Remaining useful life

Estimated operating time before an asset reaches a defined end-of-life criterion.

Abbreviated RUL. It has no meaning without a stated end-of-life criterion, which for an electrolyser stack is usually a cell voltage threshold, an efficiency loss, or an economic point at which replacement costs less than continuing.

An estimate requires a measured degradation trend, a projection of future operating conditions, and that criterion. All three carry uncertainty, so an RUL quoted as a single number without a range is presenting more confidence than the method supports.

It matters commercially because it drives the timing of the largest recurring expenditure in the plant, which is sized into major maintenance reserves at financial close on assumptions that measurement can later confirm or correct.