Commercial solar payback and ROI

Simple payback, and why it understates the return

Simple payback divides the net cost by the annual saving. For a well-matched commercial system that lands at roughly four to seven years before tax relief. But simple payback ignores two things that matter to a business: the tax relief you claim in year one, and the fact that the system keeps generating for 25 years or more after it has paid for itself. Factor in the Annual Investment Allowance, which returns up to a quarter of the cost in year-one corporation-tax relief, and the effective payback on the net cost drops to nearer three to five years.

ROI, IRR and NPV

Return on investment over the life of the system is high because the asset runs for decades on sunlight after a short payback. Internal rate of return, which annualises that return, is typically in the 10 to 20% range for an unleveraged commercial system, and higher on sites with strong daytime demand and good self-consumption. Net present value, which discounts all the future savings back to today, is comfortably positive at any sensible discount rate over a 25-year life. The reason all three look strong is the same: you are replacing power bought at 26 to 32 pence with power generated at a far lower levelised cost.

How the funding route changes the return

Owning the system, with cash or a loan, gives the highest ROI because you keep every unit of saving, all the export income and all the tax relief. Financing it on hire purchase or asset finance keeps most of that return while spreading the cost, and can be cash-flow positive from year one if the saving exceeds the payment. A PPA has the lowest ROI to you because the funder owns the asset and takes the allowances and export income, but it needs no capital and no maintenance. There is no single best answer, which is exactly why we quote every route against your numbers.

Four ways to measure the same investment

Payback and return get used loosely, but a finance team will want to separate four measures, because each answers a different question and they can point in different directions on the same system.

• Simple payback is net cost divided by annual saving: the years until the system repays its own cost. It ignores the time value of money and everything the asset earns after payback, so it understates the real return.
• Return on investment (ROI) is total lifetime saving against what you paid. Because a system generates for 25 years or more after a short payback, lifetime ROI is high, though it says nothing about timing.
• Internal rate of return (IRR) annualises the return so it compares like for like against other uses of the money. For an unleveraged commercial system it typically sits in the 10 to 20% range.
• Net present value (NPV) discounts future savings back to today's money at a rate you choose. A positive NPV means the investment beats that rate, and over a 25-year life a well-matched system is comfortably positive.

Simple payback is the number most people quote, but IRR and NPV are the ones a finance director tends to trust, because they respect timing and the cost of capital.

Why self-consumption is the biggest driver

The largest lever on all four measures is how much of the generation you use on site rather than export. Power you self-consume displaces electricity you would otherwise buy at the full commercial rate, so it is worth the whole grid price you avoid. Power you export earns a payment under the Smart Export Guarantee (SEG), which replaced the Feed-in Tariff and covers exported generation up to 5 MW, but the export price is well below the import price. In practice a self-consumed unit is worth roughly double an exported one, which is why a warehouse or factory running daytime load pays back years faster than a building that sits empty in daylight, on an identical system. Sizing the array to your actual half-hourly demand, not to the available roof, is usually the difference between a strong return and an average one.

How year-one tax relief shortens the effective payback

Simple payback measures gross cost, but the amount your business actually funds is lower once first-year tax relief is applied. Solar PV is classed as special-rate plant and machinery, so it does not qualify for 100% full expensing (that is main-rate only) or the newer 40% first-year allowance. It does qualify for the Annual Investment Allowance (AIA), which gives 100% first-year relief on qualifying spend up to £1 million a year. Because most commercial rooftop installs sit below that cap, the whole cost usually attracts full relief in year one, cutting your corporation tax bill. Where qualifying capital spend runs past the £1 million cap, the balance on the solar asset can use the 50% special-rate first-year allowance, which is available to companies, with the remainder written down at 6% a year on the reducing-balance basis.

Two further points help the cashflow. VAT on commercial solar is charged at the standard 20% rate, not the 0% domestic rate, but a VAT-registered business can normally reclaim it through the return. And rooftop solar generating power for your own use is 100% exempt from business rates in England from April 2022 to March 2035. Together, these shift the effective payback from the pre-relief four to seven years towards three to five. This is general information rather than tax advice, so confirm your position with your accountant before you commit.

How financing changes the cashflow, even when payback does not

The payback and IRR on the asset itself are a property of the system and your consumption, not of how you pay for it. What financing changes is the shape of your cashflow. Buy outright and you take the full cost up front, then keep every saving as the asset pays back on its own timetable. Fund it with hire purchase, asset finance or a business solar loan and you spread the cost over a term, so the deal can be cash-flow positive from year one whenever the monthly saving exceeds the repayment, even though the underlying asset payback is unchanged. A power purchase agreement removes the capital entirely, but the funder owns the system and takes the tax reliefs and export income, lowering your headline return.

The only reliable way to see this is to have the same system costed across every route on your own numbers. We are a comparison and quote service, not a lender or financial adviser. Start with a tailored quote and model the timing on the finance calculator.