Saturday, August 30, 2014

Electricity analysis

I received a letter from Vattenfall the other day. Two years have passed and it's time to renew my contract.

They do a good job of providing analytical information on my account webpage. I find it fun to delve into all kinds of data about my electricity use. (Everyone has their definition of "fun", I guess!)

I made the chart shown above, which compares my electrical use as measured against heating degree days. Degree days are a tool used to calculate how much heating (or cooling) is needed for a building. A day's average temperature is measured against a base of 65°F. One day with an average of 60° would count as five heating degrees. A much colder 5° day would count as 60 degrees. Adding the daily sum for a month (or a season) gives an accurate picture of how often—and, crucially, how much— a building needs to be heated over a given time. (The same process is used to measure cooling days needed, which is important in Texas, but not Sweden.)

The European Environment Agency has a great chart showing the falloff in heating days in Sweden and Finland over the last 30 years. It's pretty clear proof of global warming, if you ask me. I also found a useful website for US/Canada that shows detail on heating and cooling days for any location, going back 20 years.

My chart confirms the obvious- there's a strong correlation between how cold it is, and how much energy the house uses. Delving into the Vattenfall data provides more detail, and my stay at the house this past March was eye-opening:

This shows my energy usage that month, along with the daily temperature. It tells two things very clearly. First, I picked a bad time to visit! I was there exactly during the coldest days of the month.

Secondly, warming the whole house used about 65 kilowatts each day. This represents an incremental cost of about $9.25 just to keep the house warm on one cold winter's day.

For our infrequent visits in the cold weather, this is fine. After all, our house is fundamentally a summer cabin. But there are things we might do to provide more efficient heating. More on this subject soon!

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