February 9, 2009
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Skylit BuildingSkylights can save a great deal of energy - as long as they are sized correctly, and as long as appropriate controls for electric lights are used in conjunction with skylights.
An undersized skylighting system cannot justify the cost of lighting controls and never attains the pleasing visual effect of a well-daylit space. Similarly, an oversized skylighting system allows too much solar heat into the space and lets too much heat escape on cold winter nights.

SkyCalc™ is a simple spreadsheet tool that helps building designers determine the optimum skylighting strategy that will achieve maximum lighting and HVAC energy savings for a building. This program is a Microsoft Excel™ application that runs on a personal computer.

SkyCalc™ uses simple data inputs (either common defaults or user-supplied data) to describe a building and analyze possible skylighting strategies. It predicts the lighting and energy outcomes of a given skylighting system over a range of skylight to floor area ratios. It graphs the overall energy and cost savings for lighting, heating and cooling. This helps a designer to quickly pinpoint the optimal sizing of skylights to maximize energy or cost savings

SkyCalc™ operates at three levels of detail:

  • The first level requires little information from the user, instead relying on extensive defaults to describe the user's skylighting system and building operation.
  • The second level allows the user to modify any default in order to describe the situation more precisely.
  • The third level allows the user to enter detailed information about products and schedules (such as performance data for specific products or detailed building operation information).

SkyCalc™ also can take into account a variety of climate conditions in California. The user selects a particular climate zone, and SkyCalc™ calculates the energy impacts from skylights on an hourly basis throughout the year. (See Figure 1.)

Skycalc 3_Graphic -Results1

Figure 1: This sample SkyCalc™ daylight illumination chart illustrates average hourly room illumination (in footcandles) that would result from a given skylighting design for a particular climate.

SkyCalc™ also includes an optimization feature that helps designers identify where the energy performance of a particular design falls among a range of possible designs. (See Figure 2.)

Savings -by -Skylight -to -Floor -Ratios

Figure 2: This graph is a SkyCalc™ optimization curve for a Bakersfield grocery store. The store is equipped with dimming controls and single-glazed, white acrylic skylights that cover five percent of its gross roof area. This graph indicates that although this particular design will save significant energy, the optimum area that could be used for skylights in this building is slightly greater. SkyCalc™ also can produce a similar graph that calculates energy cost savings.

All SkyCalc™ reports can be printed easily. In addition, the user can adjust the basic spreadsheet and save new Excel templates for different building projects and/or climate zones. Since the program is installed as an Excel template, new copies can be saved for different building projects and/or climate zones.

SkyCalc™ can be used in conjunction with the EDR Skylighting Guidelines (2nd edition) to help designers select the best skylighting system for a given building.

SkyCalc™ 3.0 Users Guide

Chapter 6 of the Skylighting Guidelines is an updated Users Guide for SkyCalc 3.0. Because there are many factors of which to keep track when sizing skylights, many users find that SkyCalc can help reduce the tedium of optimizing skylight design. The users guide includes sections on 'Getting Started', 'The Inputs Tab', 'Generating a SkyCalc Weather File', Graph and Table results, 'Advanced Modifications', and more.

Download the SkyCalc™ Users Guide, (see Chapter 6)

SkyCalc System Requirements

SkyCalc™ runs on Microsoft Excel 7.0 or a later release running on a compatible Windows platform of Windows 95™ or later. Installation requires a minimum of 3 MB of free disk space, but 10 MB of free space is recommended for running the program.

Much of SkyCalc's functionality is now available in eQUEST (v3.63 and later).

SkyCalc Technical Support

EDR does not provide ongoing technical support to SkyCalc users, however you may be able to ask questions and learn more from your fellow SkyCalc users by joining one of the LinkedIn groups featured on the right.


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