The general public’s level of knowledge in the area of energy efficiency and energy saving has undoubtedly improved over the last few years. The number of government schemes intended to convince everyone to use less energy and the rise in electricity prices (domestically & within business/industry) have led to people taking a little more interest in their energy use.
Despite this, it is still not unusual to encounter confusion around the main unit of electrical energy, the Watt (W).
On TV, radio and various places around the web, people still discuss how much energy is consumed each day/month/year by a particular piece of equipment, building or site, incorrectly citing the number of kilowatts (kW) used.
Hence the below attempt to simply and succinctly explain the difference and correct usage without straying too far into the complex realms of three-phase power.
For the sake of ease & familiarity all examples here will use active/real power with kilowatt (kW) & kilowatt hour (kWh) units.
The same arguments will of course hold for apparent (kVAh & kVA) & reactive (kVArh & kVAr) power.
Firstly the ‘k’
The ‘k’ in ‘kW’ or ‘kWh’ simply donates a thousand units as it does in kilogram (kg) or kilometre (km).
1 kilowatt (kW) = 1000 Watts (W).
1 kilowatt hour (kWh) = 1000 Watt hours (Wh).
kW or kWh?
Kilowatt hours (kWh):
The kilowatt hour (kWh) is a unit of energy consumption. The ‘amount’ of energy used in a given time period is measured in kWh.
It is important to note that this is not ‘kilowatts per hour’ or kW/h.
This is probably in part where the confusion lies. In general, we are far more used to seeing units with time components occurring after a ‘per’ or ‘/’. For example in ‘miles per hour’ or ‘kilometers per hour’ (which are confusingly often written in slightly different formats, mph and km/h).
Further confusion may come from the idea of an ‘amount’ of electrical energy, which is a bit abstract.
Unlike a liquid or gas, you can not fill a vessel with electricity, so talking about an ‘amount’ of it can seem a bit strange.
So, the kilowatt hour (kWh) is to electrical energy what the litre (l) is to water.
The kilowatt (kW) is a unit of electrical load or demand, i.e. it is a rate of use of electrical energy.
kW = kWh/h
(kilowatts = kilowatt hours per hour)
1 kW = 1 kWh used per hour.
As I once heard someone say, mathematicians & physicists do not like to spend any more time writing than absolutely necessary. Why spend time to write the ‘h’ twice, above and below the line when they can simply cancel each other out?
So, the kilowatt (kW) is to electrical energy what the litre per hour (l/h) is to water, i.e. the rate of use.