As all of us involved in projects like Talybont Energy aren’t experts in that field, we sometimes get confused over our units of energy. Journalists writing about environmental projects do so frequently. Here’s a helpful reminder for us all.
kW is a unit of power
kWh is a unit of energy (where energy = power x time)
A kilowatt (kW) is a thousand Watts – and measures the rate at which energy is being generated or consumed. A kettle consumes about 3kW, for example, as does a b-bug on a flat road. Because it’s already a rate, it doesn’t make sense to talk about a kilowatt per hour, and that isn’t what a kilowatt-hour is. A kilowatt-hour is a measure of how much energy has been generated or consumed, and as the term implies, it’s the equivalent of generating or consuming one kilowatt for one hour. A 3kW kettle would consume 3 kilowatt-hours if it were on for an hour, but since it takes only 5 minutes to boil, it consumes at the rate of 3 kilowatts for only 5 minutes, or one twelfth of an hour. Since 3/12 is exactly a quarter, boiling a kettle uses a quarter of a kilowatt-hour (0.25kWh) of energy, which costs just under 4p.
It’s unfortunate that the most common electrical measure – the Watt and kiloWatt – denotes a rate rather than an amount of energy consumed. So when we want to talk about amounts – which are after all what we get charged or paid for – we have to express that in terms of a rate multiplied by a time. That would be like measuring distance in mph-hours instead of in miles. I think the confusion comes from journalists trying to find a compelling way to explain energy amounts in ways we can all understand. But to do this they use silly expressions like “enough for all the lights in Birmingham for a year”. This makes no sense at all. Either the power source can power all the lights in Birmingham, or it can’t – the time period doesn’t come into it.
For what it’s worth, physicists don’t do this. They measure energy in Joules. A Watt is actually a Joule per second.
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