The tax mistake solar salespeople make about "Full Expensing"

Ask three commercial-solar salespeople how the tax relief works. There’s a fair chance at least one will tell you your system qualifies for “100% Full Expensing.” It’s a confident, specific-sounding claim. It’s also wrong — and HMRC’s own manual says so in one sentence.

The claim, and why it’s false

Full Expensing is a real UK tax policy. It delivers a 100% first-year deduction against taxable profits, with no cap. The government introduced it for main-rate plant and machinery in the Spring 2023 Budget.

It became permanent from April 2024. It’s generous, uncapped, and genuinely a big deal for the assets it covers.

Solar panels are not one of those assets.

Since 1 April 2012, HMRC has classified solar panel spending as “special rate” expenditure, not main-rate plant and machinery. HMRC’s Capital Allowances Manual, CA22335 sets this out explicitly, and Full Expensing, by law, excludes special rate assets. So the two facts sit together plainly: solar is special rate, and special rate assets don’t get Full Expensing. There’s no ambiguity here — it’s a classification, not a judgement call.

Where the confusion actually comes from

The mistake is understandable, even if it’s still a mistake. Most solar buyers do end up with a genuine 100% first-year deduction on their system — just not via Full Expensing. The relief that actually delivers it is the Annual Investment Allowance (AIA).

The AIA also gives 100% relief in the year of purchase. That overlap makes it easy to round “100% relief” up to “Full Expensing.” Someone selling systems, rather than advising on tax, may not check which specific allowance actually applies. The two schemes produce a similar number for many buyers, which is exactly what keeps the wrong name alive.

The distinction stops being cosmetic once a buyer’s spend runs past the AIA’s cap. Above that point, AIA and Full Expensing behave completely differently, as the table below shows.

What actually happens to solar spend

Step	What it is	Rate	Applies to solar?
1	Annual Investment Allowance (AIA)	100%, up to a shared £1 million annual cap across all qualifying plant and machinery spend that year	Yes — this is the relief that usually covers the whole system
2	Full Expensing	100%, no cap	No — main-rate assets only; special rate expenditure is explicitly excluded
3	Special rate first-year allowance	50% on spend above the AIA cap	Yes — this is what solar gets once the AIA is used up
4	Special rate pool writing-down allowance	6% per year on the remaining balance	Yes — ongoing relief on whatever’s left after steps 1–3

Source: GOV.UK — Claim capital allowances and HMRC CA22335.

In practice, this means:

• Suppose your business hasn’t used its AIA elsewhere that accounting year. A solar system priced well under £1 million then typically qualifies. It gets a genuine 100% first-year deduction via the AIA, correctly named.
• Sometimes the AIA is already committed elsewhere — to a refit, new machinery, or a fleet purchase. Or the solar system itself is large enough to exceed the cap. Either way, the excess falls to the 50% special rate first-year allowance, then 6% a year after that. That’s a materially slower relief profile than an uncapped 100% deduction.
• The gap between “AIA covers it” and “Full Expensing covers it” only shows up once you’re above the cap. That’s exactly the scenario where getting the name wrong costs the most.

The 2026 Budget changed the wrong pool

Adding to the confusion, the Autumn 2025 Budget introduced two changes to main-rate plant and machinery. These took effect from January and April 2026. The first is a new permanent 40% first-year allowance. The second cuts the main pool’s writing-down allowance from 18% to 14% (Deloitte Taxscape — capital allowances, Autumn Budget 2025).

Both changes apply to the main pool. Solar sits in the special rate pool.

Neither change touches solar’s treatment at all. But a headline about “new first-year allowances” can land in the same news cycle. That makes it easy to fold into a sales pitch without anyone checking which pool it actually applies to.

Why the distinction is worth insisting on

None of this makes solar a bad investment. The AIA, correctly applied, still gets most single-site commercial systems to a genuine 100% first-year deduction. However, a business case built on the wrong allowance name can go wrong in two specific ways. First, it can overstate the relief on a large system that blows through the AIA cap.

That understates the true payback period to a board or finance committee — and they’ll spot the error later. Second, it can simply be the first domino in a wider sales conversation. From there, people round numbers up rather than checking them properly.

Either way, if a quote or information pack cites “Full Expensing” for a solar system, that’s worth noticing. It’s a fair signal to ask what else in the numbers no one has checked against the primary source.

We cover these definitions in full in our glossary entries for the Annual Investment Allowance and the special rate pool. The sourcing there matches what’s above. It’s worth bookmarking, so you have the precise wording next time someone quotes you a tax figure — not a paraphrase.

Getting the real number for your business

Three things decide which allowance actually applies. They are your accounting year, what else you’ve bought that year, and the size of the system itself. Together, they also determine how much of your spend falls into each step of the table above. That’s a modelling question, not a rule of thumb.

It’s exactly what we build into every business case before a client commits capital.

We work out the real, net-of-tax payback using the allowance that genuinely applies. That’s not always the one that sounds best in a sales deck. If you want that calculation done against your own numbers, our energy and ROI modelling service is where it happens.