Carbon factor and Scope 2 tracking for commercial solar in 2026
The UK grid carbon emission factor has been falling steadily since 2014 — from 0.50 kgCO2e/kWh down to ~0.21 kgCO2e/kWh in 2026. This affects every commercial solar carbon-savings claim. Understanding the trajectory matters for TCFD disclosure, SBTi target-setting, and long-term project carbon ROI.
The DESNZ published grid carbon factor
The UK Department for Energy Security & Net Zero (DESNZ) publishes annual emission factors for UK electricity:
Recent trajectory (kgCO2e per kWh):
- 2010: 0.51 - 2014: 0.46 - 2018: 0.28 - 2020: 0.23 - 2022: 0.21 - 2024: 0.21 - 2026 (current): 0.21 - 2030 (projected): 0.15 - 2035 (projected): 0.09
The factor is published in the DESNZ Green Book of conversion factors annually, typically in June for the prior calendar year. UK GHG reporting standards (GHG Protocol, SECR, TCFD) require use of the current published factor.
The trajectory reflects the UK grid's actual decarbonisation: closure of coal plants, growth in offshore wind, deployment of nuclear and storage. The forward trajectory depends on government policy and market dynamics — not guaranteed but reasonably central.
How carbon factor affects solar carbon savings
Solar PV carbon savings = kWh self-consumed × current grid carbon factor.
A 100kWp commercial solar system saving 60,000 kWh self-consumption per year:
- 2018 carbon factor: 0.28 × 60,000 = 16.8 tCO2e/year saved - 2026 carbon factor: 0.21 × 60,000 = 12.6 tCO2e/year saved - 2030 projected: 0.15 × 60,000 = 9.0 tCO2e/year saved - 2035 projected: 0.09 × 60,000 = 5.4 tCO2e/year saved
The headline carbon impact of solar declines as the grid decarbonises. Lifetime carbon savings need year-by-year calculation:
- 25-year cumulative savings (constant 2026 factor): 315 tCO2e - 25-year cumulative (year-by-year falling factor): ~220 tCO2e
The 30% gap matters for TCFD scenario analysis and SBTi target setting — overstating future carbon savings creates strategic risk.
Year-by-year reporting methodology
For credible TCFD disclosure and SBTi target reporting, solar carbon savings must use:
1. Current published factor for current-year reporting 2. Year-by-year projected factor for forward lifetime projection 3. Methodological transparency documenting which factor was used 4. Third-party verification for SBTi-aligned claims
For commercial solar projects:
- Year 1-3 (current): Use 2026 published factor 0.21 kgCO2e/kWh - Year 4-10: Use DESNZ projected factor; sensitivity to ±20% - Year 11+: Use 2030+ projected factor; flag higher uncertainty - Lifetime cumulative: Sum of year-by-year, not constant × 25
Many installers' marketing materials use constant carbon factor — overstating lifetime savings by 30-50%. Sensible TCFD-disclosing companies present both views.
Self-consumption vs export carbon accounting
Self-consumed solar:
- Removes grid electricity from your Scope 2 calculation - Saving = kWh self-consumed × current grid factor - Year 1 of a 100kWp system: ~12.6 tCO2e under location-based methodology
Exported solar with REGO retained:
- The renewable claim stays with you - Saving = exported kWh × current grid factor (location-based) - Or 0 emissions claim for the exported kWh (market-based)
Exported solar with REGO sold:
- No Scope 2 reduction credit - You receive cash from REGO sale (£8-£20 per certificate) - Buyer takes the renewable claim
For TCFD-disclosing companies, the REGO retention question is material. The £/tCO2e of REGO retention versus alternative offset purchase typically favours retention.
Future-proofing solar carbon claims
Three practical strategies for credible long-term carbon claims:
1. Use year-by-year factors. Don't multiply constant factor × 25 years. Use DESNZ's published projected trajectory or commission third-party scenario modelling.
2. Transparency in disclosure. Show the factor used for each year. Explain the methodology. Reference DESNZ source.
3. Bound the uncertainty. Carbon factor projections beyond 2030 have material uncertainty. Present scenario ranges, not point estimates.
4. Decouple cost and carbon. Solar's financial value (lower energy bills) is more stable than its carbon value (which declines as the grid decarbonises). For TCFD, distinguish the two narratives.
5. Combine with non-electricity decarbonisation. Heat pump retrofit (Scope 1) and process changes (Scope 1/3) have more stable carbon trajectories than pure solar PV (Scope 2 only). Combined programmes age better.
For TCFD-disclosing companies, solar + heat pump combined typically delivers more stable 25-year carbon savings than solar alone — heat pump replaces gas (a high-carbon fuel that won't decarbonise materially) rather than grid electricity (which will).
FAQs on this topic
What is the current UK grid carbon factor?
0.21 kgCO2e/kWh in 2026, per DESNZ Green Book published June 2026.
Does the falling factor reduce solar value?
Carbon value declines over time as grid decarbonises. Financial value (lower bills, SEG income) is more stable. For TCFD, distinguish the two.
Should I use constant or year-by-year factor?
Year-by-year. Constant-factor calculations overstate lifetime savings by 30-50%.
Where do I find DESNZ projections?
Government's Green Book on gov.uk; updated annually. Future projections in Energy & Emissions Projections published periodically.
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