Whole House Total Energy Calculations

Last Updated:

Thursday, September 14, 2017

Software/Versions Affected:

RESFEN›5
RESFEN›6

The Total Annual Energy result for the Whole House is calculated as "Source" energy, in MBtu (Million British Thermal Units). The "Source" descriptor means that the electric energy, in most cases the Cooling energy (from electric air conditioning), has been converted from kWh (kilowatt-hours) to MBtu and then is multiplied by a "Site to Source" factor.

The following was written for RESFEN 5.0

The "Site to Source" factor is used to take into account the fact that there is an efficiency loss between the "source" where the electricity is generated (the power plant) and the "site" where it is finally consumed, in this case a residence, due to generation and transmission losses. (Note that RESFEN currently uses an approximate Site-to-Source electricity conversion factor of 3.0. However, the US Department of Energy's Buildings Energy Data Book uses a more precise factor of 3.10. For more details, visit the US Department of Energy Buildings Energy Data Book website, and look at Chapter 1, Section 1.1 Buildings Sector Energy Consumption, Table 1.1.4.

The following was written for RESFEN 6.0

The "Site to Source" factor is used to take into account the fact that there is an efficiency loss between the "source" where the electricity is generated (the power plant) and the "site" where it is finally consumed, in this case a residence, due to generation and transmission losses. (Note that RESFEN currently uses an approximate Site-to-Source electricity conversion factor of 3.0. However, the US Department of Energy's 2006 Buildings Energy Data Book uses a more precise factor of 3.18. We plan to make this change in the next version of RESFEN). For more details, visit the US Department of Energy 2006 Buildings Energy Data Book website, and look at Chapter 1, Section 1.1 Buildings Sector Energy Consumption, the 4th link, Table 1.1.4, 2004 U.S. Buildings Energy End-Use Splits, by Fuel Type.

The rest is the same for both versions

The screen shot below, shows the RESFEN Preferences dialog box (go to the Options menu, Preferences choice), where the value for the Site-to-Source conversion factor is shown (it can't be changed, but is displayed for informational purposes). In this case, it is set to 3.0.

See the explanation below for a detailed description of the calculation based on this example screen.

Below are the details of how the Total (source) Annual Energy Whole House energy consumption is calculated for the results shown above:

Heating: 91.5 MBtu

Cooling: 706 kWh

Total (Source): 98.7 MBtu

To convert the cooling electricity in kWh to MBtu:

1 kWh = 3413 Btu

1 kWh = 0.003413 MBtu

706 kWh * 0.003413 kWh/MBtu = 2.409578 MBtu (site)

Converting the cooling site energy to cooling source energy using the Site-to-Source conversion factor of 3.0:

2.409578 MBtu (site) * 3.0 = 7.228734 MBtu (source) for cooling

91.5 MBtu (heating, source) + 7.228734 MBtu (cooling, source) = 98.728734 , which rounds to 98.7 MBtu (source)

98.7 MBtu (source) is the value RESFEN reports in the Results Tab, under the Total (source) column for Annual Energy Totals.