RUL is estimated per component, not per machine. In a photovoltaic inverter the IGBT modules, the DC link capacitors, the cooling fans and the contactors age through different mechanisms and at different speeds, so a single figure for the cabinet hides the part that will fail first.
A number without a horizon and an interval is not usable
An estimate becomes actionable when it states how far ahead it looks and how uncertain it is. A point estimate with no interval invites two failure modes: acting on noise, and discarding the model after the first miss. A range with its probability attached can be compared against the real planning cycle: parts lead time, crew availability, the next scheduled outage.
How it is estimated
Degradation models turn operating history into consumed life. Coffin-Manson relates thermal cycling to fatigue in solder joints and bond wires, Arrhenius relates temperature to the ageing of electrolytic materials, and rainflow counting extracts the cycles from an irregular temperature series. The inputs are the asset's own SCADA history plus ambient conditions, so two identical units on different sites get different answers.
It is not an alarm
A fault alarm reports a condition that already exists. RUL is a forecast, and its value is the time it buys for planning. Judge it by whether maintenance decisions changed, not by whether one prediction landed on the exact day.