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Reading a proposal, line by line

Half of a solar proposal is fact. The other half is assumption dressed as fact.

Facts and assumptions

Which lines of a proposal are facts and which are assumptionsSystem size, price and the payment schedule are facts. Production, the incentives applied and any assumed utility rate growth are assumptions, and each one can be questioned.Your proposalSystem sizeA factProduction estimateAn assumptionPriceA factIncentives appliedAn assumptionUtility rate growthAn assumptionPayment scheduleA factQuestion every orange row.
System size, price and payment schedule are facts — they are in the contract. Production, incentives and any assumed rate rise are estimates, and every one of them can be questioned without being rude.

The four assumptions worth pressing

  1. 01

    Annual production

    Ask what shading data it came from. A number from a satellite image and a number from a site survey can differ by a fifth.

  2. 02

    Utility rate escalation

    Many proposals assume the utility raises rates every year forever. Ask to see the savings at today's rate instead.

  3. 03

    Incentives applied

    Anything income-qualified or block-limited should be marked as unconfirmed until someone has actually checked your eligibility.

  4. 04

    Degradation and maintenance

    Panels lose a little output each year and inverters do not last 25. Ask whether either is in the model, and who pays.

Lines that should make you slow down

Red flag

A savings total with no year-by-year table

A single lifetime figure hides every assumption that produced it. Ask for the schedule.

Red flag

“Estimated” on the price but not on the savings

If the savings are estimates too — and they are — the document should say so in the same size type.

Read next

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When you are ready to compare a real number

Zip and a bill gets you a range in about a minute. An advisor confirms it on the roof — and you should hold us to every question on these pages.

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