Kilowatt-Hour (kWh)
A kilowatt-hour is one of the few genuinely fixed quantities in carbon accounting. It is 3.6 megajoules, everywhere, always. What is not fixed is what a kilowatt-hour costs the atmosphere — and on the current UK factor set the same unit carries at least five different numbers.
A kWh of gas is not a kWh of electricity, and a kWh of grid electricity is not a kWh of grid electricity either. The unit is constant; the factor attached to it depends on the carrier and on which question you are answering.
A kilowatt-hour (kWh) is a unit of energy equal to 3.6 MJ — one kilowatt for one hour. It is the unit most activity data arrives in and most emission factors are quoted against. The trap: emissions per kWh differ by carrier and by basis, so grid electricity and gas are not interchangeable.
The Unit That Is Never Just a Unit
A kilowatt-hour is the energy delivered by one kilowatt of power over one hour: exactly 3.6 megajoules, by definition, since a watt is a joule per second. That much is unambiguous, and it is why the kWh is the common currency of energy data — meters read in it, bills are denominated in it, and most emission factors are published per unit of it.
What makes it worth a page of its own is the second half of that sentence. Because so many factors are quoted per kWh, the unit invites a comparison it cannot actually support: that a kilowatt-hour is a kilowatt-hour regardless of where it came from. It is not. The energy is identical; the emissions attached to it are a property of the carrier and the accounting basis, not of the unit.
Definition at a glance
| What it is | 3.6 MJ — one kilowatt sustained for one hour |
|---|---|
| Why it matters here | Most activity data arrives in kWh and most factors are quoted per kWh |
| Is a kWh a fixed carbon amount? | No. It varies by carrier and by basis — see below |
| Grid electricity (UK) | 0.13096 kg CO2e per kWh, location-based |
| Natural gas (UK) | 0.18231 gross basis — about 39% more |
| Bigger unit | 1 MWh = 1,000 kWh; 1 GWh = 1,000 MWh |
A kWh Is Not a kWh
The same quantity of delivered energy, on the current UK factor set, priced five ways:
| One kWh of… | Basis | kg per kWh |
|---|---|---|
| Grid electricity | Location-based | 0.13096 |
| Natural gas | Gross (GCV) | 0.18231 |
| Natural gas | Net (NCV) | 0.20199 |
| LPG | Gross (GCV) | 0.2145 |
| Grid electricity | Lifecycle (incl. upstream) | 0.21741 |
| Grid electricity | Residual mix | 0.28064 |
Two things in that table are worth stopping on.
Gas is now the dirtier kilowatt-hour. At 0.18231 [GreenCalculus fuels.gbr.natural_gas.kwh_gcv · DEFRA 2026 'Fuels'!D42 · v2026.272] against grid electricity’s 0.13096 [GreenCalculus grid.gbr.electricity.location_based · DEFRA 2026 'UK electricity'!E25], a unit of energy from gas carries roughly 39% more than the same unit from the UK grid. That inverts the intuition a great many energy models still encode — electricity was the dirtier carrier for most of the period those models were written, and the grid has since decarbonised past gas. If a heating comparison in your organisation still assumes otherwise, it is using an old factor rather than an old fuel.
The grid kWh alone spans a factor of 2.1. The same electricity is 0.13096 location-based [GreenCalculus grid.gbr.electricity.location_based], 0.21741 [GreenCalculus grid.gbr.electricity.lifecycle_intensity · EMBER 2025] on a lifecycle basis that includes upstream fuel supply, and 0.28064 [GreenCalculus grid.gbr.electricity.residual_mix · AIB 2025 'CO2'!C15 (GB)] as a residual mix. None of those is wrong. They answer different questions — what the grid averaged, what the whole chain cost, and what is left once other buyers have claimed the renewable output — and quoting one while meaning another is the most common way a Scope 2 figure goes astray.
Quick converters: scale or restate a kWh with kWh to MWh, kWh to MJ and kWh to GJ, or take one straight to emissions with kWh to CO₂ by country. Restate a per-kWh factor itself with CO₂ per kWh to per MWh or kg CO₂/kWh to lb/MWh.
Which kWh Is on Your Bill
Before a kWh can be multiplied by anything, two questions have to be settled about the number itself. Both are covered in depth elsewhere; what matters here is knowing that the kWh on your invoice has already answered them, usually without saying so.
| Question | What it decides | Where it is settled |
|---|---|---|
| Gross or net? | Whether the kWh counts the heat in the water vapour combustion produces — about 10% for gas, and the reason gas has two per-kWh factors above | Calorific value |
| Delivered or primary? | Whether the kWh is what arrived at your meter or what entered the energy system to get it there | Primary energy |
The practical rule for an inventory: use delivered energy on a basis that matches the factor. UK gas is billed on a gross basis, so a UK gas bill pairs with a gross-basis factor — 0.18231 [GreenCalculus fuels.gbr.natural_gas.kwh_gcv] rather than the net 0.20199 [GreenCalculus fuels.gbr.natural_gas.kwh_ncv · DEFRA 2026 'Fuels'!D41]. Crossing them is roughly an 11% error in the direction that flatters nobody, because the net figure is the larger one.
Which Electricity kWh
For electricity the carrier question splits again, and the answer depends on what you are reporting rather than on what the meter recorded. A single metered kWh can legitimately be reported at more than one number in the same inventory — that is what Scope 2 dual reporting is.
| Use this | When you are answering | UK value |
|---|---|---|
| Location-based | What did the grid I drew from average? | 0.13096 |
| Market-based | What did I contract for? | Per your instruments |
| Residual mix | Market-based, with no instruments to claim | 0.28064 |
| Lifecycle intensity | What did the whole chain cost, generation included? | 0.21741 |
| Well-to-tank only | The upstream share, reported separately in Scope 3 | 0.03682 |
The electricity emission factor page covers how each is built and how to choose between them across 30 countries. The point to carry away here is narrower: the kWh did not change, and the number did.
Common Confusions
- Treating a kWh as a fixed quantity of carbon. It is a fixed quantity of energy. The emissions depend on the carrier and the basis — six numbers for one unit on the table above.
- Assuming electricity is the dirtier carrier. On the current UK set a kWh of gas is about 39% higher than a kWh of grid electricity. The grid decarbonised; a lot of internal comparison models did not.
- Quoting a location-based figure while meaning market-based. Or lifecycle while meaning operational. All are defensible; naming one and using another is not.
- Pairing a gross kWh with a net factor. UK gas is billed gross. Crossing the bases is an ~11% error, and the invoice rarely states which it is.
- Adding kWh across carriers to get an energy total, then applying one factor. The energy total is valid; a single factor over it is not, because the carriers differ by up to 70%.
- Confusing kW with kWh. A kilowatt is a rate, a kilowatt-hour is a quantity. Only the second can be multiplied by an emission factor.
Related Terms, Standards, and Tools
Frequently Asked Questions
The energy delivered by one kilowatt of power over one hour — exactly 3.6 megajoules, because a watt is a joule per second. It is the unit most meters read in and most emission factors are quoted against, which is what makes it the pivot of an energy inventory.
There is no single answer, and that is the point. On the UK set: grid electricity is 0.13096 kg CO2e per kWh location-based [GreenCalculus grid.gbr.electricity.location_based], 0.21741 [GreenCalculus grid.gbr.electricity.lifecycle_intensity] on a lifecycle basis and 0.28064 [GreenCalculus grid.gbr.electricity.residual_mix] as a residual mix; natural gas is 0.18231 gross [GreenCalculus fuels.gbr.natural_gas.kwh_gcv] or 0.20199 net [GreenCalculus fuels.gbr.natural_gas.kwh_ncv]. The unit is fixed; the factor is not.
The same energy, not the same emissions. On the current UK factors a kWh of natural gas carries 0.18231 [GreenCalculus fuels.gbr.natural_gas.kwh_gcv] against grid electricity’s 0.13096 [GreenCalculus grid.gbr.electricity.location_based] — gas is about 39% higher per unit of energy. That is the reverse of the position most comparison models were built on, because the UK grid decarbonised past gas.
A kilowatt is a rate — how fast energy is being used. A kilowatt-hour is a quantity — how much was used. A 2 kW heater running for three hours consumes 6 kWh. Only the quantity can be multiplied by an emission factor; a rate cannot.
Whichever your factor is on — and UK gas is billed gross, so a UK bill pairs with the gross-basis factor, 0.18231 [GreenCalculus fuels.gbr.natural_gas.kwh_gcv] rather than the net 0.20199 [GreenCalculus fuels.gbr.natural_gas.kwh_ncv]. Crossing them is roughly an 11% error. Calorific value explains why the two exist.
For an energy total, yes — a kWh is a kWh in energy terms. For emissions, no: apply each carrier’s own factor and add the results. A single blended factor over a mixed energy total is wrong by however far the carriers differ, which on the UK set is up to about 70% between grid electricity and butane.
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