A 2% vs CPI-Linked PPA Escalator: The 20-Year Numbers Worked Through
A fixed-rate quote and a CPI-linked quote can carry the same headline year-one price and a six-figure difference by year twenty. Here is the arithmetic, with every assumption shown.
Two providers quote you the same year-one rate for the same size system. One offers a fixed 2% annual escalator; the other offers CPI-linked. On paper they look identical on the number that gets circulated internally. Over twenty years they are not, and the gap is worth knowing before you sign rather than after.
The worked example
Take a 500 kWp on-site system generating 475,000 kWh a year, opening at 12.0 p/kWh — the mid-point of the 500 kWp band on our rates page. Year one costs £57,000 under either structure. From there they diverge:
| Escalator | Year-20 tariff | 20-year cumulative cost | Effective average rate |
|---|---|---|---|
| 0% (flat) | 12.00 p/kWh | £1,140,000 | 12.00 p/kWh |
| Fixed 2% | 17.48 p/kWh | £1,385,000 | 14.58 p/kWh |
| CPI-linked, modelled at 2.5% | 19.18 p/kWh | £1,456,000 | 15.33 p/kWh |
The fixed 2% escalator alone adds £245,000 over the flat comparator — a 21% increase on total spend from one clause. A CPI-linked deal modelled at 2.5% adds a further £71,000 on top of that. Full assumptions, the sensitivity across a 2–3% CPI range and what an RPI-linked deal does instead are set out on our solar PPA rates page.
Why CPI-linked is a range, not a number
Nobody can price a CPI-linked quote precisely in advance, because nobody knows future CPI. The Bank of England's own target is 2%; UK CPI actually ran at 2.9% in the twelve months to July 2026. Treat any CPI-linked quote as a range bounded by the target on one side and recent outturn on the other, not a single figure — and ask the provider to show you the year-20 number, not just the opening rate.
The one clause worth negotiating hardest
If a provider will not drop index-linkage entirely, negotiate a cap on the index rather than accepting it unbounded. And check whether the uplift is floored at 0% — a floor stops your tariff falling in a negative-inflation year, which protects the provider's return, not yours. Compare competing quotes on the effective average rate across the full term, not the headline year-one number: it is the only figure that makes a flat quote and an escalating quote genuinely comparable side by side.
Sources: Bank of England — Inflation and the 2% target; ONS CPI series D7G7, 12 months to July 2026, released 19 August 2026. Verified 17 September 2026.
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