500kW Commercial Solar

Warehousing and manufacturing roofs of this size combine two things that make solar pay back fastest. First, there's a very large area to spread fixed installation costs over. Second, heavy and steady daytime demand helps too — from process loads, refrigeration or materials handling. That demand lets you use most of your generation on site, rather than exporting it at a lower rate.

Two constraints actually decide viability here: structural and electrical. Structurally, can an ageing roof or lightweight sheeting take the additional load? Electrically, does the local grid connection have enough headroom, or does G99 reinforcement become a material cost? Both are feasibility-stage questions, not afterthoughts.

This is also the size at which capital allowances modelling needs real care. A 500kWp system's cost usually sits under the £1 million Annual Investment Allowance limit on its own. However, that's not guaranteed if the system shares a capital programme with other spending in the same accounting year. A re-roof, new plant purchase, or a fit-out are common examples that can use up the AIA first.

Any balance then falls to the 50% special rate first-year allowance. You then write down the remainder at 6% a year in the special rate pool. Consequently, that's meaningfully different — and slower — than the 100% 'Full Expensing' relief solar doesn't actually qualify for.