The use of degree days in environmental management

Environmental assessment: proving savings in heating fuel

By Vilnis Vesma. June 1999

Reducing the amount of heating fuel you consume will reduce your CO₂ emissions. But to prove 'continuous improvement' as required under ISO14001 and ISO14032 you have to allow for variations in the weather. 1997 and 1998 were both mild years, so most people's fuel consumption would have been lower than average anyway. But the next twelve months may well revert to something closer to long-term averages, causing an increase of perhaps 8-10% in fuel demand in the coming year. The task will be to adjust actual fuel consumption to account for this variation. Otherwise even those who have done well will appear to have lost ground when the weather turns against them.

The effect of the weather is accounted for by degree days *, figures calculated from outside air temperatures to give a single index number to indicate how cold it was in a given month (or week). The figures are published on a regional basis for 18 centres in the UK and can be used to analyse patterns of fuel consumption.

There are four major steps to the process:

Establish the existing relationship between kWh fuel consumption and local degree-day counts
Carry out your energy conservation measures
Each month or week thereafter, measure your actual kWh consumption and compare it with what it would have been if you had not improved the situation
Keep a running total of cumulative kWh savings and express these in cost terms

In order to achieve anything it is essential to have regular meter readings at monthly intervals, or for large buildings, weekly.

Establishing the existing relationship

Here's a way to fix the 'baseline' against which you will measure future improvement objectively, regardless of weather variations.

Collect a year's monthly kWh fuel consumption figures
Add up the annual total kWh
Identify or estimate the minimum monthly consumption (which will usually be in July or August). Call this 'k₀' for later reference.
Multiply the minimum monthly kWh by 12. This gives an approximation for your annual non-weather-related consumption
Subtract the result of step 4 from the result in step 2. This is your estimate of variable, weather-related, consumption
Obtain the corresponding monthly degree-day values for the region where the building is situated
Add up the total degree days for the year
Divide the kWh total (from step 5) by the annual degree days (step 7) to estimate the kWh demand per degree day, which we'll call 'k₁'

Assessing subsequent performance against the baseline

Once you have put in place your energy-saving measures, you will need to prove the savings you've achieved. Here's how (I'll talk in monthly terms, but you can do it weekly):

First let's recall that we have got two fixed numbers which describe how the building responds to changes in the weather (or used to respond - you have hopefully changed it for the better):

k₀ is the number of kWh units which will consumed every month, regardless (the 'fixed' or 'base load' consumption)
k₁ is the number of kWh units which are required per degree day (the variable part)

Each month, do the following:

Measure your consumption by means of meter readings. Let's say that you used C kWh.
Get the regional degree-day figure for the month. Let's say that the value was W degree days.
Calculate the expected consumption in kWh, E, which is simply k₀ + k₁.W
Calculate the saving in kWh: E - C

Note: occasionally the saving may be negative. Don't worry; it's equally likely that some savings will exceed expectations. If you keep a running total of the cumulative saving you will make your point. And if that running total ever settles out at a steady value, find out why - it means your energy-saving initiative has stopped working.

More information on energy management in general, and degree days in particular, can be found at Opening Page