How to calculate CO₂ emissions from your utility bill: a step-by-step example
From electricity and gas bills to tonnes of CO₂e: formula, emission factors and a complete numerical example for Scope 1 and 2.
To calculate CO₂ emissions from a utility bill, multiply the consumption shown on the invoice, in kWh for electricity or Smc for gas, by a recognised emission factor. Gas burned on site generates Scope 1 emissions, purchased electricity generates Scope 2 emissions. Here you will find the formula, where to read the data on your bill and a complete example.
The basic formula
The calculation follows the GHG Protocol method and is always the same:
Emissions (kgCO₂e) = activity data × emission factor
The activity data is the consumption, read from the bill. The emission factor tells you how many kg of CO₂ equivalent correspond to one unit of that consumption. The result is divided by 1,000 to get tonnes, the unit used in reports: tCO₂e.
Where to find the data on your bill
You don’t need the total in euros: you need the quantities. On an Italian bill you will find them in the consumption details, usually on the pages after the first.
- Electricity: the kWh consumed in the period, often split into time bands F1, F2 and F3. For the calculation, add them together.
- Natural gas: the Smc (standard cubic metres) consumed. If the invoice shows the conversion coefficient C, the volumes are already adjusted to standard conditions.
- Billing period: the start and end dates of consumption, which do not always match the month the invoice was issued.
- Actual or estimated readings: if consumption is estimated, the next adjustment bill will correct it.
Watch out for adjustments: an invoice can recalculate consumption from previous months. To avoid counting the same kWh twice, consumption must be assigned to the period it refers to, not to the invoice date.
Which emission factor to use
For electricity, the GHG Protocol requires Scope 2 to be calculated in two ways, explained in the guide on market-based and location-based:
- location-based: uses the average factor of the national grid. In Italy the reference source is the series published by ISPRA on emissions from the electricity sector;
- market-based: reflects purchase contracts. Energy covered by guarantees of origin has a factor of zero; energy without guarantees uses the residual mix factor published by AIB (Association of Issuing Bodies).
For natural gas, use national combustion factors or those from recognised databases, expressed in kgCO₂e per Smc. The rule is always the same: use the factor for the reporting year, cite the source and keep the same choice over the years. Learn more in the guide to emission factors.
Complete example: an SME with one site
Take a company with one plant that consumed 180,000 kWh of electricity and 25,000 Smc of gas for heating and a furnace during the year. Part of the electricity, 60,000 kWh, is covered by guarantees of origin. In the example we use rounded, purely illustrative factors: in your calculation, replace them with the official ones for the reference year.
| Item | Consumption | Illustrative factor | Emissions |
|---|---|---|---|
| Natural gas (Scope 1) | 25,000 Smc | 2.0 kgCO₂e/Smc | 50.0 tCO₂e |
| Electricity, location-based (Scope 2) | 180,000 kWh | 0.25 kgCO₂e/kWh | 45.0 tCO₂e |
| Electricity with guarantees of origin, market-based | 60,000 kWh | 0 kgCO₂e/kWh | 0 tCO₂e |
| Electricity without guarantees, market-based | 120,000 kWh | 0.45 kgCO₂e/kWh | 54.0 tCO₂e |
Here is how to read the result:
- Scope 1: 25,000 × 2.0 = 50,000 kg, i.e. 50.0 tCO₂e.
- Scope 2 location-based: 180,000 × 0.25 = 45,000 kg, i.e. 45.0 tCO₂e.
- Scope 2 market-based: 60,000 × 0 + 120,000 × 0.45 = 54,000 kg, i.e. 54.0 tCO₂e.
The market-based figure can be higher than the location-based one even if part of the energy is renewable: the residual mix excludes green energy already “sold” with guarantees of origin, so fossil sources weigh more heavily than in the grid average. This is normal, and the report must show both values.
The most common mistakes
- Using the euro amount instead of consumption. Spend-based estimates are the last resort, not the starting point when you have the bill.
- Counting adjustments twice. Each kWh must be assigned only once to the period it refers to.
- Mixing different years. The emission factor must refer to the same year as the consumption, or to the latest available one, stating so.
- Forgetting smaller sites. Warehouses, offices and shops have their own utility accounts that often end up outside the boundary.
- Reporting only market-based. The GHG Protocol requires both methods for Scope 2.
From one bill to a hundred
Calculating one bill takes five minutes. The problem is doing it for twelve months, multiple utility accounts, multiple sites and multiple energy carriers, while keeping track of adjustments and updated factors. This is where the spreadsheet starts losing pieces.
Envify starts from bills as they are: the AI reads consumption, billing periods and POD or PDR codes, applies the emission factors for the correct year and calculates Scope 1, 2 and 3, keeping every number linked to the invoice it was extracted from. If a bank asks where a value comes from, you open the indicator and show the document.
