3. Calculating Zero Energy Performance Index (zEPI)

The zero energy performance index (zEPI) is the ratio of energy performance of the rated building to the average energy consumption of a similar building at the turn of the millennium that is operated in a similar climate, for similar hours of and similar operating conditions. zEPI is used by some rating programs and COMNET purposes. The COMNET modeling rules and procedures produce an estimate of the energy performance of the proposed design. The energy performance of the baseline building can be determined in a variety of ways. For comparison, the rated building energy performance and baseline building energy performance need to be compared in consistent units; the units are specified for each purpose. This section provides a method for converting EPA source energy to energy costs, consistent with the COMNET default tariffs.

The process, using the COMNET tariffs, is similar except that the zEPI energy efficiency ratio is calculated as the ratio of energy cost instead of source energy. The following procedure enables the energy performance of the baseline building to be converted to COMNET energy costs:

1. Estimate the energy cost of the rated building through simulations and using the default tariffs provided in this chapter.
2. Estimate  the electricity, gas, steam and/or other fuels of the baseline building or comparator building for a similar climate and for similar operating conditions.
3. Use the EPA source energy multipliers to convert the baseline building estimates to EPA source energy. The source-site ratio for electricity is 3.34; natural gas is 1.047; fuel oil and propane are 1.01; steam is 1.45; hot water is 1.35; chilled water is 1.05; wood, coal, coke and other fuels are 1.00. The EPA source energy multipliers are static and do not vary with season, day, or hour. 1
4. Convert the EPA source energy to COMNET TOU costs using the conversion factors in [bookref id="source-energy-conversion-factors-(\$/kBtu)"].
5. Calculate the zEPI as the ratio of the proposed design energy costs divided by the baseline building energy costs, as determined in the previous step. 

A benefit of using the COMNET tariffs for zEPI is that building features such as daylighting or thermal storage that reduce energy demand in periods when the utility grid is stressed and when more expensive generating sources are on-line are given more credit. The procedure in this section enables proper credit for thermal storage, daylighting, photovoltaic production and other measures that reduce energy consumption during peak periods.

Note that if the energy performance of the baseline building is determined though simulations, the COMNET default tariffs may be directly applied and this procedure is not applicable. Note also that some rating programs may require that the ratio of the rated building and the baseline building be compared using source energy. In this case, this procedure is also not applicable.

Table 3-1: Source Energy Conversions from “ENERGY STAR Performance Ratings Methodology for Incorporating Source Energy Use”, December 2007, were used to generate this table.

 Table 3-1: Source Energy Conversions (continued)

• 1 ENERGY STAR Performance Ratings Methodology for Incorporating Source Energy use, U. S. Environmental Protection Agency.