Power & Energy: What’s the Difference?
In the solar industry, we spend a lot of time explaining how solar saves homeowners money by reducing their utility bill. Solar equipment powers the appliances in a home, so you don’t need to use as much power from the utility grid. Customers understand the difference between a $200 electricity bill and a $30 electricity bill — a savings of $170! Simple, right? But when we dive deeper and look into the details of the bill, the story gets more complicated.
Utilities charge the customer based on their energy consumption in the unit of kilowatt-hours (kWh). Home appliances typically have a power rating measured in the unit of watts (W). How are these two units related?
It is common for people to mistakenly use the words “power” and “energy” interchangeably, but they are different. I think the confusion stems from the units used to describe them, but let’s drill down to the basics to find out. Power is measured in watts, but can’t power be measured in other ways?
In the 18th century, the Scottish engineer James Watt coined the term horsepower as a way to compare the power of a train’s steam engine to that of a horse. Fundamentally, horsepower is just another unit of power that represents the rate of work by a horse. In the case of James Watt, he wanted to show an example of power that an everyday person could understand — the power of one healthy horse. So when you think of power, you can think of the capable strength of a horse, or you can also think of the capable power of a solar panel. While both can be measured by power, solar happens to be measured in “watts” while engines are measured in “horsepower.” For reference, 1hp = 745.7 watts.
What is a watt?
You can wrap your head around the idea of horsepower, but what is this mysterious unit of power called a watt? A watt (yes, it is named after James Watt) is actually a combination of other units. Confusing, right? A watt is a measurement of work per unit of time, or more specifically, one joule per second. Remember, hidden inside of the unit of a watt there is a rate of energy per unit of time, one joule per second. This is the key to the confusion!
A joule is another unit of energy, and it is equivalent to the energy released when a 100 gram apple falls one meter to the ground. You can think of energy as something generated or consumed.
Here is an analogy, power and energy are like speed and distance, respectively. Power is a ratio of energy per time and speed is a ratio of distance per time. Miles per hour is a unit of speed and watts (joules per second) is a unit of power. Alternately, energy is the amount consumed and distance is the amount traveled. Miles is a unit of distance, but watt-hours (joules) is the unit of energy.
A kilowatt-hour is not a ratio. It is not kilowatts per hour, it is kilowatt-hours. I’m referring to the two units — kilowatt and hour — which are in the numerator (top side of the fraction).
Here is an example, a light bulb uses a little bit of energy to shine for a second but uses more energy to shine for an hour, even though that light bulb always uses the same amount of power.
How much energy does a light bulb use? It depends on the power measurement of the light bulb and the length of time it is used. If a 15-watt light bulb is used for ten seconds, it will use 150 watt-seconds of energy. Here’s the part where I explain the key to the confusion: we stated earlier that a watt is equal to one joule per second. In this example, a light bulb uses 150 joules per second-seconds! The double use of seconds cancels out and we are left with 150 joules, so that light bulb uses 150 joules or 150 watt-seconds. The equation below shows how the units cancel out.
So why doesn’t your utility company charge you in joules? Joules are used in physics but are not practical for utility companies, they prefer to use kilowatt-hours (kWh). This may seem convoluted and confusing based on all that I have told you, but they have their reasons. Electricity is measured in voltage and amperage, which shares the unit watts.
1 watt = 1 volt × 1 amp, I know mind blowing, what is the magical unit not related to?! Volts, amps, and watts are interrelated and taking the time to explain that is best for another article. After reading, you should know that power is the ratio of energy per time and energy is either generated or consumed.