ECW -- Resources -- Tools

BACK-OF-THE-ENVELOPE CALCULATOR (version 1.0)

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 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?

Play and learn!


How to download and use
Back-of-the-Envelope Calculator screenshot

You must have Microsoft Excel to use this tool.

To download:

Because of the large filesize (31MB), it's best to save a copy to disk and use it locally.

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

To use:

  • 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.


Assumptions

Building Level
  • The building is a single story 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
  • 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
  • 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]

Customized version

To inquire about a customized version of this tool, please contact Kathryn Schiedermayer at kschiedermayer@ecw.org or 608.238.8276 x147.