## Degree days: some frequently-asked questions

Submit a question

*Er... Silly question, but what exactly***are**degree days?The short answer is they're a measure of how cold it has been. Naturally, there is a longer answer as well.

*How are degree days calculated?*For

**heating**degree days the procedure is based on the difference between the daily outside air temperature and some notional 'base' (the outside temperature at which no heating would be required). Daily results are added to a running total, commonly over the course of a calendar month. For**cooling**the opposite applies and the accumulated*excess*temperature is calculated. The full procedure is somewhat more complex for cases when the outside air temperature range overlaps the base temperature.*How do the Degree Day tables take into account the varying number of days in each month? Do they simply give a total by calendar month?*The monthly degree-day total is aggregated from daily results and therefore depends upon the length of the month (as too does the fuel consumption, of course).

*I have been plotting cumulative gas consumption against cumulative degree days, and I don't get a straight line, even though there is nothing we can find wrong with the building*It's your method which is wrong, not your building. There is a mathematical reason why cumulative consumption and cumulative degree-days often have a wavy-line rather than straight-line relationship. We suggest a different approach to the analysis.

*What's the significance of 20-year average degree days?*Historically, 20-year average figures were used for projecting fuel budgets. They were also sometimes (incorrectly) used to provide a fixed basis against which to

**normalise**energy consumption to account for differences in prevailing weather. Standard degree days are now available for this purpose. They have the advantage that they do not change with time.For

**budget forecasting**, whether for heating or cooling, it is now generally accepted that a moving average of less than 20 years is reasonable. Regional ten-year moving averages are available as monthly tables on this web site.*Can I calculate cooling degree days from heating degree days?*No. Although there is a vague relationship, in that cooling degree days tend to be high when heating degree days are low, the actual heating and cooling figures for a given month are not linked. In simple terms, a spell of very hot weather (which increases the cooling degree day value) will have little or no effect on the heating degree days during that period. Likewise a cold month, with a low cooling degree-day value, could clock up virtually any number of heating degree days. Nor is there any relationship with average air temperature. When the weather is very unsettled and changeable, both heating and cooling degree days will tend to be higher for a given average temperature.

*I plotted fuel consumption against degree days using a scattergram format, but my straight line trend cut the "Y" axis at a negative intercept. Any ideas?*It means your building balances at a temperature below the assumed 15.5C. This can happen when a building is highly insulated, has high casual gains, uses heat recovery, or runs at a low occupancy temperature. Your scattergram will look as if the normal diagonal characteristic has shifted vertically downwards; in fact it has shifted to the right. Try repeating the exercise using HDDs to 10C base temperature.

*How soon do results become available?*Weekly results, which run to midnight on Friday, are published by 18:00 on the first working day of the ensuing week. Monthly results for the 18 'standard' UK regions are normally on-line and distributed by email by 18:00 on the second working day of the ensuing month. There are occasional exceptions; see the planned target publication dates

## Post a question on degree days

Please use this form if you would like to ask a question on degree days to Vilnis Vesma.Degree Days Direct opening page