Units, Scales & Conversions
Joules, watts, kWh, therms and tonnes of CO2 — the units you'll meet every day and how to move between them.
10 min read
Energy management runs on numbers, and those numbers come with units. Mix them up and you can be out by a factor of a thousand. This lesson gives you the handful of units you'll meet every day and how to move between them with confidence.
The base units
- Joule (J) — the scientific unit of energy. One joule is a small amount: roughly the energy to lift an apple one metre.
- Watt (W) — the unit of power, meaning energy per unit time. One watt is one joule per second. (We dig into the energy-vs-power distinction in the next lesson — it matters more than any other.)
- Kilowatt-hour (kWh) — the practical unit of energy on every bill. It is the energy used by a 1 kW appliance running for one hour.
Because a joule is so small, real-world figures use prefixes: kilo (k, thousand), mega (M, million), giga (G, billion) and tera (T, trillion).
The kilowatt-hour is your currency
Almost everything in energy management is easiest to express in kWh. Your electricity is billed in kWh. In the UK, gas is metered in cubic metres but converted to kWh on the bill, so you can compare fuels directly. Get comfortable thinking in kWh and most calculations fall into place.
The key conversion to memorise:
1 kWh = 3.6 MJ — because 1 kW (1000 J/s) running for 3600 seconds delivers 3,600,000 J.
A conversion table to keep handy
| Unit | In kWh | Notes |
|---|---|---|
| 1 kWh | 1 | The everyday unit |
| 1 MJ | 0.278 | 1 kWh = 3.6 MJ |
| 1 therm | 29.3 | Older gas unit (100,000 Btu) |
| 1 GJ | 278 | Common in district heating |
| 1 toe (tonne of oil equivalent) | 11,630 | Used for large energy totals |
Don't forget carbon
Increasingly you'll convert energy into carbon, expressed as kilograms or tonnes of CO₂-equivalent (CO₂e). You multiply energy used by an emission factor published each year by the UK government (DESNZ/DEFRA). As a rough guide, grid electricity and natural gas each sit somewhere around 0.2 kg CO₂e per kWh, but the exact figures change every year and electricity's is falling as the grid decarbonises.
The most common — and most expensive — mistakes come from prefixes and time. Is that figure kWh or MWh? Per day or per year? Before trusting any number, sanity- check its scale against something you already know.
A worked conversion
Suppose a process uses 500 therms of gas a week. How many kWh per year?
- 500 therms × 29.3 kWh = 14,650 kWh per week
- × 52 weeks ≈ 762,000 kWh per year (about 762 MWh)
- At ~0.18 kg CO₂e/kWh for gas, that's roughly 137 tonnes of CO₂e a year
Once everything is in kWh, energy, cost and carbon all line up — which is exactly what you need to build a business case.
Pick one unit — kWh — and convert everything into it before you start comparing or adding. It removes a whole class of errors.