Economizer Savings Calculator

This calculator estimates the payback on installing an air-side economizer, which lets a building use outdoor air for "free cooling" instead of running the compressor. Annual Cooling Cost is what mechanical cooling costs today with no economizer: Compressor Power Draw times Annual Cooling Hours times Electric Rate. Annual Savings with Economizer is Compressor Power Draw times Economizer Hours times Electric Rate -- the electricity the compressor would have used during the hours the economizer runs it instead -- except Economizer Hours is capped at Annual Cooling Hours first, since free-cooling hours can't logically exceed the total hours the building needs cooling for in the first place; at the default inputs (800 economizer hours, 2,000 cooling hours) that cap doesn't bind, but pushing Economizer Hours past Annual Cooling Hours stops increasing the savings estimate. Within that capped regime, Electric Rate, Economizer Hours, and Compressor Power Draw all move Annual Savings by the same proportion, since the formula is a straight product of the three.

Simple Payback Period is Estimated Install Cost divided by Annual Savings, so it moves in the opposite direction from all three of those inputs -- a higher electric rate, more economizer hours, or a larger compressor all shorten the payback period. System Cooling Capacity affects only the estimated install cost ($2,000 plus $100 per ton) and has no effect on Annual Savings, since compressor energy use in this model comes from Compressor Power Draw directly rather than from tonnage; a bigger system with a fixed compressor draw would even see a slightly longer payback, since a larger system costs more to fit an economizer to without generating any extra savings on its own. This is a simple, single-rate payback estimate -- it doesn't account for maintenance costs, demand charges, or economizer hours that vary by climate zone beyond whatever figure is entered.