Energy Intensity is the amount of energy used per unit of output. For buildings, the output would refer to floor area.
The formula for determining the Energy Intensity for a building would be
If Energy Intensity is generated for 1 year, say 2008, the formula would be
If the intensity is generated for 2008 and 2009, the formula becomes
If the floor area didn’t change in 2008 and 2009, then the energy consumption of multiple years is being divided by the same floor area value as the energy consumption of one year. This would appear as a two-fold difference in energy intensity between the report for one year and the report for two years. The consumption has doubled but the divisor (floor area) has not.