This is an area chart showing the contributions of free heat gains and useful solar energy to gross space heating demand in a typical poorly insulated UK house of the 1970s. At the left is a vertical y axis marked from 0 to 100 and labelled ‘daily energy in kilowatt-hours’. It has a horizontal x axis running in months from July at the left, marked ‘summer’ 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: walls and windows 1.8 watts per square metre per degree kelvin, roof 0.6 watts per square metre per degree kelvin , floor 1.0 watts per square metre per degree kelvin’. The gross space heating demand is shown as a line starting at approximately 20 kilowatt-hours per day in July, falling to about 15 in August and then rising to a peak of about 95 at the end of January, before falling back to 20 in summer. At the bottom of the chart 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, 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 the 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 3000 kilowatt-hours per year’ and starts at 10 kilowatt-hours at the beginning of July, falling to 3 kilowatt-hours at the end of July, and rising to 14 kilowatt-hours in mid-September. Useful solar heat then falls to 2 kilowatt-hours at the end of December, rises to 22 kilowatt-hours at the end of May, and falls to 10 kilowatt-hours at the end of June. A band of data shown in white, the difference between the gross space heating and the sum of the free heat gains and useful solar heating is labelled ‘net space heating 13 thousand kilowatt hours per year’. It starts at 0 kilowatt-hours in September, rises to a peak of 75 kilowatt-hours at the beginning of February and falls to zero kilowatt-hours at the end of May.