If your emissions change sharply from one year to the next and you can't tell whether the change is real, the cause is rarely missing data. Most errors come from three things: boundaries, units and emission factors.
The Greenhouse Gas (GHG) Protocol gives carbon reporting a common framework, but it leaves many choices to you: which control approach to use, which emission factors to apply, how to estimate Scope 3 and when to recalculate a base year. When those choices aren't written down, they change each year with whoever prepares the data.
Across the thousands of data submissions made in KEY ESG each year, the same 12 errors come up again and again. This article walks through each one, how to avoid it, and where KEY ESG can help. There is a checklist at the end to run before you submit.
Please note: this article covers the most common errors. For a full introduction to carbon accounting, start with Getting Started with GHG Scope 1, 2 and 3.
Boundaries, units and emission factors
These three errors sit underneath every scope. Get them right first and you stop mistakes spreading through your whole footprint.
1. An undefined or inconsistent organisational boundary
What goes wrong: The GHG Protocol lets you set your boundary using operational control, financial control or equity share. Many companies never state which one they use, or use different approaches for different entities. Acquisitions and disposals during the year are also missed.
How to avoid it:
Choose one consolidation approach, document it and state it in your report.
Keep a list of every entity and site in scope.
Update that list whenever your group structure changes.
In KEY ESG: Make sure every site and entity is set up in the platform before data collection starts. See Defining Reporting Boundaries: Practical Guidance for Carbon Accounting.
2. Unit and conversion errors
What goes wrong: These are the errors most likely to make a figure wrong by an order of magnitude. Common examples:
Fuel entered in litres when the factor expects kWh.
Kilograms confused with tonnes.
Gross and net calorific values mixed (in the US, gross is called higher heating value, or HHV).
US units such as therms, MMBtu, US gallons and short tons mixed with metric units.
Refrigerant top-ups reported in kg of gas without applying the gas's global warming potential (GWP), which can be in the thousands.
How to avoid it:
Record the unit with every data point.
Convert in one controlled place, not in separate spreadsheets.
Check each factor matches the unit and calorific basis of your data.
If you operate in the US and Europe, pick one unit system and convert everything into it before applying factors.
Compare each site with the previous year and investigate any large swing.
In KEY ESG: Select the unit your data is in when you enter it, and the platform converts it for you. See KEY ESG Supported Units for the full list. For refrigerants, see Understanding R404A and R452A Refrigerants in Simple Terms.
3. Outdated or mismatched emission factors
What goes wrong: Factors from the wrong year or country are applied, such as UK factors for operations in Poland. Some factors cover CO2 only when the report needs CO2e, and GWP values from different IPCC assessment reports are mixed in one footprint.
How to avoid it:
Keep a factor register with the source, year, geography and GWP basis of every factor.
Update it once a year.
Use country-specific factors for operations outside your home market.
In KEY ESG: The platform applies emission factors for you, matched to the reporting year and the country of each site. See KEY ESG Emission factor methodology for our sources.
Scope 1 and Scope 2
Scope 1 covers direct emissions from sources you own or control. Scope 2 covers emissions from the electricity, heat and steam you buy. Both are usually well documented, but they still carry some of the most frequent errors.
4. Missing Scope 1 sources
What goes wrong: Gas and fleet fuel are reported, but refrigerant leakage, backup generators and company-owned cars are left out. Leased buildings and vehicles end up in the wrong scope.
How to avoid it:
Run a source inventory for every site: combustion, vehicles, refrigeration and air conditioning, and any process emissions.
Ask your facilities team for service records showing refrigerant top-ups.
Decide how leased assets are treated under your consolidation approach.
In KEY ESG: See Carbon 101: Scope 1 for a full list of sources to check, and Leased office space: which category should I categorise my emissions under? for leased assets.
5. Reporting only one Scope 2 method
What goes wrong: The GHG Protocol requires dual reporting: a location-based figure using average grid factors, and a market-based figure reflecting your electricity contracts. Many companies report only one. Others calculate the market-based figure using grid average factors, which understates emissions.
How to avoid it:
Calculate both figures every year.
For market-based, use this order: contractual instruments first, then supplier-specific factors, then residual mix factors.
Please note: the GHG Protocol is revising its Scope 2 Guidance, with proposals for stricter market-based rules such as hourly matching. Until the new guidance is published, follow the 2015 version.
In KEY ESG: The platform calculates both location-based and market-based Scope 2 for you. See Understanding the Difference: Market-Based and Location-Based Emissions and KEY ESG's electricity emission factor methodology.
6. Renewable electricity claims without valid evidence
What goes wrong: A company reports zero market-based Scope 2 because its supplier markets a green tariff, without holding the certificates behind it. Certificates are also claimed from the wrong country or period, or counted by more than one entity.
How to avoid it: Hold evidence for every renewable claim and check it matches the market, period and volume you claim:
UK: Renewable Energy Guarantees of Origin (REGOs)
EU: Guarantees of Origin (GOs)
Anywhere: the relevant power purchase agreement (PPA)
In KEY ESG: Upload your certificates or supplier evidence alongside your electricity data, so it is ready for review or assurance. See Supplier-based vs. location-based methods for calculating renewable energy consumption.
Scope 3
Scope 3 covers all other indirect emissions in your value chain, from purchased goods to business travel. It is the least precise part of a footprint, and for services, retail and other asset-light businesses it is usually the largest. That means errors here have the biggest effect on your total.
Please note: for energy-intensive businesses such as steel, cement or chemicals, Scope 1 usually dominates instead, so focus your checks there.
7. Incomplete screening of the 15 categories
What goes wrong: Companies report business travel and commuting because the data is easy to find, and stop there. Larger categories, such as purchased goods and services, are left out with no explanation. Readers can't tell whether a category was assessed or never looked at.
How to avoid it:
Screen all 15 Scope 3 categories at least once and record the result.
Explain every exclusion.
State which categories your Scope 3 total covers.
Focus measurement effort on the categories that make up most of your footprint.
In KEY ESG: See Carbon 101: Scope 3 for a breakdown of all 15 categories, and Configuring Carbon Metrics and Viewing the Results in Reports to switch on the categories you need.
8. Spend-based estimates that are never adjusted or improved
What goes wrong: Spend-based methods multiply what you spend by a factor per pound or euro. Errors arise when factors are old, the currency doesn't match, or inflation is ignored. A price rise can then look like an emissions increase, and a supplier discount like a reduction.
How to avoid it:
Match the currency and price year of your spend data to the factor.
Adjust for inflation where needed.
Over time, replace spend-based estimates for your largest suppliers with activity data or supplier-specific emissions.
In KEY ESG: See A Step-by-Step Guide on Reporting Purchased Goods & Services Emissions using the Spend-Based Method, then Moving from Spend-Based to Activity-Based Calculation Methods when you are ready to improve your data.
9. Double counting between categories
What goes wrong: The same emissions are counted twice. For example, capital equipment appears in both purchased goods and capital goods, or delivery costs in a supplier invoice are also counted under upstream transportation. Fuel for company-owned vehicles already in Scope 1 also turns up again in Scope 3.
How to avoid it:
Map each spend or activity line to one category only, using written rules.
Check your Scope 3 data against Scope 1 and 2 before you finalise the total.
In KEY ESG: See How to Account for Scope 3.4 Upstream Distribution Emissions and Carbon Accounting Primer: FAQs and Debugging the Trickiest Questions for common category overlaps.
Energy, water, waste and data process
These last three errors cover environmental data beyond carbon and how your whole dataset is managed. They are the ones most likely to come up at assurance stage.
10. Incomplete energy, water and waste data
What goes wrong:
Energy is reported for sites with their own meters, but not for shared offices where usage is in the service charge.
Water figures come from bills that combine supply and wastewater.
Waste is estimated from the number of bins collected rather than weighed.
Renewable energy percentages are calculated on electricity only, when the framework asks for total energy.
How to avoid it:
Use the same site list for energy, water and waste as for emissions.
Ask landlords and waste contractors for actual consumption and tonnage data.
Disclose where figures are estimated or allocated by floor area.
In KEY ESG: If you can't get actual data, use our estimation guides for energy, water and waste. For the difference between renewables, electricity and energy metrics, see Understanding the Difference: Renewables, Electricity, and Energy.
11. No base year or recalculation policy
What goes wrong: A company sets a reduction target against a base year but has no rule for when to restate it. After an acquisition, disposal or method change, emissions appear to rise or fall even though nothing changed in operations.
How to avoid it:
Write a recalculation policy that sets out which changes trigger a restatement.
Set a significance threshold. Many companies use 5%, the threshold the Science Based Targets initiative (SBTi) uses.
Restate the base year when the policy is triggered, and explain the change in your report.
In KEY ESG: Set up a Year-on-Year Variance validation rule on your Scope 1, 2 and 3 metrics using your significance threshold. If a rule fails, you will know to check whether a restatement is needed. See Data Validation rules.
12. Undisclosed estimates and no audit trail
What goes wrong: Missing months or sites are filled in by extrapolation, which is fine, but nobody records which figures are estimated or how. Totals can't be traced to invoices or meter readings, and don't match energy and fuel costs in the accounts. If the person who prepared the data leaves, nobody knows how the figures were calculated.
How to avoid it:
Flag every estimate and record the method used.
Link each figure to its source evidence.
Compare energy and fuel spend with the finance ledger before you submit.
Write down your calculation approach so it can be repeated next year.
In KEY ESG: Use the comments and evidence fields on each data point to record estimates and sources, so the next person can follow your workings.
Checklist: before you submit
Run through these checks before any environmental figures leave the business.
Boundaries, units and factors
The consolidation approach is stated and applied to every entity
The entity and site list reflects acquisitions and disposals during the year
Every data point has a unit, and conversions happen in one place
Refrigerant top-ups are converted using the gas's GWP
Every emission factor has a recorded source, year, geography and GWP basis
Scope 1 and 2
Scope 1 includes refrigerants, generators and company vehicles
Scope 2 is reported on both a location-based and a market-based basis
The market-based figure uses contractual instruments, supplier factors or residual mix factors
Every renewable electricity claim is backed by certificates or a PPA
Scope 3
All 15 categories are screened, included categories are stated and exclusions explained
Spend-based factors match the currency and price year of your spend data
No emissions are counted in more than one category or scope
Data and process
Energy, water and waste cover the same sites as emissions
A base year recalculation policy is written down and applied
Estimates are flagged, with the method recorded
Energy and fuel figures reconcile with the finance ledger
All failing data validation rules have been reviewed
How KEY ESG helps you catch errors
Carbon accounting is built into KEY ESG, so Scope 1, 2 and 3 are calculated consistently across every entity and site. On top of that, there are four ways the platform helps you stop errors reaching your report:
Guidance on every question. Each question explains what to include and how to provide the data.
Data validation rules. Fund managers and company admins can set rules that flag unusual data as it is entered. See Data Validation rules.
AI Root Cause Analysis. If a Year-on-Year Variance rule fails on a Scope 1, 2 or 3 metric, AI analysis suggests the likely reason, such as a unit or factor issue.
Support and training. Our team offers in-platform support throughout data collection, and contributors receive annual methodology training.
How to use AI Root Cause Analysis
Go to the Data Validation tab under the Data section.
Expand a failing Year-on-Year Variance rule for a GHG Scope 1, 2 or 3 metric.
Click Find root cause.
Wait a few moments while the analysis runs, then read the suggested cause.
Please note: AI functionality must be turned on first. Click your initials in the top right, select Company settings, then turn on AI Capabilities.
Frequently asked questions
What is the difference between Scope 1, 2 and 3?
Scope 1 is direct emissions from sources you own or control, such as boilers, vehicles and refrigeration. Scope 2 is emissions from the electricity, heat or steam you buy. Scope 3 is all other indirect emissions in your value chain, split into 15 categories. See Getting Started with GHG Scope 1, 2 and 3.
What is the difference between location-based and market-based Scope 2?
Location-based uses the average emissions of the grid where you use the electricity. Market-based reflects the electricity you have chosen to buy, using contracts and certificates. The GHG Protocol requires both.
Do I need to report Scope 3?
It depends on your framework. CSRD requires Scope 3 for companies in scope. Since the Omnibus I package came into force in March 2026, CSRD applies only to EU companies with more than 1,000 employees and over €450 million net turnover. Many investors, lenders and customers still ask smaller companies for Scope 3 data. See Do Startups Need to Report Scope 3 Emissions?
What is the most common error?
Unit and emission factor errors. They are the most frequent and have the biggest effect, as a single entry in the wrong unit can outweigh every other line in your footprint.
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Still have questions? Reach our team on support@keyesg.com.