Electrical Units and Terminology

There’s a lot of confusion about electrical units and terminology, which is unhelpful. I’ll try to tidy a few things up a bit. Some of the people for whom this is intended probably have very little prior knowledge of science and engineering, so please excuse me if it’s a bit over the top for those with more.

Energy and Power

When we talk about “stuff” and “rates of production or consumption of stuff” we usually have a simpler word for the stuff (say apples) and a compound expression for the rate of consumption or production (apples per year, or apples per year per person).

Energy is an intangible sort of stuff, but it’s still stuff – and power is the rate of consumption (or production) of that stuff. Confusingly, we have the simpler unit for rate of consumption of energy (kilowatts, kW), and a compound unit for energy (kilowatt hours, kWh). 1 kWh is the amount of energy you consume (or produce) in an hour, at a power level of 1kW – or in half an hour at a power level of 2kW, etc.

Because we’re so used to talking about consumption as so much stuff per unit time, we then tend to make things unnecessarily complicated, and talk about kilowatt hours per day (kWh/d) and similar things. A kWh/d is 1/24 of a kW – but it does make it a bit clearer that we probably aren’t using 1/24 of a kW all day, but averaging that consumption while our actual rate of consumption varies wildly as we turn things on and off or up and down (or they turn themselves on and off with thermostats and so forth). The total of all the bits of energy we use during the day adds up to that 1kWh of energy in the 24 hours.

Here are the units you’re likely to come across:

W Watt

kW Kilowatt 1,000 Watts
MW Megawatt 1,000 kW
GW Gigawatt 1,000,000 kW
TW Terawatt 1,000,000,000 kW
kWh Kilowatt hour (Energy)
kWh/d Kilowatt hours/day 42W
(Average Power)
TWh/y Terawatt hours/year 114MW

Typical values might be:

Peak Average Capacity Factor Load Factor
Domestic LED light bulb 10W 0.040kWh/d 17%
Electric kettle 3kW 1kWh/d 1.4%
Total household consumption 20kW 12kWh/d 2.5%
Large wind turbine 2.5MW 7GWh/year 32%
Nuclear power station 2GW 14TWh/y 80%
Total UK consumption 60GW 330TWh/y 63%

All the numbers in this table would vary from case to case, depending for example how much a particular light bulb or kettle was used – some light bulbs are left on permanently (load factor 100%), others used very rarely (1% or less).

The last two columns are the ratio of the average to the peak. Distinguishing between capacity factor (generators) and load factor (consumers) is useful, because their implications for the grid are different. However, you need a lot more detail about the pattern, in both space and time, of demand and of availability of generating capacity, to understand what is required of the grid. More about that in Not Melting the Grid.

(This page will probably grow. Do ask if you’d like me to explain any other units or terms – or if anything I’ve “explained” is really as clear as mud.)