This calculation tool is designed to be used for preliminary screening. By necessity, it makes simplifying assumptions. Final decisions should be based on a detailed analysis of all relevant factors, including actual cooling load shape, local weather, part-load performance of equipment, and specific utility rates.

Enter values as indicated and click on "calculate" to determine how the simple payback compares for a natural gas cooling system and an electric system. To estimate equivalent months of full-load cooling demand, consult chiller logs or compare the maximum expected temperature for each month with the maximum expected temperature in the design month. (Expected temperatures can be determined based on weather data for your location; weather data can be found at www.ncdc.noaa.gov.)

These steps will yield a peak cooling demand profile for the year. Once you have this profile, translate it into an equivalent number of months of full cooling demand by summing the values for each month and dividing by the full-load value.

Equivalent full-load hours per year for cooling can be estimated from chiller logs or cooling degree days. A discussion about estimating cooling loads using degree days can be found in the 1997 ASHRAE Handbook, Chapter 30, page 30.17.

Other values required by this calculation tool should be available from utility bills and equipment manufacturers' data.