Working at portfolio scale
Why installer proposals can’t be compared across an estate
Installer proposals can’t be ranked against each other because each carries its own yield, capex, tariff and self-consumption assumptions — and most of those are unstated. Thirty proposals are thirty quotations, not a ranking. Comparison needs one model, applied identically to every building, with every assumption held constant across the estate.
The argument
An ordering question, not a viability one
Most of what follows is method, not regulation. The point is what a quotation is for — and what it is not.
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What’s happening
At portfolio scale the question isn’t whether a roof works. It’s which roofs, in what order, and on what basis the order can be defended. Boards approve sequences and budgets — not individual roofs.
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Where estates actually are
The default is to collect proposals — building by building, often over months, often from different suppliers covering different areas. It feels like diligence. You end up with a stack of quotations and the question still open.
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What it does to the solar case
That stack does not produce a ranking. Each proposal carries its own yield assumption, its own capex basis, its own tariff and self-consumption figures, its own view on degradation and discount rate. Some of those are stated; most aren’t. So the numbers aren’t comparable. A roof that looks better may simply have been quoted by someone more optimistic. Ranking them is ranking the assumptions, not the buildings. Comparison requires one model, applied identically to every building, with every assumption stated and held constant across the estate — which means the ranking is the product, not something you assemble afterwards from thirty separate studies.
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What you can’t answer from this alone
Whether the order in front of you reflects the roofs or the people who quoted them.
Comparability
What varies between proposals
These differences cannot be normalised after the fact without rebuilding each quote from scratch.
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Yield
Different irradiance sources, different loss assumptions — often no stated basis at all.
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Capex
What’s in and what’s excluded — scaffolding, DNO works, structural strengthening, cable runs — varies by quote.
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Tariff
Import price assumed, export rate assumed, and whether either is escalated.
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Self-consumption
Frequently a round percentage rather than modelled against actual load.
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Financial
Degradation rate, discount rate, and the period modelled.
You cannot back out a comparison from figures whose assumptions were never stated. Normalising thirty proposals means rebuilding all thirty — at which point you have modelled the estate anyway, just slowly and from worse inputs.
A quotation’s job is to price work the quoter would do — not to enable comparison against a rival. That is fair to say and fairer than implying bad faith. The problem is structural: proposals are not built for ranking. Treating them as if they were is what leaves the board with an order it cannot defend.
A ranking is every building on one model, banded by return, with the assumptions stated once and applied everywhere — so the order is defensible and the question “why is this one above that one” has an answer. Without that, a board is being asked to approve a sequence whose basis cannot be shown.
The value is consistency and stated assumptions, not superior precision on any single roof. A firm installer quote still replaces a modelled capex band when you procure. That is already how the method works elsewhere on this site, and it holds here. The work is feasibility screening — not design or procurement.
At five buildings you can hold the assumptions in your head and sanity-check them. At fifty you can’t — and the errors aren’t visible. They look like differences between roofs. The optimistic yield on one quote and the thin capex scope on another cancel or compound in ways no spreadsheet of headlines will show.
That is the point where proposal-collection stops being a workable method for ordering an estate. Not because anyone acted in bad faith, but because the inputs were never built to be compared. The method has to change: one model, stated assumptions, applied everywhere.
Fit
Does this apply to you?
- You are responsible for many buildings, not one
- You have been asked which roofs to do first — or you have a stack of proposals and no defensible order
- Proposals have come from more than one supplier, or over enough time that assumptions have drifted
FAQ
Common questions
Can I compare installer quotes across buildings?
Only if the assumptions behind them are identical and stated. Each proposal typically carries its own yield basis, capex scope, tariff and self-consumption figures. Where those differ — and often go unstated — ranking the headlines ranks the people who quoted, not the roofs.
How do you rank roofs in a portfolio?
Put every building on one model, with every assumption stated once and held constant across the estate. Band by return so the order is defensible and the question why this one sits above that one has an answer. That is feasibility screening under identical assumptions — not design or procurement.
How many buildings before this becomes a problem?
At a handful of buildings with consistent proposals from a single supplier, you can often hold the assumptions in your head and sanity-check them. Once the estate is large enough that you cannot, differences between proposals start to look like differences between roofs. That is when the method has to change — one model, stated assumptions, applied everywhere.
Does a ranking replace installer quotes?
No. A ranking decides which buildings are worth getting quotes for. A firm installer quote still replaces a modelled capex band when you procure. The value of the ranking is consistency and stated assumptions across the estate — not superior precision on any single roof.