Commercial Solar Payback UK 2026: 100kWp to 500kWp Worked

Quick answer

A well-sized UK commercial solar system pays for itself in 3.5 to 6 years before tax relief — or roughly 2.5 to 4.5 years once the 100% Annual Investment Allowance is applied. For a typical 250kWp rooftop array costing around £200,000, that means clawing the net cost back in under five years and then banking 20+ years of near-free daytime electricity. Below we work through the full numbers for 100kWp, 250kWp and 500kWp installs — capex, the 100% pre-tax capital allowance, Smart Export Guarantee income, energy-bill savings and the resulting payback in months.

How commercial solar payback actually works

Commercial solar payback rests on four cash flows. Get these right and the rest is arithmetic:

1. Capex — the fully installed cost (panels, inverters, mounting, DNO connection, scaffolding, commissioning).

2. Self-consumption savings — every kWh you generate and use yourself avoids buying grid power at 22-30p/kWh in 2026. This is the biggest lever by far.

3. Smart Export Guarantee (SEG) income — exported surplus earns 4-15p/kWh from a licensed supplier, per Ofgem's SEG rules.

4. Capital allowances — solar PV is plant and machinery, so the install qualifies for the 100% Annual Investment Allowance, writing the whole cost off against taxable profit in year one (see gov.uk capital allowances).

The single most important decision is sizing to daytime demand, not roof area. A business that self-consumes 80% of generation pays back roughly twice as fast as one that exports half its output, because avoided import (≈26p) is worth far more than export (≈8p). Our commercial solar installers UK team always models your half-hourly consumption profile before recommending a system size.

100kWp commercial solar: the worked numbers

A 100kWp system suits a mid-sized warehouse, light-industrial unit or large retail premises with a working-hours load.

• Capex: ~£85,000 (≈£0.85/W)
• Annual generation: ~92,000 kWh (Scottish Central Belt yield ≈920 kWh/kWp)
• Self-consumed (75%): 69,000 kWh × 26p = £17,940 saved
• Exported (25%): 23,000 kWh × 8p = £1,840 income
• Gross annual benefit: ~£19,780
• Simple payback (pre-tax): £85,000 ÷ £19,780 = ~4.3 years (≈52 months)
• After 100% AIA (25% corp tax saves £21,250): net cost ≈£63,750 → payback ~3.2 years (≈39 months)

Over 25 years, allowing for ~5% annual energy-price inflation and panel degradation, this system returns well over £500,000 in savings and income against an £85k outlay.

250kWp commercial solar: the worked numbers

A 250kWp array is the sweet spot for larger industrial roofs, distribution centres and manufacturing sites with strong daytime demand.

• Capex: ~£200,000 (≈£0.80/W — scale brings the per-watt cost down)
• Annual generation: ~230,000 kWh
• Self-consumed (78%): 179,400 kWh × 26p = £46,644 saved
• Exported (22%): 50,600 kWh × 8p = £4,048 income
• Gross annual benefit: ~£50,692
• Simple payback (pre-tax): £200,000 ÷ £50,692 = ~3.9 years (≈47 months)
• After 100% AIA (saves £50,000 in corp tax): net cost ≈£150,000 → payback ~3.0 years (≈36 months)

This is the configuration most of our commercial clients land on, because the per-watt cost is keen and the daytime load swallows most of the generation.

500kWp commercial solar: the worked numbers

At 500kWp you are into large warehouse, factory or multi-roof estate territory. Here a Power Purchase Agreement or wholesale-linked export deal often beats standard SEG, and a G99 grid application is mandatory.

• Capex: ~£375,000 (≈£0.75/W)
• Annual generation: ~460,000 kWh
• Self-consumed (70%): 322,000 kWh × 26p = £83,720 saved
• Exported (30%): 138,000 kWh × 9p = £12,420 income
• Gross annual benefit: ~£96,140
• Simple payback (pre-tax): £375,000 ÷ £96,140 = ~3.9 years (≈47 months)
• After 100% AIA (saves £93,750 in corp tax): net cost ≈£281,250 → payback ~2.9 years (≈35 months)

Note the £1m AIA cap: a £375k system sits comfortably within it, so the full amount can usually be claimed in year one.

Commercial solar payback by system size — comparison table

The headline numbers side by side (2026 Scottish Central Belt assumptions, 26p import / 8-9p export):

Bigger systems show a slightly better per-watt capex but a marginally lower self-consumption ratio, so payback flattens out around the 3-4 year mark after tax relief across the range. Adding battery storage can lift self-consumption a further 10-15 points, shortening payback again where evening loads or time-of-use tariffs reward stored energy.

The 100% AIA tax relief — why it matters so much

The Annual Investment Allowance lets a business deduct the full cost of qualifying plant and machinery — including solar PV — from taxable profits in the year of purchase, up to £1 million per year. At the 25% main rate of corporation tax:

• A £200,000 install reduces your tax bill by £50,000
• The net effective cost drops to £150,000
• That single mechanism shortens payback by roughly a full year

This is pre-tax relief in year one — not a slow writing-down allowance spread over decades. It is the difference between a "nice ESG project" and a genuinely compelling investment. Confirm the treatment with your accountant, because eligibility depends on your tax position and whether the asset is bought outright versus leased. Leasing and PPA routes shift the tax treatment but require zero capex, which suits cash-conscious businesses — our team models all three funding routes.

What actually changes your payback (and what doesn't)

Two businesses on the same street can see payback periods a year and a half apart on identical kit. The variables that genuinely move the number are:

• Daytime self-consumption ratio. This is the single biggest driver. A site that runs machinery, refrigeration or air-conditioning through the working day might self-consume 85% of generation; an office that empties at 5pm and sits dark at weekends might manage 55%. Higher self-consumption means more electricity valued at 26p rather than 8p.
• Your current import tariff. If you are paying 30p/kWh, every avoided unit is worth more, so payback shortens. Businesses still on a fixed contract signed during the 2022 spike often see the fastest returns.
• Roof orientation and pitch. South-facing at 30-40° is ideal, but east-west splits — common on industrial sheds — actually spread generation across the morning and afternoon peaks, which can lift self-consumption even if total yield drops slightly.
• Battery storage. Adding a commercial battery captures surplus that would otherwise export at 8p and redeploys it at 26p, or arbitrages a time-of-use tariff. On the right load profile this lifts self-consumption 10-15 points and recovers its own cost in 5-7 years.

What does not materially change payback: the brand of panel (within reason), minor shading from a single flue, or the exact month you commission. Chasing a marginally cheaper per-watt price while ignoring self-consumption is the most common costly mistake we see in tender documents.

Funding routes: CapEx, lease or PPA

You do not have to find the capital up front. The three standard routes each suit a different balance sheet:

Under a PPA, a funder installs and owns the system and sells you the generated electricity at a fixed rate below grid price — you save from day one with no outlay, but forgo the AIA and the long-term free power. CapEx delivers the strongest lifetime return for businesses with the cash and the tax position to use the allowance. Our commercial solar installers UK team models all three side by side against your actual consumption data so you can see the net-present-value difference before deciding.

Bottom line

Commercial solar in the UK pays back fast and predictably in 2026. Across 100kWp, 250kWp and 500kWp the pattern is the same: a 3.5-6 year simple payback that compresses to under 4 years once the 100% Annual Investment Allowance is applied, followed by two decades of near-free daytime power. The winning formula is to size to your daytime demand so you maximise self-consumption (worth ≈26p/kWh avoided) rather than chasing export income (≈8p/kWh), and to claim the full capital allowance in year one. For a site-specific model built on your half-hourly consumption data, speak to our commercial solar installers UK team — we provide a fixed-price feasibility study with worked payback before you commit a penny.

Related Ecoaim guides:

• 🏭 Commercial solar panels — full guide
• 💷 Green mortgages & business finance
• 🔋 Battery storage for businesses
• 🛠️ Postcode ROI calculator
• 📈 Do solar panels increase home value? UK 2026 data