How to Calculate CO₂ Emissions from Electricity

Emissions (kg CO₂e) = Electricity used (kWh) × Emission factor (kg CO₂e/kWh)

Take kWh from your bill or meter. For one device, kWh = watts × hours ÷ 1,000. The result inherits everything about the factor: its gas basis (CO₂ or CO₂e), its geography, its year and its scope. State those next to the result.

• g vs kg — 1 kg/kWh = 1,000 g/kWh. Mixing them gives an answer 1,000× too large or too small.
• kWh vs MWh — 1 MWh = 1,000 kWh, so a factor in t/MWh has the same number as kg/kWh.
• lb/MWh — US data is often in pounds per MWh: kg/kWh = lb/MWh × 0.45359237 ÷ 1,000 (1 lb = 0.45359237 kg exactly).
• CO₂ vs CO₂e — CO₂e also includes methane and nitrous oxide, weighted by their warming effect. Do not mix CO₂-only and CO₂e factors in one comparison.

The consumption (4,000 kWh a year) is illustrative; the factors are published values with their scope stated.

A grid's factor reflects its generation mix — how much comes from coal, gas, nuclear, hydro, wind and solar — and imports. It differs between countries, between regions within a country, and from year to year as the mix changes. Do not use one global factor for a local calculation. Prefer the official source for your location and note the year:

• UK — Greenhouse gas reporting: conversion factors (Department for Energy Security and Net Zero), published annually.
• United States — the EPA's eGRID, with national, state and subregion emission rates.
• Elsewhere — national energy agencies, grid operators or environment agencies.

Use the factor year that matches the year of your electricity use where possible.

Published factor sets can split electricity emissions into parts:

• Generation — emissions at power stations for the electricity generated.
• Transmission and distribution losses — the extra generation needed to cover energy lost on the grid (shown separately in the UK set).
• Upstream (well-to-tank) — extracting, processing and transporting fuels before they reach the power station.

Which parts you include depends on your purpose. Pick them deliberately and keep the same choice when comparing years or options.

The GHG Protocol Scope 2 Guidance (2015) defines two methods:

• Location-based — the average emissions of the grid where the electricity is used.
• Market-based — emissions reflecting the contracts behind your supply, such as a supplier-specific factor or renewable energy certificates.

Companies in markets with contractual instruments are required to report both, labelled separately. A renewable tariff can lower your market-based figure without changing the electricity physically delivered through your local grid, so the location-based figure stays the same. Market-based claims are only as reliable as the instruments behind them; ask your supplier how its factor is calculated. Neither method is "the true number" — say which one you used.

On many grids carbon intensity changes through the day and across seasons — for example with solar output around midday or with wind conditions. Grids dominated by hydro or nuclear tend to vary less.

• Annual average — what national factor sets publish and what annual reporting normally uses. With an annual factor, shifting a load to another hour does not change the reported figure.
• Hourly intensity — services such as Electricity Maps and, for Great Britain, the Carbon Intensity API from the National Energy System Operator publish current and forecast intensity. Useful for timing flexible loads such as EV charging; check whether a service's figures are lifecycle or direct before combining them with other factors.
• Marginal emissions — the change in emissions caused by adding or removing a load. It can differ substantially from the average and is harder to estimate. If you use a marginal figure, state its source and method.

Grid factors above describe emissions from operating the grid. Lifecycle estimates also include building and decommissioning plants and supply chains. They answer a different question — how generation technologies compare over their whole life — and are not the emission factor of any country's grid:

Whether using electricity instead of a fuel lowers emissions depends on the grid factor, the efficiency of each option and the scope of both factors. Compare like with like: the same factor set, year and scope for both.

The Electricity CO₂ Calculator multiplies your kWh by a factor. Its presets are the UK 2025 and US eGRID2023 factors above, with their scope shown; for anywhere else, enter the official factor for your location and year. The methodology page documents the sources.

Related questions with their own guides:

• Your whole household footprint — the carbon footprint guide combines electricity with heating, transport and other categories.
• Travel — the transport carbon footprint guide.
• Food — the food carbon footprint guide.
• A reduction plan — the carbon reduction guide.

The Carbon Footprint hub collects all carbon tools and guides.