Commercial solar sums, shown in full
Four sample sites, each with its assumptions listed and its sums worked through. They are not clients. They show the method, so you can test a quote against it.
Please read first: these are worked examples, not case studies of real clients. The sites are made up, although the rules they use are real. Every input is labelled as an assumption, and every rule links to the body that sets it.
Why publish made-up sites? Because a payback figure means nothing until you can see how someone built it. Each example lists its inputs, does the sums one line at a time, then tests what happens if a key input is wrong.
Two lessons come up again and again, although each site is different. The share of power you use on site matters more than the size of the roof. And the person who pays for the panels is not always the one who saves money.
You can use them in two ways. First, as a check on a quote you already hold: does it state its yield, its share used on site and its tax treatment? Second, as a template. Swap in your own bills and roof size, for example, and see how far the answer moves.
None of these sums replaces a site visit. Still, they show which questions to ask before you pay for one, and which figures an installer should be able to back up.
Worked example: a 40,000 sq ft warehouse
Shows how the share of power used on site moves the payback more than the size of the roof400 kWp rooftop array (assumed)See the sums →Commercial & OfficesWorked example: a multi-let office and retail park
Shows how a landlord and tenants can split the value of solar when one owns the roof and the others use the power180 kWp across three roofs (assumed)See the sums →Public Sector & EducationWorked example: a council-run secondary school
Shows why a smaller array can pay back faster on a site that is shut for the sunniest weeks of the year80–120 kWp rooftop array (two sizes tested)See the sums →Property DevelopersWorked example: a new-build logistics shed
Shows why the value of solar on a speculative shed depends on a tenant nobody has met yet350 kWp designed in before build (assumed)See the sums →Want the sums for your own site?
Real numbers depend on your roof, your bills and your grid. A feasibility study replaces our assumptions with your figures.