Commercial Solar and Capital Allowances: Cutting the Real After-Tax Cost

If you run a profitable UK business, the sticker price of a solar install is rarely the real price. Capital allowances let you deduct the cost against taxable profits, so the true outlay is the install cost minus the tax you'd otherwise have paid. That can be substantial — but there's a widely repeated error about which allowance actually applies to solar, and getting it wrong can leave you expecting a bigger year-one deduction than HMRC will allow. This guide walks through the mechanism honestly. We sell nothing and install nothing, so we've no reason to flatter the numbers.

The Annual Investment Allowance (AIA) — the main event

The Annual Investment Allowance lets a business deduct 100% of qualifying plant and machinery — including commercial solar PV — from its taxable profits in the year of the spend, up to £1 million per year. For almost every SME solar project this is the relevant relief, because most installs sit comfortably under that cap.

The value to you isn't the allowance itself — it's the allowance multiplied by your tax rate:

Relief value = install cost × allowance rate × your business's tax rate.

So a company paying corporation tax deducts the full install cost against profit, and the cash benefit is that cost scaled by its tax rate. We're deliberately not printing a worked £ figure here, because it depends entirely on your profits, your tax rate and your business structure. That calculation belongs with your accountant — but the mechanism is the point: your real after-tax cost is meaningfully lower than the invoice.

The 'full expensing' trap — read this before you budget

Here's the catch that trips up businesses, brokers and even some installers. You may have read that the UK offers 100% 'full expensing' on plant and machinery. That is real — but solar PV is excluded from it. Solar counts as special-rate plant, and full expensing only applies to main-rate assets.

What this means in practice:

• Up to the £1m AIA cap, solar qualifies for the AIA at 100% — so most projects still get full year-one relief via the AIA.
• Above the £1m AIA cap, solar falls into the special-rate pool and attracts a 50% First-Year Allowance (available to companies), not the 100% full expensing you might have assumed.

So if someone has pencilled in "100% full expensing" for a large install that exceeds the AIA, the year-one deduction on the excess is halved. For a sub-£1m project this rarely bites; for a large multi-site rollout it absolutely can. Confirm the current treatment with your accountant — capital-allowance rules and rates are set by HMRC and can change at fiscal events.

VAT, SEG income and the Climate Change Levy

A few further points that shape the commercial case:

• VAT: Commercial solar installs are standard-rated, not the 0% that residential installs enjoy. The difference is that a VAT-registered business reclaims the VAT. Don't let anyone present a commercial quote as "0% VAT" — that's the domestic rule.
• SEG export income: The Smart Export Guarantee applies to business sites too. Rates are retailer-set and have trended down; across the market they've sat roughly in the 4–16.5p/kWh range, so treat any single figure as a snapshot and verify the current rate before relying on it. You need an MCS-certified install and a smart meter to claim.
• Climate Change Levy (CCL): Electricity you generate and self-consume avoids the CCL you'd pay on imported grid power — a per-kWh saving for eligible businesses. Check first whether your site actually pays CCL; many SMEs are on reduced or exempt rates, in which case this saving doesn't exist for you.

Why self-consumption beats export — and sizing honestly

The strongest commercial case is almost always self-consumption, not export. Every unit you generate and use yourself displaces grid electricity at your full import rate; every unit exported earns only the SEG rate, which is typically far lower. The implication is blunt: size the system to your daytime demand, not to the biggest array the roof can hold.

This matters even more for larger sites. Systems above 3.68 kW per phase need a G99 application to your DNO, and sites above around 50 kWp routinely face DNO export limits — so a design built around exporting surplus may simply not be allowed to. If you're on half-hourly metering with capacity charges and DUoS red/amber/green bands, self-consuming during the expensive red-band periods is often the single fastest saving available.

Housing associations and councils — a different route

If you're a social landlord or local authority rather than a trading company, the capital-allowance logic may not apply, but the Warm Homes Plan can fund solar, heat pumps and insulation across housing stock — via the Warm Homes: Social Housing Fund for social landlords, and the Local Grant (council-delivered) for eligible low-income owner-occupiers and private renters. Installs must be MCS-certified. We flag it here because the right funding route depends entirely on who you are.

We take nothing from any installer — not a referral fee, not a commission, nothing. Even the "free" comparison sites get paid by whoever they send you to. We charge one small fixed fee for engineered advice, and if the honest answer is that solar won't pay at your site, or that your "full expensing" assumption needs halving, that's exactly what we'll tell you. Confirm every tax figure with your accountant before it reaches a decision.

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.