Commercial Solar ROI UK by System Size: 3kW to 20kW Compared
- 1. What Actually Determines Commercial Solar ROI?
- 2. 3kW: The Micro-Business System
- 3. 5kW: The Entry Point for Most Small Businesses
- 4. 7kW: The High-Performer for Daytime-Intensive Operations
- 5. 10kW: Entry-Level Commercial With Strong ROI per £
- 6. 15kW: Where Economies of Scale Begin
- 7. 20kW: Full SME Commercial Systems
- 8. Which System Size Is Right for Your Business?
- 9. The AIA Tax Benefit in Real Terms
- 10. DNO Approval: What Larger Systems Require
- 11. FAQs
The Short Version (Read This First)
- For most UK businesses, systems between 5kW and 15kW deliver the strongest balance of upfront cost and financial return
- A 3kW system typically pays back in 8–9 years. A 15kW or 20kW system can reach payback in 4–5 years for businesses with strong daytime energy use
- The biggest driver of ROI is not system size — it is self-consumption. Electricity used on-site saves 27p/kWh. Electricity exported earns around 7p/kWh. That gap determines everything
- The Annual Investment Allowance (AIA) lets businesses deduct the full system cost from taxable profit in Year 1, reducing effective net cost by 25% for a standard corporation tax payer
- Commercial solar is subject to 20% VAT. VAT-registered businesses reclaim this as input tax. Non-registered businesses must factor it into their budget from the outset
- Larger systems often have shorter payback periods than smaller ones — not despite their higher cost, but because of economies of scale in installation and stronger annual savings
Which commercial solar system size actually delivers the best return? And how much difference does going from 5kW to 10kW — or 10kW to 20kW — really make in terms of cost, savings and payback period?
These are the right questions to ask. The answer is not simply “bigger is better” or “start small and scale up.” Instead, it depends on how your business uses electricity, when it uses it, and how much of the solar generation you can consume on-site. This guide compares every major commercial system size from 3kW to 20kW — with real numbers, real payback figures, and the factors that determine which size works for which type of business.
All figures in this guide use the current commercial electricity rate of 27p/kWh (Ofgem Q2 2026), an MCS irradiance figure of 850 kWh/kWp/year, and a conservative SEG export rate of 7p/kWh. For a broader view of what the commercial solar panel installation process involves from survey to sign-off, see our main commercial solar page.
What Actually Determines Commercial Solar ROI?
Two businesses can install the same-sized system and see completely different returns. The panels are the same. The inverter is the same. The difference, however, is how the energy is used once it is generated. Four factors drive the outcome:
Self-consumption rate
Electricity used on-site replaces power bought from the grid at 27p/kWh. Electricity exported, on the other hand, earns around 7p/kWh via the Smart Export Guarantee. Every percentage point of self-consumption is therefore worth almost four times more than export. As a result, this is the single most important variable in any ROI calculation.
Operating hours
Businesses that run during daylight hours — offices, retail, workshops, schools — naturally align with solar generation and consequently see the strongest returns. Evening-heavy businesses, such as restaurants and gyms, rely more on battery storage to capture that value.
Electricity rate
This guide uses 27p/kWh, reflecting typical SME pricing in 2026. Every unit of solar electricity used on-site avoids buying at that rate. If your contracted rate is higher, payback is therefore faster.
AIA tax relief
The Annual Investment Allowance reduces the effective cost of the system in Year 1 by 25% for a standard corporation tax payer. This has a disproportionate effect on larger systems, where the cash saving is more significant.
These four factors apply to every system size. What changes, however, is how well each size can take advantage of them for a given business type.
📋 The self-consumption point in plain numbers: On a 10kW system generating 8,500 kWh/year, moving from 65% to 80% self-consumption adds approximately £340 to your annual benefit — with no additional hardware cost. Matching system size to usage patterns therefore matters more than squeezing an extra panel on the roof.
3kW: The Micro-Business System
A 3kW system sits at the edge of what qualifies as commercial solar. It is not designed to transform energy costs — rather, it is designed to reduce them in a controlled, low-risk way. This size suits businesses with very low and stable daytime demand: home offices, small therapy practices, or single-room operations.
| Metric | Without battery | With battery |
|---|---|---|
| Annual generation | ~2,550 kWh | ~2,550 kWh |
| Self-consumption rate | ~70% | ~85% |
| Total annual benefit | ~£536 | ~£612 |
| Net cost after AIA (25% corp tax) | ~£4,500 | ~£6,750–£8,250 |
| Simple payback | ~8–9 years | ~11–13 years |
At this size, most generation is used directly during the day, which keeps returns stable. Installation is also straightforward — a 3kW system falls under G98 and does not require pre-approval from the DNO.
However, the limitation is clear. Once daily usage grows beyond a modest level, the system becomes undersized quickly. The cost difference between 3kW and 5kW is relatively small, but the generation increase is significant. Battery storage at this size rarely improves the financial outcome — it adds enough cost to push payback well beyond what most businesses would consider acceptable.
5kW: The Entry Point for Most Small Businesses
A 5kW system is where commercial solar starts to align properly with typical small business usage. It moves beyond marginal savings and, as a result, delivers a meaningful reduction in electricity costs. This size works well for small offices, salons, retail shops and professional services where energy use is steady throughout the day.
| Metric | Without battery | With battery |
|---|---|---|
| Annual generation | ~4,250 kWh | ~4,250 kWh |
| Self-consumption rate | ~75% | ~88% |
| Total annual benefit | ~£935 | ~£1,019 |
| Net cost after AIA (25% corp tax) | ~£6,000 | ~£8,625–£10,500 |
| Simple payback | ~6–7 years | ~8–10 years |
Moving from 3kW to 5kW increases generation by around two-thirds, but the additional cost is relatively modest. Because most of that extra energy is still used on-site, the value per kWh therefore remains high. For many businesses, this is the point where solar starts to feel like a practical operational decision rather than a small efficiency upgrade.
Battery storage becomes more relevant at 5kW, particularly for businesses with mixed usage patterns. FoxESS or GivEnergy suit moderate demand at this scale. Larger options like Sigenergy or Tesla are more relevant where EV charging or extended evening usage is expected.
7kW: The High-Performer for Daytime-Intensive Operations
A 7kW system is particularly effective for businesses with consistent, equipment-heavy daytime demand — cafés, medical practices, dental surgeries and busy retail environments. In these settings, electricity use overlaps strongly with solar generation. Consequently, the system operates at high self-consumption levels.
| Metric | Without battery | With battery |
|---|---|---|
| Annual generation | ~5,950 kWh | ~5,950 kWh |
| Self-consumption rate | ~75% | ~88% |
| Total annual benefit | ~£1,368 | ~£1,488 |
| Net cost after AIA (25% corp tax) | ~£8,250 | ~£11,250–£13,500 |
| Simple payback | ~6 years | ~8–9 years |
At this scale, the system makes a visible impact on operating costs. However, usage patterns matter more here. For businesses that run into the evening — restaurants, bars — a larger share of energy may be exported unless battery storage is included. Furthermore, because this system exceeds the G98 export threshold, it requires a G99 application to the local DNO before installation begins. This adds lead time but does not affect viability.
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10kW: Entry-Level Commercial With Strong ROI per £
A 10kW system marks the transition into true commercial solar. It no longer just reduces costs — it actively reshapes how much electricity the business needs to buy from the grid. For a dedicated guide to 10kW system economics, see our 10kW commercial solar system guide.
| Metric | Daytime-heavy business | With battery storage |
|---|---|---|
| Annual generation | ~8,500 kWh | ~8,500 kWh |
| Self-consumption rate | ~75% | ~88% |
| Total annual benefit | ~£1,785 | ~£2,040 |
| Net cost after AIA (25% corp tax) | ~£10,125 | ~£13,500–£16,125 |
| Simple payback | ~5–6 years | ~7–8 years |
Why 10kW is a turning point
The most important shift at 10kW is scale efficiency. The cost per kW begins to stabilise while total savings increase significantly. This is also where the AIA tax benefit becomes a major factor — reducing the effective cost before the system generates a single unit of electricity. For many businesses, therefore, 10kW is the point where solar moves from an efficiency improvement to a strategic financial decision.
💡 The scale efficiency point: Moving from 5kW to 10kW almost doubles the annual generation and benefit. However, the cost increase is proportionally smaller. At 5kW, the cost per kW installed is typically higher than at 10kW — because fixed costs like scaffolding, DNO applications and commissioning are spread across more capacity at larger system sizes.
15kW: Where Economies of Scale Begin
At 15kW, the economics change in a way that surprises most business owners. Despite a higher upfront cost, payback periods become shorter than those of smaller systems. The installation cost per kW drops, while total annual savings increase substantially.
| Metric | Without battery | With battery |
|---|---|---|
| Annual generation | ~12,750 kWh | ~12,750 kWh |
| Self-consumption rate | ~75% | ~88% |
| Total annual benefit | ~£2,932 | ~£3,187 |
| Net cost after AIA (25% corp tax) | ~£12,375 | ~£16,125–£19,125 |
| Simple payback | ~4–5 years | ~5–6 years |
This size suits medium offices, gyms and high-usage retail environments where electricity demand is both high and consistent. A 15kW system illustrates a key principle: larger systems can outperform smaller ones financially — not just in total savings, but also in how quickly they recover their cost. The payback at 15kW is therefore faster than at 5kW, despite the higher price tag.
20kW: Full SME Commercial Systems
A 20kW system represents the upper end of what most SMEs can realistically install within standard roof space. At this level, solar becomes a significant part of the business’s energy infrastructure — capable of offsetting a large proportion of annual demand.
| Metric | Daytime-heavy business | With battery storage |
|---|---|---|
| Annual generation | ~17,000 kWh | ~17,000 kWh |
| Self-consumption rate | ~75% | ~88% |
| Total annual benefit | ~£3,910 | ~£4,158 |
| Net cost after AIA (25% corp tax) | ~£16,500 | ~£21,000–£24,750 |
| Simple payback | ~4–5 years | ~5–6 years |
This size suits larger restaurants, hotels, light manufacturing units and multi-site operations where electricity use is high throughout the day and into the evening. At 20kW, solar is no longer a supplementary system. Instead, it becomes a core part of how the business manages its energy costs.
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Which System Size Is Right for Your Business?
The right system size is determined by how much electricity you use during the day — not how much roof space you have. Sizing based on roof space leads to oversized or undersized systems. Sizing based on actual daytime consumption, however, leads to the strongest ROI.
| Business type | Daily kWh usage | Recommended system | Key reason |
|---|---|---|---|
| Home office / small practice | 10–25 kWh | 3–5kW | Low, stable daytime demand |
| Small office / salon / retail | 25–50 kWh | 5–7kW | Consistent daytime usage |
| Café / medical practice | 40–70 kWh | 7kW | Equipment-heavy during trading hours |
| Gym / school / larger office | 55–110 kWh | 10–15kW | Long operating hours, mixed demand |
| Restaurant / hotel / light manufacturing | 80–137 kWh | 15–20kW | High demand, extended hours |
How to use this table
As a rough guide: below 40 kWh/day, a 5–7kW system is usually sufficient. For 50–80 kWh/day, moving to 7–10kW improves returns noticeably. For 100+ kWh/day, 15kW+ systems become more cost-effective per kW generated.
The mistake to avoid is sizing based on what fits the roof. The better question is how much of your electricity you can realistically use during daylight hours. Once that is clear, the right system size usually becomes obvious. For a broader introduction to available funding, our commercial solar grants guide covers tax relief and funding options in full.
⚠️ Battery storage and ROI: At every system size, battery storage improves self-consumption but extends payback. For daytime-heavy businesses (offices, retail, schools), battery storage is optional — the system pays back well without it. For evening-heavy businesses (restaurants, hospitality, care homes), however, battery storage is worth the additional cost because it captures generation that would otherwise be exported at 7p/kWh. See our Sigenergy battery review for a detailed look at one of the leading commercial battery options.
The AIA Tax Benefit in Real Terms
System size matters more than most businesses expect — and a large part of why is how the Annual Investment Allowance changes the real cost of scaling up. AIA allows UK businesses to deduct the full cost of qualifying capital equipment from taxable profit in Year 1. For solar, this means a 25% corporation tax payer effectively gets 25% off the ex-VAT system cost before the system generates a single unit of electricity.
| System size | Installed cost (ex-VAT) | AIA tax saving (25% corp tax) | Effective net cost |
|---|---|---|---|
| 5kW | ~£8,000 | ~£2,000 | ~£6,000 |
| 10kW | ~£13,500 | ~£3,375 | ~£10,125 |
| 15kW | ~£16,500 | ~£4,125 | ~£12,375 |
| 20kW | ~£22,000 | ~£5,500 | ~£16,500 |
What this means in practice
Moving from 5kW to 10kW adds £5,500 on paper. After AIA, however, the real additional cost is closer to £4,125 — while almost doubling annual generation. That is why larger systems often outperform smaller ones on payback: you are increasing output at a lower effective cost per kW.
One important point: solar installations qualify under the Annual Investment Allowance, not Full Expensing. These two mechanisms are frequently confused. AIA is the correct mechanism for solar PV — so confirm this with your accountant before completing a purchase.
DNO Approval: What Larger Systems Require
The grid approval process is one of the most commonly overlooked factors in commercial solar planning. It directly affects project timelines, so understanding it before committing to a system size helps avoid delays.
The threshold is based on inverter output per phase, not panel capacity:
- G98 (smaller systems): if the inverter output is ≤3.68kW per phase, the system can be installed and the DNO notified within 28 days afterwards. No pre-approval is required. This applies to most 3kW systems and some 5kW configurations on single-phase supplies
- G99 (larger systems): if the inverter output exceeds 3.68kW per phase, a G99 application must be submitted to and approved by the local DNO before installation begins. This applies to 7kW systems and above in most configurations
What G99 involves in practice
A G99 application covers system specifications, export management approach, and MCS installer certification. The DNO has up to 45 working days to process it — in practice, most SME applications complete within 4–12 weeks. However, some areas have backlogs extending to 3–6 months. For a full explanation, see our DNO application guide.
In some cases, the DNO will approve the system subject to an export limitation. For most commercial installations, however, this makes little practical difference — most solar energy is consumed on-site or stored in battery rather than exported at levels where limits apply. For a full overview of planning permission, building control and grid approval, see our guide to commercial solar panel regulations in the UK.
Frequently Asked Questions
What is the payback period for commercial solar panels in the UK?
Payback periods for commercial solar in the UK currently range from 4 to 9 years depending on system size, self-consumption rate, and whether the Annual Investment Allowance is applied. Smaller systems (3–5kW) typically pay back in 6–9 years. Larger systems (15–20kW) with strong daytime self-consumption can pay back in 4–5 years after AIA tax relief. The average across the commercial sector in 2026 sits around 4–6 years for well-matched installations.
Which commercial solar system size has the best ROI?
For most UK businesses with consistent daytime electricity use, systems between 10kW and 15kW typically deliver the strongest ROI — balancing lower cost per kW installed with high annual savings. Larger systems (15–20kW) can achieve faster payback than smaller ones (5–7kW) because economies of scale reduce the installed cost per kW while total annual savings increase. The right size depends on your actual daytime energy consumption, not your roof space.
How does self-consumption affect commercial solar ROI?
Self-consumption is the single biggest driver of commercial solar ROI. Electricity used on-site replaces grid power at 27p/kWh. Electricity exported earns around 7p/kWh via the Smart Export Guarantee. That is nearly a 4:1 difference in value. A business with 80% self-consumption will therefore achieve substantially better returns than one with 60% self-consumption on the same system. Daytime-heavy businesses (offices, retail, schools) naturally achieve high self-consumption; evening-heavy businesses need battery storage to match.
Does the Annual Investment Allowance apply to commercial solar?
Yes. Commercial solar PV systems qualify under the Annual Investment Allowance (AIA), which allows the full system cost to be deducted from taxable profit in Year 1. For a business paying 25% corporation tax, this consequently reduces the effective net cost of the system by 25% before any electricity savings are counted. Note: solar qualifies under AIA, not Full Expensing — these are different mechanisms and are frequently confused. Confirm with your accountant before purchase.
Does a commercial solar system need DNO approval?
Yes, if it connects to the grid. Systems with inverter output up to 3.68kW per phase fall under G98 — the DNO is notified within 28 days after installation. Systems above this threshold (virtually all commercial installations above 7kW) require a G99 application to be approved by the local DNO before installation begins. G99 approval typically takes 4–12 weeks but can extend to 3–6 months in high-demand areas. Solar4Good manages the G99 application process as part of every commercial installation.
Is a 20kW solar system worth it for a small business?
A 20kW system is worth it if your business uses 80–140 kWh of electricity per day during daylight hours. At that usage level, the system’s annual generation of approximately 17,000 kWh translates to £3,900–£4,200 in annual savings and SEG income, with payback in 4–5 years after AIA. If your daytime usage is significantly lower, however, a 10–15kW system will deliver a stronger ROI without the additional upfront cost.
What solar system size is right for an office?
A small office using 25–50 kWh/day typically suits a 5–7kW system. A medium office using 50–80 kWh/day is better served by a 7–10kW system. A larger office or business park consuming 100+ kWh/day during operating hours should consider 15–20kW. In all cases, sizing should be based on actual metered consumption data, not estimated figures — a half-hourly data download from your smart meter gives the most accurate picture.
About the Author
Manan Shah
Co-Founder & Solar Expert. Leads Solar4Good's commercial strategy and customer relationships, and is quoted as a clean-energy authority by some of the UK's most trusted news publications. Quoted in the Financial Times and The Times on UK solar.