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February 10, 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 computer 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™ spreadsheet 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 calculates the lighting and whole-building energy impacts of each design, and produces graphs and charts that describe annual energy-use patterns.

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 Illuminance Table
(Click on this image to see a larger version.)

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.)

SkyCalc Savings Graph
(Click on this image to see a larger version.)

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 Skylighting Guidelines to help designers select the best skylighting system for a given building.

SkyCalc™ Users Guide

As described in Chapter 5, there are many factors of which to keep track when sizing skylights. We have reduced the tedium of optimizing your skylight design with a simple-to-use spreadsheet that identifies the energy and cost savings, and shows the effects of changing various aspects of the system. This Microsoft Excel® spreadsheet, named SkyCalc™, will predict the lighting and energy outcomes of a given skylighting system over a range of skylight-to-floor area ratios. It will graph the overall energy and cost savings for lighting, heating, and cooling. This will help you to quickly pinpoint the optimal sizing of skylights to maximize your energy or cost savings.

Download the SkyCalc™ Users Guide

SkyCalc™ Weather Files

SkyCalc™ weather files (.wea3) can now be created using eQuest 3.61b (a simulation program available for download here). With this added feature, SkyCalc™ users now have the ability to create a weather file for any location in the world, if they have a DOE2 compatible weather file. These weather files (.wea3) are not compatible with earlier versions of SkyCalc™.

Download instructions for generating Weather Files

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.

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 Earlier Comments

by yuanjue717, Tuesday, May 24, 2011 at 9:14:20 AM


by wquirk, Wednesday, July 25, 2012 at 4:10:25 PM

Could you assist with how the Ltg. Energy without Skylights
and the Lighting Energy w/ Skylights is calculated.

by tconlon, Wednesday, July 25, 2012 at 5:34:17 PM

see section 6.8 of the "SkyCalc Users Guide" (above).

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