A more likely scenario is that the green roof will be
added and provide additional insulation, not used as
replacement for traditional insulation. This additional
insulation value creates energy savings for the building
owner. The authors used the building energy savings
modeled from a single-story 929 m2 building (Hilten, 2005).
This type of building was selected because it represents the
majority of flat-roofed buildings in the watershed. The
energy load reduction from the green roof system was
modeled at 4222.56 kWh/year. This is an energy savings of
3.3% which is less than half of the 8% used in the Wong
et al. (2003) study. Residential rate surveys for the 2005
year were acquired from the Georgia Public Service
Commission and the 2005 average rate of $0.082/kWh
was applied to the energy savings modeled in the building.
This current price is used for the conservative base case
BCA, but we believe that assuming electricity prices will
remain constant in real terms over the next 40 years is
extremely optimistic. Policies to limit air pollution and
climate change are likely to bring about significant
increases in this price. For the sensitivity analysis, it is
assumed that the actual rate of increase in energy prices
will vary on a uniform distribution between 0% (the base
case assumption) and 8% (a pessimistic but plausible
assumption under significant future environmental regulation).
All buildings in the watershed were estimated to
have the same energy savings, although savings may vary
based on the number of stories and orientation of each
structure. The unit energy savings for current energy rates
was $0.37/m2 (Table 6).