The Car Analogy
Consider two cars. The first car is a Ferrari Enzo driven by a speed demon. The second is a 1990 Volkswagen Golf driven by someone who just wants to get from A to B in one piece.
The Kilowatt (kW) = Speed (mph)
Both cars go on a 7-hour drive. Once it gets on the highway, the Enzo’s speed ranges from 80 to 140mph. That’s the “demand” the driver’s putting on her car. If it were a building, we’d say it were using between 80 and 140kW at any given moment.
Once the Golf gets on the highway, it tops out at 60mph and goes as slow as 45mph. If it were a building, we’d say it has a maximum demand, or “peak demand”, of 60kW, and a minimum demand, or “baseline”, of 45kW.
The kilowatt, then, is a measure of energy use at a given moment, not over time. When we speak about the amount of energy a building requires to operate—it’s electrical “load”—we speak in kilowatts (e.g., my building’s load ranges from 250kW to 800kW).
The Kilowatt-hour (kWh) = Distance Travelled (mi)
Even though both cars were driving for the same number of hours (the “h” in kWh), the amount of distance they covered was drastically different. The Enzo went nearly 1,000 miles on its journey, versus the Golf’s 420 miles.
If you think of distance covered as usage (that’s kWh), you can see how two buildings can operate for the same amount of time but use drastically different kWh. In this example, the Enzo “building” used 980kWh over the same period of time that the Golf “building” used 420kWh.
So the kilowatt-hour is, in essence, a measure of total energy you use over a specific period of time, not at a given moment. When we speak about the amount of energy a building used over a month, we speak in kilowatt-hours (e.g., my building used 20,000kWh more in July than it did in June).
To sum it up, the kilowatt is a building’s demand, or how much energy it’s using at a given moment. The kilowatt-hour is a building’s consumption, or how much energy it’s using over a given period of time.
Determining the difference between kW and kWh is essential, but so is understanding how they’re treated differently on your electricity bill. For that breakdown, read the second post in this series.