Back of the Envelope Calculator

  • OVERVIEW
  • DOWNLOAD
  • ASSUMPTIONS
  • IN ACTION

Play and learn

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. 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.

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?

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. Back of the Envelope is here to help.

This tool is free. After you register, you will receive links and instructions.

We have adapted the tool to every major climate zone in the United States. After using the tool, please let us know what you think—send a note to Scott Schuetter at sschuetter@seventhwave.org.

Building level

  • The building is an office, 12' floor-to-roof, with square footprint.
  • Climate is Madison, Wisconsin using hourly TMY2 weather data. (Other climate data is available.)
  • 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]

Tell your story

If you used the calculator on a project, please let us know by emailing Scott Schuetter.

Oregon Energy Trust

The Oregon Energy Trust used the BOE tool to approximate whole-building baseline energy use intensities (download the breakdown calculations in Excel).

Riverview Elementary School (Snohomish, WA): NAC Architecture used the BOE tool to further verify the efficiency upgrades to the Riverview Elementary School in the Snohomish School District of western Washington.

Riverview Elementary SchoolThis 82,810 square foot new construction project aggressively pursued a sustainable design.

For example, the lighting efficiency measures included high efficiency fluorescent fixtures, daylighting controls, and LED site lighting.

A ground loop heat exchanger system coupled with heat recovery coils satisfied the school's heating and cooling requirements. The school district also installed a 100 kW photovoltaic array.

The calculator was used by the design team at NAC Architecture to perform quick sanity checks on their envelope upgrades.

  • Walls: replaced batt insulation with a continuous layer of 6" spray-applied closed cell polyurethane foam insulation
  • Glazing: specified 1.5" thick, triple-pane insulated glazing with low-e coatings for windows and upgraded curtainwall to 2" thick triple-pane glazing
  • Roof: added rigid insulation, bringing the overall R-value to 45 from 30

Through the combination of all of these strategies, the Riverview Elementary School is expected to achieve 1,386 MBTU in annual energy usage.