Kilowatt hours
A lot of people don’t seem to understand the units for energy and power. Here’s a quick tutorial.
The metric system unit of energy is the joule. A joule is about a quarter of a calorie, which is defined as the energy required to heat one gram of water by one degree centigrade (1.8 degrees Fahrenheit). A joule is also about how much energy you expend by lifting an apple from the floor onto a table. It’s defined as the exertion of a force of one newton over a distance of one meter. Now, that energy can be delivered quickly or slowly, and it’s the same amount of energy, but if it’s quicker, it’s more power. The SI unit of power is the watt, which is defined as a delivery of one joule every second.
Power * time = energy Energy / time = power
Now, for some reason, utitilies have chosen to meter energy in kilowatt hours, rather than megajoules (1 kWh = 3.6 MJ). This seems to cause no end of confusion. This post was prompted by someone, upon being told that a system delivered 100kW, who clarified “you mean 100 kW per hour?” This was the third person I had heard reference kilowatts per hour, and two of them were supposed to be energy experts. Kilowatts per hour does not make sense as a unit. Kilowatts are already units of energy per unit time (kilojoules per second). You can multiply kilowatts by hours to get kilowatt hours, a unit of energy, but that’s almost certainly not what you meant if you said kilowatts per hour. Now, you could define a unit of power based on kilowatt hours per second, wherein the hours/seconds divides out to a dimensionless factor of 3600, but this is rare.
What are these units for? Well, the capacity of a power source, or the demand of a power load, is measured in power - watts or megawatts, joules per second or megajoules per second. But cost is measured based on energy - dollars per megajoule or dollars per kilowatt hour. This is because it costs more to run a system for longer, but it doesn’t affect the system’s capacity.
Any questions? If you’re still confused (or more confused than you were…) please ask.
