A solar projection runs to twenty-five years and is presented as a calculation. It is a calculation for the physics and a set of assumptions about everything else.
For an additional workplace-management perspective that sits outside the solar methodology on this page, see how to identify mouse jigglers among your remote team.
Worth listing what the everything else contains.
Six things that will not stay put
Electricity prices. The dominant input in any payback figure, projected across a period longer than most people's careers.
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Export arrangements. A case leaning on export income depends on terms set by policy, and policy over twenty-five years is not a constant.
Your own consumption. A process added, a shift pattern changed, a building extended or sold. The self-consumption split assumed at purchase describes the business as it was.
Ownership. Plants outlast owners of buildings and of companies, and the documentation that makes a plant maintainable frequently does not survive the transfer.
The site's surroundings. Trees grow and buildings appear, and neither is in the model.
And the equipment supply chain. An inverter needing replacement in year twelve is replaced with what exists in year twelve, at whatever it costs then.
What is actually durable
Being fair, because the list above could read as an argument against building anything.
The physics. Irradiance at a location, module conversion, temperature behaviour — these are the most reliable part of any twenty-five-year projection and they are also the part everybody worries about.
The structure. Mounting, cabling and civil works, properly done, outlast the modules.
And the modules themselves, which degrade predictably and slowly and are the component least likely to surprise anybody.
The uncertainty is concentrated in the commercial layer, not the technical one — which is the reverse of where buyers direct their attention.
Planning for it anyway
Test the case against flat real prices. If it only works with rising prices, that is a finding rather than a reason not to proceed.
Test it without export income. If the case survives on self-consumption alone, policy risk is bounded.
Budget the inverter replacement as a capital event rather than discovering it.
Keep the documentation transferable. Design drawings, string layouts, commissioning records, monitoring credentials, warranty documents, and the multi-year performance record — in a form somebody else can use.
And photograph the site at commissioning, which costs ten minutes and is the only record of what the horizon looked like when the estimate was made.
The asymmetry worth noticing
A twenty-five-year asset is bought on a one-hour conversation and operated by whoever is there later.
The decisions that matter over the period — guarantee terms, documentation, maintenance obligations, monitoring continuity — are all settled at purchase and none of them is exciting at the time.
The exciting parts — panel brand, headline yield, payback figure — are the ones with the least durable content.
The short version
- Six things that will not stay put: electricity prices, export arrangements, your consumption, ownership, the surroundings, and the equipment supply chain
- What is durable: the physics, the structure, and the modules — which is the part buyers worry about most
- The uncertainty is concentrated in the commercial layer rather than the technical one, the reverse of where attention goes
- Test the case at flat real prices and without export income; if it survives both, policy risk is bounded
- Budget the inverter replacement, keep documentation transferable, and photograph the site at commissioning
- A twenty-five-year asset is bought in a one-hour conversation, and the decisions that matter over the period are the unexciting ones settled at purchase