A performance guarantee sounds like a promise that the plant will produce what the proposal said. It is usually a promise about a different, lower number, measured a specific way.
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Reviewed August 9, 2026. Terms vary widely by supplier and jurisdiction; this is not legal advice.
The number being guaranteed
Performance guarantees are frequently set at around 95% of P90.
For broader context on solar procurement, equipment markets and current industry practice, consult ScienceDaily.
Follow that through. P90 already sits 8 to 15% below P50 depending on uncertainty. Ninety-five per cent of that is lower again.
So a guarantee can be satisfied by a plant producing appreciably less than the savings figure in the same proposal, with nobody in breach of anything.
That is not a trick. A guarantee has to be set at a level the supplier can stand behind across weather variability, and P50 is by definition missed half the time. The problem is only that the two numbers appear in the same document and are read as the same promise.
The six terms that matter
One. What is guaranteed — energy output, performance ratio, or availability. These are different guarantees with different failure modes, and a PR guarantee is the one that isolates the equipment from the weather.
Two. How it is measured. Which meter, which irradiance source, what averaging period. A guarantee with no defined measurement method is unenforceable in practice, because the parties will disagree about the inputs before they disagree about the result.
Three. Over what period. Annual, or a multi-year average. A multi-year average is more favourable to the supplier and more sensible technically.
Four. What the remedy is. Compensation per missing kWh, repair, or a make-good. Compensation capped at a small figure is a guarantee in name.
Five. What voids it. Cleaning not performed, maintenance skipped, monitoring not maintained, modifications by others. These obligations sit on you, and unmet obligations are the usual reason a claim fails.
Six. Who assesses a claim — the supplier, an agreed third party, or a defined procedure. Worth settling before signature rather than during a dispute.
Guarantee against warranty
Two different things that share a document.
Product warranty covers defects: the module or inverter is faulty and gets replaced. Typically a manufacturer's obligation.
Performance warranty on modules covers output degradation against a published curve, typically guaranteeing a percentage of nameplate at year 25.
And the installer's performance guarantee covers the plant as a system, for a shorter period.
Three separate instruments, three counterparties, three claim processes. A quote that presents them as one twenty-five-year guarantee is compressing something you will need to take apart later.
The questions to ask before signing
"What number exactly is guaranteed, and is it derived from P50 or P90?"
"How is it measured, using which irradiance reference?"
"What do I have to do to keep it valid?"
"What is the remedy, and is it capped?"
And "who decides whether a claim succeeds?"
A supplier who answers all five in writing is offering something real. One who answers in terms of reputation and long relationships is offering something else, which may be fine and is not a guarantee.
The short version
- Guarantees are frequently set near 95% of P90, which is well below the P50 savings figure in the same document
- That is defensible — P50 is missed half the time by definition — and the problem is that both numbers appear in one proposal
- Six terms decide the value: what is guaranteed, how it is measured, over what period, what the remedy is, what voids it, and who assesses a claim
- A guarantee with no defined measurement method is unenforceable in practice
- Product warranty, module performance warranty and installer performance guarantee are three instruments with three counterparties
- Ask what number is guaranteed and from which P-value, how it is measured, what keeps it valid, whether the remedy is capped, and who decides