Using Science and Math to Reveal Weather Impacts on Energy Bills

Using Science and Math to Reveal Weather Impacts on Energy Bills

by Mason PUD 3 on

January 12, 2017

Let's get this out of the way: Christmas lights don't have that big of an impact on your utility bill!

Don't do it! Scroll through these pictures to see just how cold it's been...

Originally published in The Journal on January 12, 2017
READING TIME: 3-5 minutes

The largest driver of Mason PUD 3’s power bills is the same thing that drives your power bills: the weather. Because it’s so impactful, we monitor it pretty closely. December 2016 was the coldest December since 2009, and January 2017 started off in the deep freeze!


When it comes to a building’s energy usage, just looking at the high and low of outside air temperature doesn’t tell the whole story. That’s why we use a formula that gives us “Heating Degree Days”. Warm climates (like Arizona) focus primarily on “Cooling Degree Days”, which are just the opposite. Like most things scientific, it’s a brilliant tool that has a really confusing name! (It really has nothing to do with a 24-hour “day”!)

It was established that a 65-degree indoor air temperature is “adequate for human comfort”. While that is debatable (comfort is subjective), it has become the basis of the calculation. In theory, during a constant 65-degree day, a building will not require any heating or cooling to be comfortable. The further your average daily temperature is away from 65 degrees; the more energy you need to add to a building (READ: the longer your heating system has to run) to get it to an average level of comfort.

Of course, every building Mason County is different. There are different types of heating systems with varied levels of efficiency; different types of homes that have varying levels of insulation; and different expectations of comfort, which impact how we control our thermostats.

Heating Degree Days helps us compare temperature differences, from a building energy use perspective, across multiple years. A normal, or average, heating degree day value is provided for each month based on our region. Mason County uses the Olympia regional averages. The data is reliable, even though temperatures on the Tahuya Peninsula, Matlock, and Lake Cushman can vary greatly from the South Puget Sound area.


Once an average is calculated, we compare it with “normal”, to see how much colder (or warmer) it is. If a year is 100% of normal, we had a pretty normal year, temperature-wise.

The more you zoom in on data, to a month or a day, the better your picture for energy usage. For example, 2016 overall was 11% WARMER than normal. However, December, 2016 was 11% COLDER than normal. Looking closer, eleven days in December were over 20% colder than normal. Throughout the month, we had four separate times where temperatures were greater than 20% colder than normal for at least six days. Brrrr!


This has a very significant impact on energy bills. Heat is lost through a building’s envelope (exterior roof and walls) at a rate directly proportional to the difference in temperature between the inside and outside of your home. In other words, the colder it is outside, the faster your precious heated air leaves!

You can’t stop heat from leaving your home, but you can slow it down. Having a well-insulated home is like having a warm winter coat for your house. The coat itself doesn’t generate any heat, but it does slow down the loss of heat so that your home stays warmer longer. This means your thermostat makes less calls for heat, your heater runs less, and your energy bills are lower. 

Looking at the depth of cold is another benefit to heating degree days. Many customers in Mason County have a conventional heat pump. If it’s just 5% colder than normal (for example) for the whole month, chances are their heat pump will continue harvesting heat from the outdoor air in an efficient manner like it normally would. When it gets to be really cold, like we saw in December (e.g. 20% or more colder than normal), a heat pump isn’t able to pull heat from the outside air and it has to rely on its expensive back-up heating elements. This is the same technology found in a toaster and isn’t very efficient at all. If you have a heat pump, watch this short video to learn more about auxiliary heat:


PUD 3 is available to help customers eliminate energy waste and keep their power bills manageable. We’ve been helping customers to understand their energy usage, and providing free energy audits and rebates on energy efficient equipment since 1983. To learn ways to help keep energy bills lows during this very cold weather, review our energy savings tips. Our Energy Efficiency Advisors may also be able to help answer questions you have about your energy usage. Give them a call in the Conservation department: (360) 426-0777.

You also might consider enrolling in Budget Billing. This program provides a sense of security in having your electricity bill be a consistent amount each month. It’s not a money saving program; a customer still has to monitor their energy usage. The program estimates a customer’s yearly bill based on past usage. It divides that amount into equal payments over 12 months, which helps to level-out high winter heating bills.