Heating Degree Days:1: A Brilliant Idea

Friends, on learning recently about the wonderful idea behind Heating Degree Days (HDDs) I found myself torn between two conflicting emotions.

  • On the one hand, I feel delighted at the cleverness of the concept and I rejoice in my new-found ability to save so much time on calculations about heating houses.
  • But on the other hand, I feel like an idiot for not having known about the idea previously!

Being the positive person that I am, I am writing this gripped by the more positive sentiment and have written four articles on the subject. This is first article in which I try to keep things simple-ish. I deal with complicated questions is this next article and the one that follows that.  In the final article I summarise the calculations that HDDs make easy.

HDDs and HTCs?

Heating Degree Days (HDDs) make it easy way to calculate the ‘thermal leakiness’ of a dwelling – a quantity technically called its overall Heat Transfer Coefficient (HTC).

The HTC is the most important number to know if you are considering any type of retrofit – insulation, draught-proofing or installing a heat pump. It allows you to answer the question:

When it’s (say) 8 °C outside, how much heating power (in watts or kWh/day) do I need to keep my house at (say) 20 °C“.

The answer is just the temperature difference (12 °C in this example) multiplied by the HTC.

So if the HTC of a dwelling is 300 W/°C then it would require 12 °C x 300 W/°C = 3,600 W or 3.6 kW to keep that dwelling warm.

But how do you find the HTC? This is normally quite hard work. It usually requires either extensive surveys and calculations or or prolonged measurements. But the idea of heating degree days HDDs makes it really easy. There is just one sum to do. Let me explain.

Degree Days in General

The idea of degree-days  is commonplace in agriculture.

For example, in viticulture, the number of Growing Degree Days (GDD) is calculated to allow farmers to estimate when the grapes will flower, or ripen, and when certain pests will emerge.

Click on Image for a larger version. Illustration of the concept of Growing Degree Days (GDDs). See text for more details.

GDDs are calculated as follows:

  • If the average daily temperature is below some Base Temperature – usually 10 °C – then one adds 0 to the number of GDDs
  • If the average daily temperature on a day is above the Base Temperature, then one subtracts the base temperature from the average temperature, and adds the result to the number of GDDs.
    • So if the average temperature on a particular day is 15 °C, and the base temperature is 10 °C, then one adds 5 °C to the GDD total.

Each geographic region has a characteristic number of GDDs available per year, and each grape-type requires a certain number of GDDs for a successful harvest. So using GDDs is a simple way to match vines to regions,

Alternatively, in any particular year, one can use the number of GDDs to discuss whether the grapes are likely to mature earlier or later.

Heating Degree Days

Heating Degree Days (HDDs) work in a similar way to GDDs, but count days when the temperature falls below a base temperature.

Over a winter season, the number of HDDs provides an estimate for the overall ‘heating demand’ that you want your heating system to meet.

Click on Image for a larger version. Illustration of the concept of Heating Degree Days (HDDs). See text for more details.

To keep things specific in this article I will mainly work with a base temperature of 16.5 °C, and the heating degree days are then known as HDD(16.5)s.

I’ll explain the choice of base temperature in the next article, but the choice corresponds to a thermostat setting of approximately 20 °C which is typical of many UK dwellings.

  • For much of the south of the UK – basically anywhere south of Manchester – the number of HDD(16.5)s per year typically lies in the range 2,150 ± 150 °C-days/year.
  • For regions north of Manchester up to Edinburgh in Scotland, the number of HDD(16.5)s per year typically lies in the range 2,350 ± 150 °C-days/year.
  • You can look up the exact number of HDD16.5s for your location for the last three years using the outstanding Heating Degree Days web site. At my home in Teddington, the annual number HDD(16.5) is typically 2,000 °C-days/year

What now?

In order to estimate the heat leak from a dwelling – its Heat Transfer Coefficient (HTC) – you also need to know one more number: how many kWh of heating the dwelling requires in a year.

  • If it’s heated with gas, you can use the annual number of kWh of gas used.
  • If it’s heated with oil, multiply the number of litres of oil used annually by 10.
    • e.g. 2,500 litres of heating oil per year is ~25,000 kWh.

So for example, before I did any work on our home, we used 15,000 kWh of gas each year. I looked up the annual number

So to calculate the HTC for my home I divide 15,000 kWh/year by 2,000 °C days/year to give 7.5 kWh/day/°C. This tells me that:

  • To heat my home 1 °C above the outside temperature required an additional 7.5 kWh of gas per day.
  • Or if I reduced the temperature in my home by 1 °C, I would save 7.5 kWh of gas per day.

Equivalently, if we divide by 24 and multiply by 1000, we can convert 7.5 kWh/day/°C into the more common units of watts i.e. 313 watts/°C.

  • So to heat my home 1 °C above the outside temperature required an additional continuous 313 W of heating.

Thinking about a heat pump?

Knowing the HTC, one can change a qualitative sense that “it’s a really cold house” into a quantitative measurement “It has a HTC of 400 W/°C“. that can help one to choose which refurbishments are likely to be effective.

Suppose, for example, we want to work out the size of heat pump required to heat our dwelling in the depths of winter.

Typically the coldest temperatures encountered routinely in the UK are around -3.5 °C i.e. around 20 °C colder than the base temperature.

So to estimate the heat pump power required for my house before insulation, I would simply multiply the heating demand (20 °C) by the HTC (313 W/°C) to yield 6.26 kW.

Additionally, if we make changes to the dwelling, such as adding triple-glazing, we can estimate the change in HTC by dividing fuel use (in kWh) by the number of HDD16.5s – a number which can be found for any location at the Heating Degree Days web site.


This article introduced the idea of using Heating Degree Days as an estimate of overall demand.

When combined with a measure of heating energy supplied over the same period, dividing one by the other magically yields the Heat Transfer Coefficient (HTC) for a dwelling.

Knowing the HTC one can measure the effect of any improvements one makes – such as triple-glazing or installation. And additionally, one can calculate the amount of heating required on a cold winter day.

But you may have some questions. For example:

  • I set my thermostat to 20 °C: Why did I recommend using 16.5 °C as a base temperature?
  • Does it really work?
  • How do HDD(16.5)s vary from one location to another and from year-to-year?

8 Responses to “Heating Degree Days:1: A Brilliant Idea”

  1. Chirag Sharma. Says:

    Hey Michael hope you are doing great! I wanted to ask like would I be correct in saying transmission of energy causes heat loss it is not loss it is more like the energy is taking energy (its own) to transmit itself( minimise and reduction) as in using energy in its initial form, like rather than heating water through petroleum gas

    I rather add a substance to it which might result in lower heat loss, transmission of energy is like friction for energy

    • protonsforbreakfast Says:


      I can’t quite grasp some of the points you are making, but your last statement I think says that

      Energy losses are like friction for energy transmission.

      I think this is roughly correct. When we try to transmit energy in any form from one place to another, some of the energy can be transformed in ways we don’t want, and it is ‘lost in transmission’.

      Best wishes


  2. Jack Chidley Says:

    Thanks for the useful instructions in this series: I have looked at degree days before but now I understand how to use it better. I tried out the ‘regression’ ‘Data type’ and discovered that my personal ‘base temperature’ is 17C. This is incredibly helpful and has prompted me to think about how I heat my house and what the temperature of the rooms should be. I remember that last year I would always turn up the heating to 25C in the living room. This year I will only turn it up to 22C as that, I am pretty sure, is actually all I really need.

  3. Greg Says:

    I turn my electric boiler off sometime in May and I turn it back on in October. Should I calculate my HDD based on the period of time the boiler is turned on?

  4. Greg Says:

    On the subject of HDD: Would this be the same as temperature demand? If so, is the Temperature Demand expressed as c-day. I ask because I have having difficulty reconciling the HTC (expressed as w/c? or w/c-day?) algebraically.

  5. protonsforbreakfast Says:

    Greg, Good Evening. It’s late and I have had a glass of wine, but I will try to answer coherently.

    I am not familiar with the operation of electric boilers, but I assume you are using this like a gas boiler to heat water for a hot water central heating system.

    The temperature demand – expressed as Heating Degree Days (HDDs) is based on the difference between internal and external temperatures.

    I made some measurements of heating power supplied to my house (in kWh) versus heating demand (°C) day-by-day during the December 2022 cold spell. This is discussed here:

    Cold Weather Measurements of Heat Transfer Coefficient

    If you are using an electric boiler as I think you are, then plotting the amount of electricity used per day (kWh) versus the difference between internal and external temperatures – the heating demand – then you should see a straight-line dependence.

    Is this what you see?

    Contact me at house@depodesta.net if you need to send detailed data.

    All the best


  6. Greg Says:

    Thank You. I have forwarded to your email my excel sheet with the demand temperature vs daily kWh heating as well as the HDD from your link to calculate ASHP size using 2 methods

  7. protonsforbreakfast Says:

    Greg: I received your spreadsheet, but it’s been a busy day. It looks impressive, but I’ll get back to you presently.

    All the best: M

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