Commercial Solar Payback, ROI and IRR Explained

The three numbers a finance director actually asks about

When a solar proposal lands on a finance director’s desk, three questions decide its fate. How fast do we get our money back. What do we earn in total. And how does the annualised return compare with everything else competing for the same capital. Those three questions map exactly onto payback, ROI and IRR. Most installer quotes give you a headline payback figure and stop there. That is not enough to sign off six figures, and any board worth its salt will send it back.

This guide explains all three properly, using real UK figures, so you can read a proposal like an analyst rather than take a salesperson’s word for it. If you want the underlying pricing first, our commercial solar cost guide breaks down per-kWp figures across the full 30 kW to 1 MW range.

Commercial solar payback, defined

Simple payback is the crudest of the three measures and the one everyone quotes. It answers a single question: how many years of savings does it take to recover what you spent. Divide the net installed cost by the annual saving and you have it.

Most UK commercial solar installations pay back in 5 to 8 years. That range is not marketing rounding, it reflects a genuine spread driven almost entirely by how much of the generated power you consume on site. A factory running a steady daytime process load consumes 80% or more of what its panels produce, so almost every unit displaces a 25p to 45p grid unit at full value. That sort of site lands at the fast end, 4 to 6 years. An office or a retail unit that sits empty at weekends exports more surplus at a lower Smart Export Guarantee rate, so it runs 6 to 8 years.

The number that matters underneath payback is self-consumption. A daytime-occupied building without a battery typically uses 55% to 75% of its solar directly. Add storage and that rises to 80% to 95%. Because on-site use is worth two to four times more than export, self-consumption, not roof size or panel brand, is the single biggest lever on your payback. You can model your own figures with our savings calculator.

Why payback alone is not enough

Simple payback has two blind spots. It ignores everything that happens after the break-even point, and it ignores the time value of money. A system that pays back in six years and then generates near-free power for another 15 to 20 under its 25-year performance warranty is a very different asset from a six-year payback that stops there. Payback treats them identically. It does not.

This is why ROI and IRR exist. Payback tells you when you stop losing. ROI and IRR tell you how much you win.

Return on investment across the whole 25 years

ROI is the total lifetime return expressed against your outlay. Take every pound of saving and export income across the panels’ 25-year warrantied life, subtract the capex and any operating cost, and express the surplus as a percentage of what you put in.

Commercial solar looks strong on this measure precisely because the tail is so long. Consider a mid-size warehouse system. Say a 250 kW array costs roughly £190,000 to £240,000 installed, before tax relief. It generates in the region of 230,000 kWh a year and, at typical commercial rates and self-consumption, saves £45,000 to £55,000 annually in early years. Even holding the saving flat and ignoring energy inflation, that is well over £1m of gross saving across 25 years against a sub-£250,000 outlay. Real ROI over the life of a well-sized commercial system commonly lands in the 300% to 500% range, and rises every time grid prices climb.

Two things make the headline capex misleading, both in your favour. First, 100% Annual Investment Allowance lets a profitable limited company deduct the full capex from taxable profit in year one. For a company paying corporation tax that is an effective saving of roughly 25% of the price, so the net cost of that 250 kW system is closer to £145,000 to £180,000. Second, VAT is reclaimable for VAT-registered businesses. Commercial solar is not zero-rated the way domestic installs are, but you reclaim the input VAT in the normal way, so it is not a real cost to you. The grants and funding page covers AIA, the Smart Export Guarantee and sector schemes in detail.

IRR, the number that lets you compare projects

Internal rate of return is the measure a serious board uses to rank capital projects against each other. It is the annualised percentage return that accounts for the timing of every cash flow, effectively the interest rate the project earns on the money tied up in it. A project with a 15% IRR beats one with a 10% IRR even if the headline ROI looks similar, because the cash comes back sooner and works harder.

Well-designed commercial solar typically returns an IRR in the low-to-mid teens for a self-funded system, and higher for sites with excellent self-consumption. A manufacturing site consuming 85% of its generation at a 5-year payback can push into the high teens or beyond. Compare that with a business savings account or the return on holding cash, and the case usually makes itself. Crucially, the IRR climbs with every rise in grid electricity prices, which have already run 100% to 150% above 2021 levels. Solar is one of the few capital projects whose return improves when your input costs get worse.

A proper proposal gives you the IRR and the net present value, not just a payback figure. If a quote will not show you the IRR, treat that as a warning sign about the modelling behind it.

Payback by building type

Payback varies by sector because load profiles vary. These are the typical figures we model by building type, drawn from real UK installs.

Building type	Typical system size	Typical payback	Why
Manufacturing and factories	200 kW to 2 MW	5 years	High, steady daytime process load drives 80%+ self-consumption
Warehouses and industrial units	100 to 500 kW	6 years	Large unshaded roofs, forklift charging and refrigeration create strong daytime demand
Retail and showrooms	40 to 250 kW	6 years	Long daytime trading day, but lighter weekend use lifts export share
Agricultural buildings	50 to 500 kW	6 years	Big south-facing barn roofs, often with dairy or cold-store baseload
Offices	30 to 150 kW	7 years	Daytime occupancy suits solar, but empty weekends export more surplus
Hospitality and leisure	50 to 300 kW	7 years	Evening-weighted demand means storage often improves the case

How you fund it changes the return profile, not the physics

The panels generate the same power whichever way you pay for them. What changes is how the cash flows and where the return sits. There are three routes, and an honest proposal models all three side by side.

Funding route	Upfront capex	Who owns the system	Effect on cash flow	Best when
Cash purchase	Full capex	You, from day one	Largest lifetime ROI and IRR, but capital is tied up until payback	You have the cash and want the highest total return
Asset finance	None, spread over 5 to 7 years	You, after the term	Usually cash-flow positive from month one, as the finance payment is less than the bill saving	You want the asset but prefer to keep capital free
Power Purchase Agreement (PPA)	None	The funder, for the contract term	No capex and immediate saving, but you buy the power rather than own the plant	You cannot commit capex, or you lease and want a transferable arrangement

Cash purchase gives the strongest IRR because you keep every pound of saving. Asset finance spreads the cost over five to seven years and is typically cash-flow positive from the first month, because the monthly repayment sits below the electricity bill it replaces. You own the system outright at the end. A PPA needs zero capex at all: a funder installs and owns the array, and you simply buy the power it produces at a fixed rate below grid. Your saving is smaller because the funder takes a margin, but your outlay is nil. See our FAQs for more on how each route settles if you sell or relocate.

PV alone versus PV plus battery

Storage is the other decision that shifts the numbers. A battery does not generate anything, it just moves solar from when you make it to when you need it, lifting self-consumption. Whether it pays depends entirely on how much of your demand falls outside daylight hours.

Configuration	Self-consumption	Effect on annual saving	Effect on payback	Best when
PV only	55% to 75%	Baseline	Shortest payback per pound spent	Daytime-weighted demand, weekday operation
PV plus battery	80% to 95%	Adds 25% to 40% to annual saving	Longer payback overall, higher lifetime saving	Significant evening, weekend or overnight load

For a straight weekday, daytime building, PV on its own usually gives the shorter payback because you are not paying for storage you barely use. Where a meaningful share of consumption falls in the evening, at weekends or overnight, a battery earns its keep by capturing surplus that would otherwise export at a low rate, and it opens up resilience and grid-service revenue. We model both, and design every system to be battery-ready even if you add storage later.

A worked example

A logistics operator running a 2,800 sqm distribution unit near Birmingham was paying £96,000 a year for electricity, driven by lighting, forklift charging and refrigeration. We modelled a 182 kW rooftop system on non-penetrative clip-fix mounts, generating around 168,000 kWh a year. At 78% self-consumption from daytime forklift charging, the annual saving came in at £38,000.

Funded on a six-year asset-finance agreement, the deal was cash-flow positive from month one. Simple payback landed at 5.5 years. But the number that persuaded the board was not the payback, it was the IRR across the full 25-year life, sitting comfortably in the mid-teens, plus the fact that the renewable disclosure helped retain a Tier-1 retail contract worth far more than the panels. That is the difference between reading payback alone and reading all three measures together.

Getting numbers you can defend

Every figure in a credible commercial proposal should trace back to a PVSyst yield model built from your half-hourly meter data and your roof drawings, not a per-square-metre estimate. Ask for the model. Ask for the IRR and NPV, not just the payback. And be sceptical of any quote that promises to wipe out your bill entirely, that is not how grid-tied commercial solar works.

When you are ready, request a fixed-price quote and we will model cash, finance and PPA side by side, with the payback, ROI and IRR for each, from your actual consumption data. No site visit is needed for the initial proposal, and we will tell you honestly if your roof, load profile or tenure do not suit solar.