AIA vs FYA detailed comparison — when each wins for commercial solar
Published 2026-03-12 · 9 minute read · By Commercial Solar Finance editorial team
AIA gives 100% year-one deduction on the first £1m of qualifying capex; FYA gives 50% year-one with the residual in the special-rate pool. AIA wins on year-one cash where headroom is available; FYA wins where AIA is exhausted by other capex. We walk through the full decision framework with worked numbers.
The AIA vs FYA decision is one of the most common questions in commercial solar tax planning, and the answer is more nuanced than the headline comparison suggests. Both allowances are available on commercial solar PV; the right choice depends on your wider capex position for the period and your long-term tax planning approach.
The mechanics
AIA (Annual Investment Allowance): 100% deduction in the year incurred on the first £1m of qualifying capital expenditure per accounting period. Applies to plant, machinery, fit-out, and qualifying solar PV. Shared across all qualifying capex in the period.
50% FYA (First Year Allowance): 50% deduction in the year incurred on qualifying expenditure. Residual 50% enters the special-rate pool at 6% writing-down allowance per year. Specifically extended for new commercial solar PV until 31 March 2026.
Special-rate pool: 6% writing-down allowance on a reducing-balance basis. Long-life assets including commercial solar PV qualify. Where AIA or FYA isn't claimed, the full qualifying capex enters the special-rate pool.
Year-one tax saving comparison
For a £200k commercial solar system at the 25% main corporation tax rate:
| Route | Year-1 deduction | Year-1 tax saving | Long-term tail |
|---|---|---|---|
| AIA | £200,000 (100%) | £50,000 | None — fully claimed year 1 |
| 50% FYA + special-rate pool | £100,000 year 1 | £25,000 | £100,000 in pool, ~£15,000 PV over 25 years |
| Special-rate pool only (post-FYA-deadline) | ~£12,000 (6% × 200k) | ~£3,000 | Declining annually for 25+ years; ~£18,000 PV |
AIA delivers materially higher year-one cash than FYA where AIA headroom is available. Lifetime tax value: AIA route ~£50k (single hit); FYA route ~£40k (split across years); special-rate-pool-only ~£21k.
The headroom question
AIA is shared across all qualifying capital expenditure in the accounting period. The £1m cap applies to total spend, not per-asset. If your business is doing other capital investment in the period — vehicles, plant, IT, fit-out — AIA headroom may already be partly committed.
Decision framework:
- If total qualifying capex ≤ £1m and AIA headroom available: claim AIA on full solar capex. Strongest year-one cash.
- If total qualifying capex ≤ £1m but AIA partly committed elsewhere: claim AIA on the residual headroom, FYA on the remainder of solar capex. Mixed route.
- If total qualifying capex > £1m: claim AIA on the first £1m (whatever combination of assets fits), FYA on the residual above £1m. Maximises combined year-one relief.
- If timing-impractical for AIA capture (year-end after 31 Mar 2026 with FYA missing): special-rate pool only. Plan around AIA where headroom available.
Long-term tax planning considerations
Year-one cash isn't the only consideration. Long-term planning factors:
Future profitability uncertainty. Where future tax-paying capacity is uncertain (loss-making years possible), claiming AIA in a profitable year captures the value before potential losses arrive. Lock in the relief while you can.
Marginal rate considerations. Where the company is at the marginal-rate band (£50k–£250k of profits), claiming larger deduction in one year may shift the band threshold. Tax adviser should model the exact effect.
Other capex priorities. If the business has other AIA-eligible capex with stronger commercial priority (e.g. critical machinery replacement, vehicle fleet renewal), preserve AIA headroom for those. Use FYA on solar.
FYA route preserves more total deduction over time. Where year-one cash isn't the binding constraint, FYA route + special-rate pool delivers slightly less PV value than AIA in cash terms but spreads the deduction across years — useful for businesses managing tax position across multiple periods.
Worked numbers across project sizes
| Project size | Capex | AIA year-1 | FYA year-1 | AIA premium over FYA |
|---|---|---|---|---|
| 100 kWp | £90k | £22.5k | £11.25k | £11.25k year-1; ~£3.75k PV lifetime |
| 250 kWp | £200k | £50k | £25k | £25k year-1; ~£8k PV lifetime |
| 500 kWp | £400k | £100k | £50k | £50k year-1; ~£17k PV lifetime |
| 1 MWp | £800k | £200k | £100k | £100k year-1; ~£33k PV lifetime |
| 2 MWp (over £1m AIA cap) | £1.6m | £250k AIA on first £1m; £75k FYA on £600k residual = £325k | £200k FYA on full £1.6m | £125k year-1 (combined route premium) |
Where the FYA still matters in 2026
For projects above the £1m AIA cap (sub-£1m AIA + above-£1m FYA), the FYA still delivers meaningful incremental tax saving — for a £2m project the combined AIA + FYA route is £125k of year-one cash above pure FYA-only.
For projects under the £1m AIA cap where AIA is already committed to other capex, FYA fills the gap as a secondary deduction route.
Post-31 March 2026, FYA disappears and only AIA + special-rate pool remain available. Project economics shift accordingly — most projects still produce 12-15% IRR but with 2-3 percentage points lower returns than the FYA window.
Engagement with your accountant
The AIA vs FYA decision is a tax decision; your accountant should advise on the exact treatment given your full year capex position. Standard advice flow:
- Confirm AIA headroom remaining for the period
- Model AIA + FYA + SRP combined route for the project
- Confirm timing — whether commissioning falls within the FYA window
- Consider future-period implications (carry-forward of unused AIA, special-rate-pool tail)
- Document the decision rationale in the capital allowances claim
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