Full Expensing and Data Centre Solar in 2026: How the 25% First-Year Tax Relief Changes the Investment Case

The UK government made Full Expensing permanent in the Autumn Statement 2023. If you are a data centre operator who hasn’t revisited your solar PV investment case since then, you’re working with outdated numbers. This article explains what Full Expensing is, how it applies to solar PV specifically, what it’s worth in actual cash, and how to structure a data centre solar project to capture the maximum benefit.

What Full Expensing is — and isn’t

Full Expensing is a capital allowance regime that allows businesses to deduct 100% of qualifying plant and machinery expenditure from their taxable profits in the year the expenditure occurs. It was introduced as a permanent measure in November 2023, having been available on a temporary basis from April 2021.

What qualifies as plant and machinery for solar PV:

• Solar photovoltaic panels
• Inverters
• Mounting structures
• DC cables and junction boxes
• AC switchgear and metering
• Battery storage systems (where installed)
• Associated electrical infrastructure (MCBs, Protection Relay, export limiting equipment)
• Monitoring hardware

What does not qualify:

• Structural roofing work (this is a building improvement, not plant)
• Asbestos removal required prior to installation
• Scaffolding (treated as a revenue cost)
• DNO external network reinforcement (this belongs to the DNO, not to you)

A typical data centre solar project splits approximately 85–90% qualifying plant and 10–15% non-qualifying structural/civils. Your tax adviser should confirm the qualifying proportion for your specific project.

What it’s worth in real cash

Full Expensing generates a corporation tax deduction equal to 100% of the qualifying plant expenditure. At the main UK CT rate of 25%, this is a cash tax saving of 25p per £1 of qualifying expenditure, received when you file your CT600 for the year in which the expenditure occurs.

For a £500,000 solar installation with £450,000 of qualifying plant:

Calculation	Amount
Total project cost	£500,000
Qualifying plant element (90%)	£450,000
Full Expensing deduction	£450,000
CT saving at 25%	£112,500
Effective net cost after tax	£387,500

The £112,500 CT saving is received within 9 months of your accounting year end (or in quarterly instalments if you pay CT quarterly, i.e., from month 7 of the accounting year). For a project completing in October 2026 in a December accounting year, the CT saving arrives by September 2027.

This cash saving directly reduces the net investment required, accelerating payback from 6.2 years (pre-tax) to approximately 4.6 years (post-tax, at £87,000/year avoided electricity cost on this example).

The Annual Investment Allowance — for smaller projects

Below £1 million of qualifying expenditure per accounting year, the Annual Investment Allowance (AIA) applies the same 100% deduction as Full Expensing. The practical difference is accounting treatment: AIA has historically been the standard mechanism for sub-£1m plant expenditure; Full Expensing is the specific regime introduced in 2021.

For data centre solar projects below £1 million (which covers most single-building installations up to approximately 900–1,000 kW), AIA is the operative mechanism. The tax benefit is identical.

For projects above £1 million, the expenditure in excess of the £1m AIA cap qualifies for the 50% First Year Allowance (FYA) on the excess, with the balance going into the standard 18% per annum writing-down allowance pool. On a £2 million project: £1m at 100% AIA (£250k CT saving) + £500k at 50% FYA (£125k CT saving) + £500k in pool (generating £90k per year declining) = total CT saving of approximately £440k over the first 3 years.

AIA headroom: sharing the cap across capital expenditure

The £1 million AIA is shared across all plant and machinery expenditure within a single accounting period — not just solar. If your data centre is also investing in UPS upgrades, CRAC replacement, or data hall fit-out in the same year, those expenditures compete for AIA headroom with the solar project.

Example: A data centre operator spends £600,000 on a CRAC replacement programme in the first half of the year, then commissions a £500,000 solar installation in the second half. Total qualifying expenditure: £1.1 million. AIA covers £1 million (100% deduction = £250k CT saving); £100,000 goes into the 18% pool (£18,000/year deduction). The optimal strategy would have been to defer either expenditure to a different accounting period, or to use Full Expensing for the solar (which is not capped) and AIA for the CRAC (which would have been below the cap alone).

This interaction is worth planning carefully. We work with clients’ finance teams at the feasibility stage to model capital expenditure across accounting periods and maximise the combined tax relief.

Finance lease vs hire purchase: the ownership trap

Capital allowances — both AIA and Full Expensing — are claimed by the person who owns the plant and machinery. In a finance lease (the most common form of off-balance-sheet solar finance), the lessor retains legal ownership and therefore claims the allowances. The lessee (you) cannot claim.

This is the single most common tax structuring mistake we see on data centre solar projects. An operator who structures a solar project as a finance lease:

• Does not claim AIA/Full Expensing (loses £112,500 on a £500k project)
• Pays slightly lower monthly payments than hire purchase (because the lessor has captured the tax benefit and reflected part of it in the rate)
• But loses more in tax benefit than they save in financing cost

Hire purchase (where ownership transfers to you at the end of the term) preserves your AIA/Full Expensing entitlement. The financing cost is slightly higher than a finance lease, but the tax saving more than compensates.

The comparison for a £500,000 solar project:

	Finance Lease	Hire Purchase
Monthly payment (5yr, 6.5% APR)	£7,100	£8,200
Total payments over 5 years	£426,000	£492,000
AIA/Full Expensing tax saving	£0	£112,500
Net cost (payments − tax saving)	£426,000	£379,500

Hire purchase wins by £46,500 — despite its higher headline monthly payment. This calculation makes clear why the ownership structure matters.

Timing the expenditure correctly

AIA and Full Expensing apply to the year in which the asset “first comes into use.” For solar PV, this is the date of commissioning — when the system is energised and producing electricity, with the G99 protection relay commissioned and the MCS certificate issued.

Practical implications:

• If your accounting year ends 31 December, a system commissioned on 30 December 2026 gets the full AIA/Full Expensing benefit in the 2026 tax year. A system commissioned 2 January 2027 gets the benefit in 2027.
• Where a project spans two accounting periods (deposit in year 1, commissioning in year 2), the allowances are claimed in year 2 — the year of commissioning, not of payment.
• If your company has a very profitable year followed by a less profitable year, you want the allowances in the high-profit year. We can adjust commissioning timing if there is flexibility, to maximise the tax relief value.

Combining Full Expensing with the Smart Export Guarantee

The SEG — the per-kWh payment for electricity exported to the grid — is a separate income stream unrelated to capital allowances. SEG income is taxable as trading income (for a data centre operator’s main trade) or as investment income. It does not affect the capital allowance entitlement.

For data centres (where export is typically near zero), SEG income is minimal — perhaps £500–£3,000/year. Include it in your financial model but don’t let it drive structuring decisions.

What finance directors need to know

For a £500,000 data centre solar project at a profitable company paying CT at 25%:

• Year 1: Pay £500,000 capex. Receive £112,500 CT saving (via reduced CT liability or CT repayment if paid in advance). Net cash out in year 1: approximately £387,500.
• Year 1 onwards: Avoid £87,000+/year in electricity cost (and growing with grid inflation). Net cash-positive after year 4.5 (post-tax).
• 25-year NPV at 8% discount rate: approximately £680,000 positive (including tax relief and inflation-escalated savings).
• 25-year IRR: 19–22% depending on grid rate assumptions.

These are the numbers to put in front of a finance director or investment committee. They represent a better risk-adjusted return than most capital market alternatives for a profitable UK corporation in 2026.

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We provide a detailed post-tax financial model for every feasibility study, including AIA/Full Expensing modelling, hire purchase vs cash analysis, and sensitivity tables for grid rate escalation. Request your feasibility or contact us with questions.