Historically, the cost of most commercially-purchased energy has increased over time as fuel, equipment and maintenance costs increase. Residential natural gas and electricity price escalation rates, e, over various periods are shown in the tables below (Annual Energy Review 2006, Energy Information Agency, U.S. Department of Energy). Natural Gas Residential Residential Residential Nominal Nominal Nominal 1967-2006 1996-2006 2001-2006 Input Input Input n 39 N 10 n 5 P 1.04 P 6.34 P 9.63 F 13.76 F 13.76 F 13.76 Calculations Calculations Calculations e 0.068461 E 0.08057 e 0.073986 Electricity Residential Residential Residential 1967-2006 1996-2006 2001-2006 Input Input N 39 N 10 n 5 P 2.3 P 8.36 P 8.58 F 10.4 F 10.4 F 10.4 Calculations Calculations Calculations E 0.039448 E 0.022075 e 0.039224 Over any period of n years, the annualized (levelized) cost of energy can be calculated by: Defining the current cost of energy as A, then finding the present P of future costs escalating at rate e with discount rate i over n years as: P = A ESPWF(i, e, n) Finding the annualized (levelized) cost, A’, of the present as: A’ = P / SPFW(i, n). Where: SPWF(i,n) = series present factor = ESPWF(i,e,n) = escalating series present worth factor = if i e = if i = e 
Historically, the cost of most commercially-purchased energy has increased over time as fuel, equipment and maintenance costs increase. Residential natural gas and electricity price escalation rates, e, over various periods are shown in the tables below (Annual Energy Review 2006, Energy Information Agency, U.S. Department of Energy). Natural Gas Residential Nominal Residential Nominal Residential Nominal 1967-2006 1996-2006 2001-2006 Input Input Input n 39 N 10 n 5 P 1.04 P 6.34 P 9.63 F 13.76 F 13.76 F 13.76 Calculations Calculations Calculations e 0.068461 E 0.08057 e 0.073986 Electricity Residential Residential Residential 1967-2006 1996-2006 2001-2006 Input Input N 39 N 10 n 5 P 2.3 P 8.36 P 8.58 F 10.4 F 10.4 F 10.4 Calculations Calculations Calculations E 0.039448 E 0.022075 e 0.039224 Over any period of n years, the annualized (levelized) cost of energy can be calculated by: Defining the current cost of energy as A, then finding the present P of future costs escalating at rate e with discount rate i over n years as: P = A ESPWF(i, e, n) Finding the annualized (levelized) cost, A’, of the present as: A’ = P / SPFW(i, n). Where: SPWF(i,n) = series present factor = ESPWF(i,e,n) = escalating series present worth factor = if i  e = if i = e
NetZeroEnergyBldgs.docx