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:
How to download and use
You must have Microsoft Excel to use this tool.
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).
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.
- 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]