Plotting energy use for heating against degree days

Comparing energy consumption from year to year is interesting but to make it really useful, it's good to be able to take account of variations in winter temperatures from year to year.

Heating degree days are calculations that reflect how much energy is needed to heat a building, plotted in relation to the outside temperature.

How is it done:
To establish a degree day value, the first requirement is to calculate an average outside temperature over the 24 hour period. This number is then subtracted from 15.5 to give the degree-day value for that day. 15.5
o C is chosen because when the average outside temperature is above this most houses do not require heating to feel comfortable.

Next, measure daily heating fuel energy consumption.

Finally, plot heating energy consumption against degree-day data on a graph (ie vertical axis with heating energy values and the horizontal axis with degree day values). The slope of a line forming the best fit to the data points will show that more energy is consumed for heating on cold days (which have a high degree-day number), than on warmer days (which have a lower degree day number). So the slope will be upwards to the right of the graph. Thus, if you maintain a constant temperature in your house the graph should show a straight line. The slope of this graph tells you how much energy per day (in kWh, generally) you need to heat your house by 1
o C. NB It's the slope that matters, not the scatter of individual points. The shallower the slope, the better the energy efficiency.

If you are able to measure this slope before and after you have carried out some insulation improvements you can tell how successful these have been in reducing your energy requirements.

Given that the energy consumption data probably includes energy used for domestic hot water, there is an interesting further possible calculation. The straight line graph will probably show an intercept with the zero degree day line which is positive. This positive intercept provides an estimate of the average amount of energy required each day to heat water.

Degree day data is available to download from the Environmental Change Institute  and the  Vesma website , and the  and the Carbon Trust provide detailed information and guidance.

If you enter weekly meter readings in iMeasure, the calculations are done for you.