Field Notes · Common mistakes
No spare engine? At full load, short a third of the time
Installing exactly as many gas engines as the load needs looks efficient. The availability arithmetic says otherwise.
Five engines for five engines' worth of load sounds efficient. The spare is often the first line cut in a capital review, because on a good day it does nothing.
The arithmetic
The US Department of Energy lists a minimum availability of 92% for packaged reciprocating-engine units (CHP Technologies fact sheet, 2023). Many fleets do better, but the shape of the result holds. Assuming outages are independent:
- Five needed, five installed: all five are available together only about 66% of the time.
- N+1, six installed: at least five are available about 92% of the time.
- N+2, seven installed: about 98.6% of the time.
Without a spare, for roughly a third of the time at full load the site is at least one unit short, and the result is shed load, diesel, or a trip.
The fix
Design to N+1 as standard, and N+2 on islanded grids without back-up. Stage maintenance so the spare also covers planned outages.
Availability figures are our own arithmetic from the DOE minimum, assuming independent outages. Planned maintenance can be scheduled, so real shortfalls are somewhat lower, but the gap between N and N+1 holds.
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