The formula
capacity = the greater of (total × diversity) and the largest single appliance
Where it comes from
Adding up the rating of everything plugged in gives an absurd figure, because it does not all run at once: nobody uses the oven, the hob, the washing machine, a hairdryer and the air conditioning in the same instant. So a diversity factor is applied, the fraction of the total that can realistically coincide, which in a home sits between 40 % and 60 %. But there is a floor you cannot go below: the largest single appliance has to fit on its own, because if it trips the breaker when it starts, the diversity factor has achieved nothing.
How to work it out by hand
- Add up the rating of every appliance
- Multiply by the diversity factor
- Compare that with the largest single appliance and keep the greater
- Step up to the next contractable capacity
What is worth knowing
The standing charge is paid every day of the year whether or not you use anything, so every step above what you need is fixed money going out. It is worth reviewing: plenty of people carry the capacity the previous tenant contracted or whatever the builder put in by default. The honest way to tell whether yours is oversized is to ask whether the main breaker has tripped at all in the last year; if it never has, you can probably drop a step. Two cautions: appliances with motors draw considerably more on starting than in normal running, and an induction hob is the single appliance that most often forces a home to go up a step.