Most of the disputes, disappointments and misunderstandings in this subject reduce to one thing.
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The supplier produces a distribution. The buyer reads a promise. Neither party is wrong about their own document, and the mismatch generates everything else.
What the supplier produced
A model of a distribution, summarised by its median and by a conservative exceedance level. P50 and P90.
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The distribution is the honest output. Weather varies year to year and no assessment can remove that; the best it can do is characterise the variation and say how confident it is.
And it comes with a stated uncertainty, which is what the gap between the two figures encodes.
What the buyer read
A number the plant will produce.
Not through carelessness. Every other capital purchase behaves that way — a machine has a rated throughput, a vehicle has a load capacity, a building has a floor area. Nothing in ordinary commercial experience prepares somebody for a purchase whose output is a probability distribution.
So the buyer applies the only reading they have, and it is the wrong one.
What follows from the mismatch
Year one below P50 reads as a failure. It has roughly a 50% probability by construction.
Guarantees look like betrayals. A guarantee near 95% of P90 is much lower than the savings figure, and to somebody expecting a promise it looks like fine print concealing a retreat.
Comparison between suppliers goes wrong. A conservative assessor with honest uncertainty loses to an optimistic one, because the buyer compares point figures and the point figures are not comparable.
And genuine underperformance gets excused. Once "it varies" is the frame, a real PR shortfall can hide inside it for years.
That last consequence is the expensive one, and it is caused by the same mismatch as the first.
What would fix it
On the supplier side: leading with the distribution. Presenting a range with its probability rather than a headline figure with a footnote. Costs a sentence.
Stating which P-value every derived number uses. Savings, payback, debt cover — each should carry its basis, and most proposals state it once if at all.
On the buyer side: asking for both figures and the ratio between them. Three numbers instead of one, and the third is free.
And treating one year as one observation. Which is obvious stated plainly and almost never done in practice.
The frame that works
A solar plant is not a machine with a rating. It is a machine plus a weather series.
The machine part can be specified, tested and guaranteed. The weather part can only be characterised.
A buyer who holds those separately asks the right questions of each: is the equipment performing as designed, and was the weather what the model assumed. Performance ratio answers the first and an irradiance record answers the second.
A buyer who holds them together has one number, no diagnostic, and an argument every time the year is poor.
The short version
- The supplier produces a distribution and the buyer reads a promise, and the mismatch generates most of the disputes in this subject
- The buyer is not careless: no other capital purchase has a probability distribution as its output
- Consequences: year one below P50 reads as failure, guarantees look like betrayals, conservative assessors lose comparisons, and real underperformance hides inside "it varies"
- The last of those is the expensive one and has the same cause as the first
- Fixes: lead with the range, state the P-value behind every derived number, ask for both figures and their ratio, and treat one year as one observation
- A solar plant is a machine plus a weather series — specify and guarantee the first, characterise the second, and never merge them