Energy and computing units: a reader glossary

Use this glossary when you read claims about energy, power or computing. Each definition and prefix value cites a source we opened. Units we have not yet checked against a source are listed under Not covered.

Units

Watt (W). The SI derived unit for power, radiant flux. The NIST table of SI derived units gives its expression as J/s (NIST SI Chapter 4).

Joule (J). The SI derived unit for energy, work and amount of heat. NIST gives its expression as N · m, or m²·kg·s⁻² in SI base units (NIST SI Chapter 4).

Megajoule (MJ). One million joules. NIST lists the prefix mega with the symbol M and the value 10⁶ (NIST SI prefixes), so 1 MJ is 10⁶ J.

SI prefixes

Values as listed in the NIST table of metric SI prefixes (NIST SI prefixes). The NIST table lists 24 prefixes; the eight used in this glossary are shown.

Prefix	Symbol	Power of ten
kilo	k	10³
mega	M	10⁶
giga	G	10⁹
tera	T	10¹²
peta	P	10¹⁵
milli	m	10⁻³
micro	μ	10⁻⁶
nano	n	10⁻⁹

Not covered

We have not yet checked these units against a source, so they are left out: watt-hour (Wh), kilowatt-hour (kWh), TOPS, TOPS per watt, FLOPS, bit and byte. Do not rely on this glossary for them until they are added with a cited source.

How to use this in a claim

1. Check the prefix letter. NIST states that submultiple prefixes are written with lowercase symbols and that multiple prefixes use uppercase symbols, with three exceptions: kilo (k), hecto (h) and deka (da). It lists k for kilo, M for mega and m for milli (NIST SI prefixes). A capital M and a lowercase m differ by a factor of 10⁹, since 10⁶ divided by 10⁻³ is 10⁹.
2. Check whether the figure is power or energy. NIST gives the watt as J/s, so a figure in watts is a rate (NIST SI Chapter 4). To turn a rate into a total energy, you also need to know the time period over which it applies.
3. Convert before comparing. NIST gives the example that one kilometre, 1 km, is equal to one thousand metres, 1000 m or 10³ m (NIST SI Chapter 4). Put both figures in the same unit first, for example 2 MJ as 2,000,000 J, using the prefix values above (NIST SI prefixes).
4. Check the prefix name in the source. If a claim gives a prefix name without its symbol, convert it with the table above and confirm the result is the size the claim implies (NIST SI prefixes).