The total daily energy volume is the total generation volume for the previous day expressed on an INDO basis, i.e. excluding station transformer, pumping and interconnector export demand. It is calculated from summing the half hourly INDO demands (divided by two to convert to MWh). Three sets of reference daily energy values are displayed - these are low, normal and high reference daily transmission energy. The daily reference energy values are calculated as the levels exceeded on 88%, 50% and 12% of days based on historical daily values from the past 30 years. The calculation includes indexation to allow for underlying demand growth for historic years .