What Realistic Commercial Solar Payback Looks Like in 2026

The four assumptions that decide your payback

Most commercial solar proposals you'll see in 2026 quote a payback period between 4 and 8 years. The honest answer is that the actual payback varies wildly with four underlying assumptions, and any installer who quotes you a single number without disclosing those assumptions is selling you the easier-to-sell version of the truth.

1. Self-consumption ratio

This is the percentage of generated kWh that you consume on-site rather than exporting back to the grid. For a typical UK business it varies from 35% (Saturday-and-Sunday office) to 92% (24-hour food processor). The single biggest lever on commercial solar ROI.

2. Import tariff assumed

Are you on a fixed 25p tariff for the next 3 years? Or a flex-tracking SVT that has averaged 31p across 2025? The same system pays back vastly differently depending on which one you assume — and most quotes use the lower number to make the maths look better.

3. Electricity price inflation

UK commercial electricity prices have averaged 9.2% annual inflation from 2010 to 2025. Conservative modelling uses 4%. Marketing modelling uses 7%. The choice shifts the year-10 cash position by hundreds of thousands on a £200k system.

4. Tax position

Limited companies paying 25% corporation tax recover up to 25% of capex via the Annual Investment Allowance (AIA). Limited companies in losses recover £0. Sole traders are different again. The headline number on the same install can vary 30% by tax position alone.

How to sense-check a quote

Ask any commercial solar installer for the four numbers above. If they cannot or will not give them, the payback figure they have quoted is fiction. We publish all four on every KMM proposal as standard.