Leave that Thermostat Alone!

2015 Home Energy Usage vs Outdoor Temperature

Last year my electricity bills totaled just over $2000, with the individual bills ranging between $77 in June and $318 in March. In June I would have been paying for my May electricity usage, and in March I would have been paying for February. May in Maryland is typically mild, with temperatures in the high 60s and low 70s, and February is fridgid, with temperatures below freezing more often than not.

It's obvious that a large difference between the outdoor temperature and a comfortable indoor temperature would drive up an electric bill since if it's too hot or too cold out, the heat (our system is electric, not gas) or air conditioner will have to run to keep the house comfortable. I wanted to see just how much of a difference it could make on a fine-grained level, so I plotted my home energy usage against the outdoor temperature for 2015 on an hour-by-hour basis. The resulting charts are below.

Most of the time, when my wife and I leave home, we adjust the thermostat so the heat and air conditioner won't run as heavily, and we bump it back to a more comfortable setting when we return home. It turns out that running the heat and air conditioner makes such an extreme difference in our home energy usage that in a lot of cases I can tell exactly what time we adjusted the thermostat. The difference is particularly noticeable during the summer and fall, when we took several multi-day trips. In the charts, these trips stand out as periods of very low energy usage in otherwise "normal"-looking months.

Yup. Not being home is a good way to keep your home energy costs down.

How to read the charts

Each vertical axis is the day of the month, and each horizontal axis is the hour of day. The air temperature for each day of the month and hour of the day is represented by the color, and the amount of energy used is represented by the size of the square.

Roll your mouse over (or tap on) each cell to see details. Noteworthy events (leaving home, returning home, etc.) are called out with a red border and an explanation at the right. Saturdays and Sundays have a slightly different background color to make them stand out better.


Data collection

The energy usage data comes from my energy provider, BGE. They have a tool that will export your account's per-hour usage data as a CSV file.

The temperature data comes from Weather Underground, which has a nice API for extracting fine-grained historic weather data. Weather Underground reports multiple readings per hour, so I averaged the temperatures across each hour to map them onto the energy usage data. I used their data from Tipton Airport, the nearest weather station to my house, when the data was available, and I used their data from BWI when they were missing readings from Tipton.

To work out at exactly what events were occuring at specific hours of the year, I reviewed old emails, text messages, calendar events, and the timestamps from photos.