What counts as end of design life for a wind turbine?
Design life is a certification number, not a prediction. When a turbine model gets type-approved under IEC 61400-1, the manufacturer runs load calculations against an assumed wind climate and arrives at a design life, almost always 20 years for anything built before the mid-2010s. That number tells a certifying body the structure, drivetrain and foundation can withstand the fatigue loads of a typical site for that period. It doesn't tell you when a specific machine on a specific ridge actually needs to come down.
Where the 20-year figure comes from
IEC 61400-1 defines design classes (I through IV) based on average wind speed and turbulence, and manufacturers size blades, towers and gearboxes to survive 20 years of fatigue cycling at that class. Some newer platforms are certified to 25 or even 30 years. But the certification is a structural calculation, not a site-specific forecast. A turbine installed on a Class I site with higher-than-assumed turbulence may rack up its design fatigue budget faster than the nameplate suggests. One sited more gently can run well past the paperwork date with the right component swaps.
This is why "turbine design life years" and "when this site needs repowering" are two different questions, and why asset managers who treat the certified number as a deadline end up either pulling consent applications too early or missing the window when O&M costs start outrunning revenue.
End of design life vs end of life, in practice
End of design life is the certified number expiring. End of life, in the way a portfolio manager means it, is the point where keeping the turbine running costs more than it's worth: spare parts for a discontinued model getting harder to source, blade and gearbox failure rates climbing past what the warranty years ever saw, insurers asking for a life-extension assessment before they'll renew cover, or the land lease and grid connection simply ageing out alongside the hardware.
A site can hit practical end of life years before its certified design life runs out, if the turbine model was pulled from production and parts lead times stretch past what an outage budget tolerates. Equally, a well-maintained site with a still-supported model can run a formal life extension study and keep spinning past the 20-year mark with no change to the hardware at all.
What ages a fleet, from a portfolio view, is a mix of three things: how long ago the model stopped being sold, how the blades and nacelle look in recent imagery against known model wear patterns, and how much runway is left on the land agreement and grid connection. Cross-referencing all three by hand, site by site, across a few dozen locations, is the part that eats a repowering team's week.
That's the gap Repowering Candidates is built to close: a single annual VHR pass over the fleet, matched against turbine model footprints, turned into a ranked list of sites nearing the end of their useful consent-worthy life rather than their certified one.
A rough way to think about the number
If you need a working rule of thumb rather than a structural engineer's report, treat the IEC-certified design life as the earliest point a repowering conversation becomes worth having. Start tracking a site at year 15, not year 20. By the time the certificate expires, what matters is how far down the consent-priority list the site sits compared to the rest of the portfolio.
Fleet age alone won't tell you that. Model obsolescence, visible wear, and what's left on the lease will.
If you're sitting on a portfolio and want that ranking without sending a team to every substation, that's the list we build.