Energy Units Cheat Sheet: kWh, MJ, BTU, Therms, toe and More

Energy comes in a confusing range of units because each industry grew up with its own. Electricity uses kilowatt-hours, physics uses joules, US heating uses BTU, gas bills use therms or cubic metres, national statistics use tonnes of oil equivalent and oil markets use barrels. This cheat sheet gathers the conversions you actually need and explains the two distinctions behind most mistakes: power versus energy, and gross versus net calorific value. It ends with worked examples for bills, cars and countries. For instant conversions between any of these units, use our energy unit converter.

Power versus energy: the one distinction that matters

Energy is a quantity, the ability to do work. Power is a rate, energy per unit of time. Mixing the two up is the most common error in energy writing.

• A kW tells you how hard something works: a 2 kW heater, a 400 W solar panel, a 7 kW EV charger, a 1 GW reactor.
• A kWh tells you how much was produced or used: 10 kWh per day, 3,500 kWh per year, 100 TWh per year for a mid-sized country.

People often write “kW per hour”. That is wrong: a kWh is not a rate per hour, it is kW multiplied by hours. Our guide to kW vs kWh covers this in more depth.

SI prefixes

Mind the capitals. mW is a milliwatt and MW a megawatt, a billion times larger.

Master conversion table: energy

Every factor is either exact by definition or rounded to four significant figures.

Power conversions

In North America, air conditioners are often sized in “tons” or BTU/h. A 12,000 BTU/h unit delivers about 3.5 kW of cooling, but with a coefficient of performance of 3 it draws only around 1.2 kW of electricity.

Fuel energy content

These are typical values; real calorific values depend on grade and origin. LHV = lower (net) heating value; HHV = higher (gross).

HHV versus LHV

Burning a fuel turns the hydrogen in it into water vapour. The HHV counts the latent heat released if that vapour condenses; the LHV leaves it out. Condensing boilers recover part of that latent heat, which is how they can claim “efficiencies” above 100% on an LHV basis. Check which basis a price, an efficiency figure or a fuel table uses. Mixing them produces errors of 5–11%.

Worked examples

1. A gas bill in cubic metres

A meter shows 1,200 m³ used in a year. The supplier’s calorific value is 39.5 MJ/m³ (gross), and a volume correction factor of 1.02 applies.

At €0.10/kWh that comes to about €1,343. To compare it with a heat pump, multiply by the boiler efficiency (say 0.90) to get roughly 12,090 kWh of useful heat, then divide by a seasonal COP of 3.2: about 3,780 kWh of electricity. The heating cost comparison calculator runs these numbers with your prices.

2. Petrol versus electricity per 100 km

A petrol car using 6.5 L/100 km burns 6.5 × 8.9 ≈ 58 kWh of fuel energy per 100 km. A typical EV draws 15–20 kWh/100 km from the socket, about a third as much. Electric drivetrains turn 80–90% of battery energy into motion; a petrol engine manages 20–35%.

3. A country’s electricity in perspective

A country generating 60 TWh per year has an average power output of 60,000,000 MWh ÷ 8,760 h ≈ 6.85 GW. A 1 GW nuclear plant running at a 90% capacity factor produces 1 × 8,760 × 0.9 ≈ 7.9 TWh per year, about 13% of that country’s supply. The capacity factor guide explains why that multiplier matters.

4. Converting to CO₂

Emissions depend on the fuel and on how it is used. Burning natural gas releases roughly 0.18–0.20 kg CO₂ per kWh of fuel (about 0.20 on an LHV basis, about 0.18 on HHV). Grid electricity ranges from under 0.05 kg/kWh (Norway, France, Sweden) to over 0.7 kg/kWh on coal-dominated grids. Get your country’s figure from the electricity CO₂ calculator and compare grids in the rankings.

Primary energy accounting: three methods

When statistics say “solar provides X% of primary energy”, the figure depends on the method behind it:

The physical method makes wind and solar look smaller than their contribution to final energy services; substitution makes them look larger. Neither is wrong. They answer different questions, so always state the method when you quote shares.

Quick reference: rules of thumb

• 1 kWh ≈ 3.6 MJ ≈ 3,412 BTU ≈ 860 kcal
• 1 m³ natural gas ≈ 10–11 kWh (gross)
• 1 litre diesel ≈ 10 kWh, 1 litre petrol ≈ 9 kWh
• 1 kg hydrogen ≈ 33 kWh (LHV), see the green hydrogen calculator
• 1 toe ≈ 11.6 MWh, 1 boe ≈ 1.7 MWh
• 1 GW running all year = 8.76 TWh
• 1 EJ ≈ 278 TWh
• 1 kW of cooling ≈ 3,412 BTU/h ≈ 0.28 tons of refrigeration

To see these units in context, look at national energy balances in the country profiles, or click any spot on the Energy Explorer to see its solar and wind resource in kWh and W/m².

Figures in this guide are typical ranges drawn from the sources above; your own numbers depend on location, prices and equipment. Use the linked calculators for site-specific estimates.