Tool · Calculator
Solar PPA Savings Calculator
Last updated 15 October 2026 · Figures indicative
Short answer: Enter your region, roof area or system size, electricity use and rates to estimate how much a commercial solar PPA could save your business. The calculator shows system size, annual generation, year-one and lifetime savings and carbon avoided — all as indicative ranges, with the formulas explained below.
Indicative estimate
- System size
- 270 kWp
- Annual generation
- 255,150 kWh
- Demand met by solar
- 51%
- Carbon avoided
- 51.0 tCO₂e/yr
Year-1 saving
£17,146–£24,647
Lifetime saving over 20 years
£569,018
Indicative only — not a quote. Excludes export income, batteries and VAT. Range reflects ±15–20% uncertainty in yield and usage.
How the calculator works
Every figure is produced by five simple steps, so you can check the arithmetic yourself:
- System size: kWp ≈ usable roof m² × 0.18 (allowing for spacing, access routes and edges).
- Generation: kWh/yr = kWp × your region's specific yield (kWh per kWp).
- Self-consumed: the lower of generation × self-consumption % and your annual use.
- Year-1 saving: self-consumed kWh × (grid rate − PPA rate).
- Lifetime saving: each year's saving summed over the term, with 0.5%/yr panel degradation, your grid price rise assumption and the chosen PPA indexation.
Carbon avoided = generation × 0.2 kgCO₂e/kWh. The year-1 range runs from 80% to 115% of the central estimate to reflect real-world variation.
Assumptions
| Assumption | Default | Source / note |
|---|---|---|
| Roof to kWp | 0.18 kWp per m² | Typical usable density on commercial roofs |
| Grid rate | 26p/kWh | Typical UK business 1-year fixed unit rates (market averages ≈25–27p, Aug 2026); varies by size, region and contract. |
| PPA rate | 12p/kWh | Indicative UK on-site commercial rooftop PPA range; subject to site survey, term and credit. |
| Self-consumption | 60% | Edit for your operating hours |
| Degradation | 0.5%/yr | Typical monocrystalline module warranty assumptions. |
| Grid price rise | 2%/yr | Editable assumption, not a forecast |
| Carbon factor | 0.2 kgCO₂e/kWh | UK Government GHG conversion factor for grid electricity — confirm latest DESNZ year before launch. |
Regional solar yields
| Region | PVGIS location | kWh per kWp per year |
|---|---|---|
| South East England | Brighton | 1,160 |
| South West England | Exeter | 1,060 |
| East of England | Cambridge | 1,035 |
| Wales | Cardiff | 1,030 |
| London | London | 1,025 |
| Midlands | Birmingham | 970 |
| Yorkshire | Leeds | 945 |
| North East England | Newcastle | 945 |
| Northern Ireland | Belfast | 920 |
| North West England | Manchester | 885 |
| Scotland | Glasgow | 850 |
PVGIS v5.2 (European Commission JRC) for a representative city per region: south-facing, 35° tilt, 14% losses. Low-pitch or east–west roofs typically yield 10–15% less; your site's orientation and shading will change this.
Worked example
A 250kWp warehouse in Yorkshire-like conditions (900 kWh/kWp), using 80% of generation on site, paying 26p/kWh for grid power and offered a fixed 12p/kWh PPA over 20 years:
- Generation: 250 × 900 = 225,000 kWh/yr
- Self-consumed: 180,000 kWh
- Year-1 saving: 180,000 × 14p ≈ £25,200
- 20-year saving with 2%/yr grid rises: ≈ £669,040
- Carbon avoided: ≈ 45 tCO₂e/yr
Illustrative — your site will differ. Compare with buying in our PPA vs buying guide, and check current PPA rates and purchase costs.
Understand the numbers behind the calculator: our complete guide to solar power purchase agreements, typical UK solar PPA rates, installed costs by system size, PPA vs buying over 20 years and personalised eligibility guidance.
Calculator FAQs
How accurate is the solar PPA calculator?
It gives an indicative range, not a quote. It uses regional average yields and your inputs; real results depend on roof orientation, shading, your half-hourly load profile and the contract offered. Expect a site-specific proposal to differ — sometimes by 20% or more.
What self-consumption percentage should I use?
Sites running mainly in daylight hours on weekdays and weekends (warehouses, cold stores, factories) often use 70–90% of solar generation. Offices and schools, closed at weekends and in holidays, are often 50–70%. If unsure, use 60% and refine with half-hourly data.
Why does the calculator ignore export income?
Under most on-site PPAs, electricity you don't use is exported and the export income belongs to the provider. The saving to you comes only from solar you use on site, so that is what the calculator counts.
What PPA rate should I enter?
UK on-site commercial PPAs typically price at 10–14p/kWh in 2026, and large industrial systems at 8–12p/kWh. The default of 12p is a mid-range figure for a 100–250kWp rooftop system.
How is carbon saving calculated?
Total generation is multiplied by the UK grid electricity conversion factor (0.2 kgCO₂e/kWh). This is a location-based Scope 2 estimate; your sustainability team may use a different factor or method.
Does the calculator include batteries?
No. Batteries can raise self-consumption by storing surplus solar, but whether they pay back depends on your load profile and tariff. Ask for battery options to be modelled in a site-specific proposal.
Sources
- GOV.UK — Greenhouse gas reporting: conversion factors (DESNZ)
- legislation.gov.uk — The Non-Domestic Rating (Plant and Machinery) (Amendment) (England) Regulations 2022, SI 2022/405
- HMRC — Capital allowances: Annual Investment Allowance and first-year allowances
- Energy Networks Association — G98 and G99 connection requirements
- European Commission JRC — PVGIS photovoltaic yield tool
Price ranges for larger systems, running costs, batteries and PPA rates are indicative market ranges from UK installer, funder and market sources; no official series exists above 50kWp.
Turn the estimate into a real proposal
Describe your site for personalised eligibility guidance — the first step towards a site-specific PPA offer.
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