Simple Payback Period
How many years an energy or capital retrofit takes to pay back its first cost from annual savings.
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The engineering
Simple payback is the fastest screening metric in project economics: divide what you spend by what you save each year and read off the years to break even. Engineers reach for it first to reject obvious losers before setting up a full net-present-value analysis.
The catch: simple payback ignores the time value of money and anything that happens after the break-even point. A two-year payback on a device that dies at year three is worse than a four-year payback on one lasting twenty. Use it to rank, then confirm survivors with discounted cash flow.
Where this math comes from
Payback thinking is as old as commerce, but it entered engineering-economy coursework as a formal (if flawed) decision rule in the mid-20th century. It became the workhorse screen for plant managers and facilities engineers who needed a defensible number on a single page before capital committees would look at anything.
Leland Blank and Anthony Tarquin's Engineering Economy, first published in 1976 and reprinted through many editions, codified the payback method alongside its warnings — presenting it as the quick filter that must be backed by present-worth or rate-of-return analysis for any decision that actually matters.
- 1930Industrial cost analystsPayback ratio adopted as a rule-of-thumb capital screen in manufacturing.
- 1959Eugene Grant & W. Grant IresonFormalize payback in Principles of Engineering Economy with its limitations noted.
- 1976Leland Blank & Anthony TarquinEngineering Economy standardizes payback as a first-pass screen alongside NPV and IRR.
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