Is commercial solar zero-rated for VAT?

No. The zero rate applies to installations in residential accommodation and runs to 31 March 2027 before reverting to 5%; a commercial roof is standard-rated. For a VAT-registered business that is usually a cash-flow point rather than a cost, because the VAT is normally recoverable. The allowance that does matter is capital allowances: solar is special-rate expenditure, so it goes to the annual investment allowance or the 50% first-year allowance and never to full expensing. Confirm your own position with your accountant before it goes in a business case.