Methodology for ENERGY STAR Savings Estimates for Windows

The ENERGY STAR savings estimates for windows are based on energy calculations performed by D+R International using Lawrence Berkeley National Laboratory’s (LBNL) energy use and cost model based on EnergyPlus 9.5 for typical homes in 132 U.S. cities. EnergyPlus is a simulation program used to model home energy consumption using many different features and conditions.

For this analysis, EPA looked at improvements to existing and new homes using a 2006 IECC model home (PDF, 621 KB) from Pacific Northwest National Laboratory (PNNL). The "typical home"[1] is a 2,376 square foot, two-story, single-family house and is assumed to be the default new construction and existing construction assemblies in each location. Additionally, the window area of the home is assumed to be equal to 15 percent of the floor area with windows equally distributed facing all four cardinal directions.

The model home was used to create a profile for each city that considers different heating and foundation types using LBNL’s consolidated heating system and consolidated foundation approaches.

• The heating approach uses the weighted average of four heating system types:
  • electric resistance
  • gas furnace
  • oil furnace
  • heat pump
• The foundation approach uses the weighted average of four foundation types:
  • slab
  • crawlspace
  • heated basement
  • unheated basement.

Representative windows were selected from the RESFEN[2] window library. For ENERGY STAR certified windows, types were selected for each city based on the city's ENERGY STAR climate zone. See the table below for default assumptions for the single and double pane clear window performance baselines used to estimate savings. TMY3 weather data was used to model baseline performance.

Savings calculations in all climate zones assume whole-home replacements with ENERGY STAR Version 7.0 windows. Energy usage is on an annual basis per home according to the model home characteristics. Annual heating and cooling savings for each city were calculated by modeling performance improvements of windows meeting the Version 7.0 U-factor and SHGC criteria for each city’s respective climate zone. Climate zone savings estimates represent the average of energy savings and cost savings among the cities in each climate zone. National savings ranges are based on the rounded average savings among homes in modeled cities in the continental U.S. with outliers removed. Each city’s cost savings calculations assume the applicable state’s average natural gas and electricity prices from the Energy Information Administration (EIA). Percentage savings[3] in the City Savings Estimates Spreadsheet (EXCEL, 59 KB) represent the energy cost savings as a proportion of the average consumer’s annual whole-home utility bill.

Actual savings will vary based on local climate conditions, utility rates, and individual home characteristics.