From utility bill to audit trail: evidence rules for carbon numbers

The question always arrives after the number

Nobody asks for evidence before you quote a figure. The sequence is always the same: you report 41,004 kgCO₂e of Scope 2 emissions for the year, someone downstream — a customer's procurement team, a bank's ESG questionnaire, an assurance provider, a board member who reads carefully — asks "how do you know that," and only then does the evidence question land. If the answer is a spreadsheet cell with no trail behind it, the number does not survive the conversation, however correct it actually was.

This is the gap a provenance-first approach to carbon accounting is built to close: not better arithmetic, but a number that already carries its own proof before anyone asks for it.

What actually counts as evidence

Auditors, assurance providers, and sharp-eyed customers converge on the same hierarchy, and it is worth internalising because it is not what most first-time reporters assume:

1. Primary documents — the utility bill, the fuel receipt, the fleet-card statement, the freight invoice, the aircon servicing invoice showing a refrigerant top-up. These are generated by a third party (a utility, a supplier, a fuel station) at the time the activity happened, which is exactly what makes them credible: nobody can quietly edit history in a document someone else issued and dated.
2. Meter photos and readings — a timestamped photo of a meter, or a sub-meter log, when no formal bill exists (common for tenanted spaces split off a landlord's master meter).
3. Your own spreadsheet or calculation — useful, necessary even, but it is working, not evidence. A spreadsheet is something you typed. It proves you can add; it does not prove the underlying activity happened.

The order matters because it is also the order of what gets asked for first when something looks off. A reviewer who doubts a number does not ask for a better-formatted spreadsheet — they ask to see the bill.

Retention discipline: three simple rules

• Keep the primary document, not just the total. If your electricity spreadsheet says "38,000 kWh for the year," keep the twelve bills that sum to that number, not just the annual figure. A total with no underlying bills is a claim; a total with twelve bills attached is a reconciliation.
• File by period, not by project. Bills and invoices belong in a structure organized by month and year, so that "show me Q3" is a folder lookup, not an afternoon of searching email.
• Match your carbon evidence retention to your accounting retention. Most organizations already keep financial records for a fixed number of years to satisfy tax or audit requirements. There is no single carbon-specific retention law that overrides this; the practical answer is to keep carbon evidence at least as long as you keep the financial records it was derived from, since the two are the same underlying documents viewed through different lenses.

Factor citation practice: the four elements

A number without a cited factor is unreviewable. A defensible factor citation states four things, every time:

• Value — the number itself, to full precision (0.4020, not "about 0.4").
• Source — who published it (Energy Market Authority, UK DESNZ/DEFRA, US EPA, Ember).
• Data year — which year's activity the factor was calculated from, which is not always the year it was published in. Singapore's factor for the 2024 data year appeared in the 2025 edition of Singapore Energy Statistics — a one-year lag that catches people who assume "2025 edition" means "2025 data."
• Licence — the terms the data was released under (Singapore Open Data Licence, Open Government Licence v3.0, CC-BY, public domain). This matters for anyone reusing your report's numbers downstream, and it is also the detail that separates a tool that can show its sources from one that cannot.

Every calculated result on tc02e.com states these four elements next to the number, not buried in a methodology appendix — see the Singapore grid emission factor guide for a full worked example of what a proper factor citation looks like in practice.

A worked comparison: the same electricity number, two ways

Take one real fact — an SME used 102,000 kWh in FY2024 — and show it two ways.

Indefensible version:

"Scope 2 emissions: approximately 41 tonnes CO2e."

Nothing here can be checked. No factor is named, no source, no data year, no underlying bills. If challenged, the preparer has nothing to point to except "I calculated it," which satisfies nobody.

Defensible version:

"Scope 2 emissions, FY2024: 102,000 kWh × 0.4020 kgCO₂e/kWh (Energy Market Authority, Singapore Energy Statistics 2025, 2024 data year, Singapore Open Data Licence) = 41,004 kgCO₂e ≈ 41.0 tCO₂e. Twelve monthly electricity bills for the reporting entity are held on file and available on request."

Same activity, same result, radically different survivability. The second version answers the evidence question before it is asked — it names the factor, the source, the data year, the licence, and points to where the underlying bills live. Nobody has to trust the number; they can check it.

The honest gap: free-tier deletion vs your own evidence copies

The free calculator and upload tool on tc02e.com are built for fast, anonymous, no-signup calculation: three documents a day without an email address, ten a day with one, and — deliberately — uploaded files are auto-deleted after 30 days. That deletion window is a privacy and storage-cost decision, not an evidence-retention feature, and it is worth being direct about what that means in practice.

If you calculate a number here and need to defend it in six months or a year — for an annual report, a customer questionnaire, or an audit — the 30-day window will already have closed on the uploaded document. The tool gives you the calculated result and its factor citation to keep; it does not hold your bills for you long-term. Practically, that means:

• Download your results and save the source documents yourself before the 30-day window closes, filed the way described above.
• Treat the calculator as the arithmetic engine, not the archive. The provenance line it prints — factor, source, data year, licence — is exactly the citation you need to paste into your own report; the document custody is still your job.
• Teams users drafting a full IFRS S2, GRI 305, or SGX report get persistent storage as part of that workflow — which is the difference between a one-off calculation and a maintained evidence trail across reporting cycles.

Being upfront about this trade-off is itself part of the evidence discipline: a provenance-first product should be honest about the limits of its own retention, not just the strength of its own factors.

What good evidence practice doesn't require

• It does not require a paid audit tool. A well-organized folder of bills, invoices, and a spreadsheet with proper factor citations satisfies most first-round questionnaires and internal reviews.
• It does not require re-deriving every supplier's number. For Scope 3 data your suppliers provide, you can evidence the document they gave you; you are not expected to audit their internal calculation unless a formal assurance engagement specifically asks for it.
• It is not the same as third-party assurance. A well-evidenced, properly cited number is a strong estimate with a clear trail behind it — that is the standard this guide describes. Formal assurance (limited or reasonable) is a separate, more expensive engagement performed by an accredited third party, and most SME reporting does not require it yet.

Where this fits with the rest of your reporting

Evidence discipline is not a separate task bolted onto carbon accounting — it is the same document trail that produces the number in the first place. Start from the scope method in Scope 1, 2 and 3, explained in 10 minutes, calculate with a business electricity calculator or organization footprint calculator that shows its factor citation automatically, and keep the bills behind every field. If your numbers eventually need to sit in a formal filing, SGX-listed issuers and their supply chains face specific deadlines where this trail stops being optional.

When someone actually challenges your number

The test of an audit trail is the day a customer's sustainability team, a grant assessor, or an auditor replies with "can you show us how you got this?" The response that ends the conversation quickly has three parts: the primary document (the bill or invoice, not a spreadsheet made from it), the factor citation (value, source, data year, licence), and the arithmetic — quantity times factor, shown, with any allocation stated. Practise the reply before anyone asks: pick one line of your footprint, and see whether you can assemble those three parts in under ten minutes. If you can't, the gap is almost always the primary document — which is exactly the thing you cannot reconstruct after the fact. That ten-minute drill, run quarterly, is cheaper than any assurance engagement and catches the rot while the paper trail is still recoverable.

Sources

• GHG Protocol — Corporate Standard, Chapter 10 (verification and assurance considerations): ghgprotocol.org
• EMA — Singapore Energy Statistics (Singapore Open Data Licence): ema.gov.sg
• UK Government — greenhouse gas reporting: conversion factors collection (OGL v3.0): gov.uk
• SGX RegCo — sustainability reporting requirements: sgx.com/regulation