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# Long description

This is an area chart showing the contributions of free heat and useful solar gains to gross space heating demand in a house of normal design but reasonably well-insulated. At the left is a vertical y axis marked from 0 to 75 and labelled ‘daily energy in kilowatt-hours’. It has a horizontal x axis running in months from July, marked ‘summer’ at the left, through to December, marked ‘winter’ in the centre and on to June, marked ‘summer’ at the right. A key at the top left states: ‘average U-values of windows 2.5 watts per square metre per degree kelvin; walls, roof, floor 0.3 watts per square metre per degree kelvin’. The gross space heating demand is shown as a line starting at approximately 15 kilowatt-hours per day in July, falling to about 10 in August and then rising to a peak of about 55 at the end of January, before falling back to 15 in summer. A band of daily energy data shown in orange is labelled: ‘useful free heat 5000 kilowatt hours per year’ and starts at 10 kilowatt-hours at the beginning of July, falls to 8 kilowatt-hours at the end of July and rises to 14 kilowatt-hours at the end of August. The data then rises to peak at 16 kilowatt-hours in December, and falls to 8 kilowatt-hours in June. Data labelled ‘total solar heating’ is shown as an addition to free heat gains and starts at 30 kilowatt-hours at the beginning of July, falling to a minimum of 4 kilowatt-hours in January, and rising to 30 kilowatt-hours at the end of June. A band of data shown in yellow is labelled ‘useful solar heat 2000 kilowatt hours per year’ and starts at 0 kilowatt-hour at the beginning of September, rises to 7 kilowatt-hours at the end of September, and falls to a minimum of 2 kilowatt-hours at the end of December. The useful solar heat then rises to 17 kilowatt-hours in mid-April, and falls to 0 kilowatt-hour at the end of June. A band of data shown in white is labelled ‘net space heating 4000 kilowatt hours per year’. This is the difference between the gross space heating and the sum of the free heat gains and useful solar gains. It starts at 0 kilowatt-hours at the beginning of October, rises to peak at 30 kilowatt-hours in January, and falls to zero kilowatt-hours in April.