We have mentioned U Values many times over the previous paragraphs and so what we thought we should do at this point is just give a simple example of how U Values work and their impact on heat loss. If we take a standard 3 bedroomed, 2 storey, semi-detached house as an example, it will typically have about 120M^{2} of external walls (not counting the boundary wall with the attached house) and will also have about 20M^{2} of windows and doors. We will now do a comparison of heat loss through an uninsulated wall and an equivalent insulated wall to show both the difference in heat loss and the energy cost. We have been using statements like 0.2 and 2.0W/M^{2}/K so to explain what this means, 2.0 refers to the amount of heat lost, in watts (W), through every square metre (M^{2}) of wall, for every one degree Celsius of temperature difference (K), every hour. So to explain we will say that our wall is 120M^{2}, the temperature inside in winter is 20^{o}C, outside it is 5^{o}C, the wall has a U value of 1.5W/M^{2}/K, it costs €0.10 per kilowatt of energy and the heating is on for just 8 hours. Calculation is 120 X 15 X 1.5 X 8 = 21,600 watts of heat loss. Cost is (21,600/1,000) X €0.10 = €2.16. If the heating was then on for 8 hours per day for just 150 days in winter then the cost would be €2.16 X 150) which is €324 and that is only the wall calculation.

If we now insulate the wall to a U Value of 0.2W/M^{2}/K then the calculation is 120 X 15 X 0.2 X 8 = 2,880 watts of heat loss. Cost is (2,880/1,000) X €0.10 = €0.29. If the heating was then on for 8 hours per day for just 150 days in winter then the cost would be €0.29 X 150) which is €43.50 a saving of almost €300 in this example. This is just an example to show how a U Value calculation works and the actual calculation of heat loss from buildings has many elements and needs to be very carefully approached.