A solar quote is a calculation resting on assumptions, and the person who chose the assumptions is paid on the outcome. That is not an accusation — it is the structure, and it means the assumptions are what you check.
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Reviewed August 9, 2026.
The seven assumptions
One. Which yield figure the savings use. P50 or P90, and quotes frequently do not say. A savings figure built on P50 describes an average year.
For broader context on solar procurement, equipment markets and current industry practice, consult Nature.
Two. The specific yield. Annual kWh divided by installed kWp. Check it against a public reference for your location — a free irradiance calculator gives a defensible range in minutes, and a quote implying materially more needs an explanation.
Three. The loss assumption. A standard stack runs around 14% for shading, soiling, wiring, inverter conversion and mismatch. A quote assuming almost none is promising electricity the system will not deliver.
Four. Degradation. 0.5 to 1% per year. A twenty-five-year savings total that applies year-one output to every year overstates by a wide margin.
Five. The electricity price path. Savings depend on what you would otherwise have paid, projected over decades. This is the assumption with the widest honest range and the one most often stated as a single confident number.
Six. Self-consumption share. How much you use directly against how much is exported, which are usually worth very different amounts.
Seven. Incentives. Which ones, for how long, and whether they are secured or expected.
Six of the seven push the total the same way when set optimistically. That is what makes the direction predictable.
What is outside the quote
Grid connection, where it is not included — and it frequently is not.
Structural work, roof strengthening, civil works for ground mount — and how much of this is yours depends on whether the contract is turnkey.
Your own time, through permitting and commissioning.
Insurance, monitoring subscriptions and maintenance, which are ongoing.
And inverter replacement, which is not a maintenance item but a capital one, arriving somewhere in the second half of the plant's life.
Rebuilding the number
Half an hour, no engineering required.
Take the quoted annual yield and check the specific yield against a public calculator for your location.
Apply degradation across the projection years rather than assuming year one repeats.
Substitute an electricity price range for the single figure, and see how wide the outcome gets.
Add the excluded costs listed above.
And ask for the same calculation at P90. It takes the supplier minutes and it tells you how sensitive the case is.
What a good quote looks like
Being fair, because they exist.
States which P-value the savings use.
Itemises the loss assumption rather than presenting a net yield with no derivation.
Applies degradation explicitly, year by year or as a stated stack.
Gives the electricity price as a range or a scenario, not a point.
Lists what is excluded.
And produces a smaller number than the marketing material, which is the strongest signal in any technical proposal.
The short version
- Seven assumptions carry the savings figure, and six push it the same way when set optimistically
- Check which P-value the savings use, the specific yield against a public reference, the loss stack, degradation, the price path, self-consumption share and incentives
- Outside the quote: grid connection, structural work, your time, insurance and monitoring, and inverter replacement as a capital item
- Rebuild it in half an hour: verify specific yield, apply degradation, substitute a price range, add exclusions, and ask for the P90 version
- A good quote states its P-value, itemises losses, applies degradation explicitly, gives price as a range, and lists exclusions
- The strongest signal is a specific calculation that comes in below the marketing material