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Energy Center of Wisconsin | Back of the Envelope Calculator
  Energy Center of Wisconsin  

Friday, Apr 18, 2014



If you used the calculator on a project, please let us know!

For example:


Scott Schuetter
Scott Schuetter
PE, LEED AP BD+C, Senior Energy Engineer

what we do ::

Back of the Envelope Calculator

Play and learn!

When it comes to building energy consumption, nearly everything affects everything. It's very difficult to get a feel for energy efficiency among the complicated interactions.

Our Back-of-the-Envelope calculator is a learning tool that allows anyone to interact with a building as an energy system. You can see real-time energy connections between building components, isolate the effects of changing a single energy parameter, or produce concept-level energy and CO2 emissions estimates.

You might be pondering these questions:

  • What happens to cooling costs if I double my roof insulation?
  • Does lighting efficiency also affect heating energy?
  • Does it cost a lot to keep my building open longer?
  • What if my building was made of glass?
  • How much CO2 will this building produce annually?

How to download and use

You must have Microsoft Excel to use this tool.

Because of the large filesize (31MB), it's best to save a copy on your computer.

To do this on a PC, right-click the download icon above and choose "save target as" (or the comparable selection if using a browser other than Internet Explorer).

  • Navigate to the saved file on your computer and double-click it to open (it might take a moment). As Excel opens the file, "enable macros" when prompted.
  • Use the tool by moving the slider bars to change your settings. Click the reset defaults button for the original settings.
  • If you wish to capture your calculations, use Excel's print function. The tool is formatted to print onto one sheet.

Alternate climate versions
You may also download versions of every major U.S. climate zone.

As the name suggests, Back-of-the-Envelope provides only rough estimates. Don't use it for design decisions! In particular, Back-of-the-Envelope models an office building in a southern Wisconsin climate zone.

See Assumptions for further limitations of this tool.


Mouse over image to view video controls.


Building Level

  • The building is an office, 12' floor-to-roof, with square footprint.
  • Climate is Madison, Wisconsin using hourly TMY2 weather data.
  • The building is treated as a single thermal zone.
  • Occupied hours first fill weekdays symmetrically around noon, and then fill weekends.
  • Lights and plug loads are assumed to be at 1/20 power density during unoccupied times.
  • There is no exterior lighting considered.
  • There are no process loads in the building other than plug loads.


  • Envelope convective heat transfer is not considered.
  • Envelope solar gain is not considered, except for windows.
  • Infiltration is assumed zero during occupied times due to fan pressurization.
  • Window solar
    • Direct radiation is based on hourly solar angles and hourly direct normal radiation from local weather data.
    • Diffuse radiation on windows is assumed equal to hourly horizontal diffuse radiation from local weather data.
    • Each wall faces directly North, South, East, or West.
    • Window area is equally distributed on North, South, East, or West exposures.
    • Latitude is 43 degrees for solar angle calculations.


  • HVAC system is rooftop VAV with hydronic reheat coils.
  • Cooling is through air-cooled direct expansion.
  • Supply fan system is variable air volume using variable frequency drives.
  • Heating is through a natural gas hot water boiler.
  • There is no pump energy calculated for hot water hydronic heating.
  • There is no energy calculated for domestic hot water heating.
  • Interior relative humidity of 50% is maintained year-round.
  • There is no unoccupied thermostat setback schedule.
  • Airside economizer operates only when full cooling load can be met. There is no partial economizing with mechanical cooling assistance.
  • People sensible and latent loads are both 250 [Btu/hr]
  • Indoor temperature is 72 [degF]
  • Supply Air Temperature is 55 [degF]
  • HVAC Fan Static Pressure is 3.5 [in water]
  • Peak supply fan energy consumption is 0.000351 [kW/in*cfm]