As weather conditions and temperatures change throughout the year, it's a given that the amount of energy needed for heating changes too. This makes comparing your energy efficiency season to season difficult — no one month or year is the same as the next.
This is where 'degree days' can help: Degree days allow you to take the weather into account when looking at your energy use for heating or cooling.
A degree day number is worked out by calculating the average temperature of a day, and taking this from a base heating or cooling temperature. So if you are aiming to heat to 20 deg C and the outside temperature has averaged 5 deg C then you'll have accumulated 15 degree days. If you summate this for an extended period, say a month you'll then get the number of degree days for the month. For example, this year, in January we have accumulated about 485 degree days for a base heating temperature of 15.5 deg C.
Degree days, by themselves, are not much use, but match them to energy used in a period, and they can help you to predict and compare efficiency. So if you used 30,000 kWh of energy this month when degree days were say, 400. And the next month, degrees days go down to 300, you'd expect energy use to be 300/400ths of last months figure (22,500 kWh). It its lower you are getting moe energy efficient; higher and something maybe going wrong.
By analysing your energy use during a particular period in terms of degree day values — rather than by comparing it to your energy use during the same period in previous years — you will get a picture of how the temperatures and weather conditions could have had an impact.
You can find free degree day data for different regions online. This data presumes a 'standard' inside temperature, typically 15.5 degrees C. If your inside temperature is higher / lower, your degree day values would be different. Some sources will calculate degree days for you based on your inside temperature, but there is usually a charge for this.
Find out more about degree days:
See Using degree days to see how the weather is affecting your energy use for more information about how degree days work, or take a look at our real-life example of degree day analysis.