Twenty years of metered yield on a 2004 array

An array commissioned in 2004 is still metered every month. Here is what two decades of that data says about degradation.

The array went up in 2004 on a warehouse roof outside Salem. It has been metered without a gap since, which makes it the longest continuous production record we hold.

Twenty-two years of monthly reads let us check degradation against the number the modules were sold on rather than the number the industry quotes.

What the record shows

Measured degradation sits at 0.42% a year against a 0.5% warranty line. The array is producing 91% of its first full-year output, which is above where the datasheet put it.

The losses that matter are not in the modules. Two string inverters were replaced in year eleven, and a single mis-set combiner cost more energy in one summer than degradation has cost in a decade.

Why we keep reading meters

Monthly reads catch a failed string in weeks, not at the annual service visit.
A long record makes the next proposal an argument from evidence rather than a brochure.
Owners refinancing an older array need production history, not modelling.