Commercial Solar and Capital Allowances: The Annual Investment Allowance and the 'Full Expensing' Trap

If you run a business and someone is pitching you a rooftop solar array, sooner or later they'll mention 'tax relief' — usually with a big, confident number attached. The relief is real. But it is not a grant, it is not free money, and one very common claim about it is simply wrong. This guide explains how capital allowances actually cut the after-tax cost of a commercial solar system, where the 'full expensing' trap lies, and the honest cases where the maths still doesn't work.

Capital allowances are tax relief, not a grant

A grant hands you cash. A capital allowance lets you deduct the cost of qualifying equipment from your taxable profits, so you pay less tax. The difference matters: the value of the relief depends entirely on your profits, your business structure, and your tax rate. A loss-making year, or a business with little taxable profit, gets little or nothing from it in that year.

The key vehicle for most businesses is the Annual Investment Allowance (AIA). It lets you deduct 100% of the install cost — up to £1m per year — from your taxable profits in year one. Solar qualifies. For a typical SME investing well under £1m, the whole system cost can usually be written off against profit immediately.

The 'full expensing' trap

Here's the mistake we see most often. Since April 2023, companies have had access to 100% 'full expensing' on qualifying plant and machinery. It sounds identical to the AIA, so people assume solar can be dropped into it — sometimes to claim relief above the £1m AIA cap, or to reassure a company that's already used its AIA elsewhere.

Full expensing EXCLUDES solar. Solar is classed as special-rate plant, and special-rate assets don't qualify for the 100% full-expensing rate. This is a genuinely common error that can throw a whole business case off.

What actually applies above the £1m AIA cap is the 50% First-Year Allowance (companies only) for special-rate plant — so you deduct half the excess cost in year one, with the balance relieved more slowly over time. That's still useful, but it is not the same as writing off 100% immediately, and any projection that assumes it is will overstate your first-year saving.

If a proposal claims your solar array qualifies for '100% full expensing', treat every other number in that proposal with suspicion. Solar is special-rate — it goes through the AIA, or the 50% FYA above the cap.

The other savings that stack — and the VAT point

Tax relief is only one lever. The everyday economics of commercial solar usually rest on self-consumption: every kWh you generate and use yourself is a kWh you don't buy from the grid.

• Self-consumption — avoiding grid electricity is the biggest win. Read the actual bill for the site's unit rate before modelling anything.
• Climate Change Levy (CCL) — self-consumed solar avoids the CCL charged per kWh on grid electricity for eligible businesses. But confirm the site actually pays CCL — many SMEs are on reduced or exempt rates, in which case this saving is smaller or zero.
• Demand and DUoS bands — larger sites on half-hourly metering face capacity charges and red/amber/green DUoS bands. Self-consuming solar during the expensive red band is often the fastest saving of all.
• SEG export income — the Smart Export Guarantee applies to business sites too, though export rates are retailer-set and volatile.

On VAT: commercial installs are standard-rated, not the 0% that applies to residential solar. A VAT-registered business reclaims it, so it's not a real cost for most companies — but never let a commercial install be sold to you as '0% VAT'. That's a residential rule.

Here's the catch: when it still doesn't pay

Tax relief makes solar cheaper. It does not make a bad site good. Solar might NOT pay for your business if:

• You have little taxable profit. The AIA reduces tax you'd otherwise pay. No profit, no immediate benefit — the relief just carries forward.
• Your consumption doesn't match generation. A building that's empty at midday, or a business that runs mostly after dark, self-consumes little. You'd be leaning on volatile export income instead.
• The DNO limits your export. Systems over 3.68 kW per phase need a G99 application, and sites above roughly 50 kWp routinely face DNO export limits. Size to what you actually use — never assume you can export the surplus freely.
• You're already CCL-exempt. One of the headline savings quietly disappears.

The point of an audit is to test these before you commit, not after.

What to confirm before any figure reaches your accountant

Capital-allowance treatment and CCL rates are set by HMRC and can change at any fiscal event. SEG export rates are retailer-set and can change with 30 days' notice. So the sequence matters: model the engineering (self-consumption, export limits, demand bands) first, then let your accountant confirm the current allowance treatment and apply your actual tax rate. Anyone quoting you a precise after-tax payback before your accountant has seen it is guessing.

Note for public-sector and housing bodies: councils and housing associations have separate funding routes for solar across their stock — a different conversation from the commercial capital-allowance case above.

We don't sell solar, we don't install it, and no installer pays us a penny — not even the 'free' comparison sites can say that, because they're paid by whoever they refer you to. Our only job is to tell you whether the numbers work, including the times the honest answer is 'not for this building'. Sometimes the most valuable thing we do is stop you spending.

Figures in this guide are current at the date of publication and indicative only — SEG/export rates change with 30 days' notice, the Ofgem price cap resets quarterly, and grant terms change. Confirm the current numbers for your situation before you commit. This is general information, not personal financial advice.