For general sizing info, you typically want to look at the 1% design temp. If you’re in the US, you can look up those values here:

If sized correctly, your system should run about 100% of the time at this temperature. It can vary a bit depending on how your aux heat is set up, but I would use the Thermostat Summary chart in beestat to find a day at or close to this 1% temp, then see what percentage of the day your system ran.

What are the specs on your hvac system? Model numbers etc. Jon’s method is true only if you have a simple single stage system.

Your temperature profiles seem to indicate that your heat (orange line) does not heat? (i.e. it lies right on top of the resist, grey line). Probably something is configured wrong?