Most commercial O&M budgets are a round number per kWp per year. That is a reasonable starting point, but it hides the thing that matters: the components on a commercial array do not fail at the same rate, and the expensive failure is the one you can see coming.
The inverter is the clock
String and central inverters reach end-of-life at roughly 10 to 12 years, against panels carrying 25-year output warranties. On a commercial roof that is a planned capital event, not a breakdown — and it is the natural moment to decide between a like-for-like replacement and a re-power that lifts the whole system's output.
Budgeting for it from year one is the clearest difference between a well-run commercial array and one that limps.
DC-side faults are the silent loss
String failures, connector corrosion and degraded rooftop DC isolators rarely stop a system. They just take a slice of it away.
Without string-level monitoring, a dead string on a 250 kWp array can run for months unnoticed — the system still generates, the bill still falls, and nobody has a reason to look. This is why monitoring is the first line of O&M rather than an optional extra: you cannot maintain what you cannot see.
Soiling is real, but over-sold
On most UK commercial roofs, rainfall does the majority of the cleaning. Soiling becomes a genuine yield issue in specific settings — near agricultural operations, quarries, coastal salt, food-processing extract, heavy bird traffic — and on shallow-pitch roofs where water sheets rather than runs.
Clean where the monitoring data shows a recoverable loss. Not on a calendar.
The structure outlives everything
Mounting systems, rails and roof penetrations are the least likely to fail and the most expensive to get wrong at install. The annual inspection is largely confirming nothing has moved, penetrations are still watertight, and nothing new on the roof — plant, a flue, a neighbouring extension — is now shading the array.
What that means for the budget
Set against the real curve, an O&M budget stops being a subscription and becomes three things: monitoring that catches the invisible losses, an annual inspection that protects the warranty position, and a funded plan for the inverter event you already know is coming.
Our commercial solar maintenance page sets out the cover levels we build into commercial installs and PPAs, and what we do for arrays another installer walked away from.