Degree Days

You can’t manage what you don’t measure – an old management adage

For anyone keen to reduce the energy consumption used for home heating, a first step is to monitor usage by reading the meter once month.  The best plan is to take the readings on the last evening of the month, or in the morning of the first day of the month. 

At the end of a year you will be able to see how much your usage varies from season to season.  The difference between usage in the summer and winter may be very dramatic if the fuel you use for heating isn’t used for any other purpose (eg if your heating is by gas, but water heating and cooking is by electricity) and rather less so if the same fuel is used for all three.

Having noticed that winter usage is dramatically higher than summer usage, you may be wondering how much difference a cold winter would make to the figures.  After all, it would be annoying to invest in some insulation and then find that your usage had increased the following year because of a much colder winter!

Degree Days can help us to understand what is really going on and to draw meaningful conclusions about the effectiveness of changes to insulation, thermostat settings, or to the heating system itself.

Degree Day data are published by various bodies – Degree Days.net publish figures for a wide range of locations and there are good quality records for Newport Pagnell.  See http://www.degreedays.net/ (The Newport Pagnell weather station ID is IBUCKSNE1.

The data are usually based on 15.5° centigrade, the average-over-24-hour temperature below which most people would tend to feel they wanted to switch the central heating on.     

A typical early October day with a maximum of 17° and a minimum of 10° (average 13.5°) would score 2 (ie the difference between 15.5° and 13.5°).

The 20-year monthly average degree days for this part of the country are approximately as follows (I’ve averaged and rounded some figures from the Carbon Trust for two adjacent regions)

October	140
November	230
December	325
January	325
February	290
March	250
April	190
May	110

To give you an idea of just how cold December 2010 was, the degree day figure for that month for Newport Pagnell was 471.

The graph below shows the variances between the monthly degree day data compared with the 20-year averages. Cold months are shown in blue and warm months in red.  Summer months are not included.

  

 

 

 

 

 

 

 

 

 

 

 

 

The maths which describe the relationship between energy usage and the degree day data are shown on the Degree Days net website. 

For a well insulated house, a lower base temperature may give a better correlation than 15.5°.

Here is a graph showing the moving annual degree day total (13.5° base) plotted against the moving annual total of gas usage for the author's detached house in Bletchley.  The correlation between the two sets of figures is remarkable:

Lisa's 2008 and 2009 footprint

Lisa has retrospectively calculated her household carbon footprint for the last two years (and puts me to shame – her figures are very low!). 

Here is a selection of the actions she has already taken, and what she plans to do in future

HOW WE CUT DOWN 2008-2009

Gas  

• We fitted radiator thermostatic controls;
• We no longer heat bedrooms when not in use;
• We increased our loft insulation,
• We made better use of wall thermostat and timer switch,
• Regular boiler servicing to maintain efficiency,
• Remembered to close internal doors to retain heat
• Use blankets, vests and jumpers instead of turning up the heating, especially of an evening.

Electricity

• Our very old dishwasher broke down and we decided not to replace it. 
• We no longer have nappies to wash so we have less laundry. 
• We wash more at 30º (though not exclusively as kids are still at messy ages!). 
• We try to use the oven to capacity when on, we take care to shut the fridge door when filling/removing things and keep kettle water minimal.

Car

• Steven began car sharing to work,
• The kids are older so can walk/cycle/scooter further,
• We plan so as to consolidate multiple journeys,
• We’ve taken less long journeys (less holiday travel)

PLANS FOR 2010

• Improve our windows and doors (now done),
• Finish loft insulation to make it all to the recommended depth,
• Fit reflectors behind radiators,
• Learn to drive more fuel efficiently (our car currently runs at 31mph.  I think we can improve this).

THOUGHTS

I use more electricity and perhaps gas than I might otherwise because of the way I cook.  I could use less by microwaving everything but I choose not to on health and taste grounds.  I use the bread machine a lot which uses power, but I save in terms of disposing of plastic bags and frequent shopping trips.  I mostly bake my own biscuits and snacks, make my own soup. All require more domestic power than buying pre-prepared but are arguably more eco-friendly when you consider the bigger picture.

David’s CRAG progess

Here’s a chart of my progress over the last four years:

 

 

 

 

 

 

 

 

 

 

 

I've managed to reduce car mileage and electricity use, but have used more gas due to increasingly cold winters.

In 2010 I installed solar PV panels which have reduced my net electricity usage considerably.  Still to do in the next few years are:

• Change my 37-year old(!) central heating boiler
• Change to a (much) more economical car, but only when my present 12-year old one has worn out!   NB Even my present one does 44mpg, but there are now many models with a  fuel consumption better than 60mpg, and a few better than 70mpg.
• Change the lighting in the main living areas to LED when the appropriate bulbs become available.

My challenge will come when one of my sons decides to move out at which point the household energy use will have to be apportioned to three people instead of four.  

This is an example of the environmental impact of us choosing to carry on living in homes that are bigger than we need…..