What are energy and power? Putting these questions into terms more relevant to our daily lives, what is a kilowatt? A kilowatt-hour? A calorie? How are they related?
First for the short answers. As most of us learned from school, energy is the capacity to do work—when more energy is available, more work can be done. Meanwhile, power is the amount of energy produced per unit time.
A kilowatt, which is equal to a thousand watts, is a measure of power. Meanwhile, a kilowatt-hour is a measure of energy. So is a calorie. When we pay our electricity bills, we pay for energy, not power. This is why our electricity consumption is measured in kilowatt-hours, not kilowatts.
Now let us expound on these concepts a little bit more.
I said above that energy is the capacity to do work, but what is work? In physics, work is defined in terms of how much a force affects the motion of an object.
There are many different kinds of work. When a bulb lights up, work is done in producing the light. When a heater boils water, work is done in making the water molecules move faster. Whenever you see something useful done, there is some form of work involved.
Work can also be seen in the living things. When you move your body, you are exerting force to take your body from one place to another—that’s work! But even while you’re sitting down, a lot of work is going on inside your body, from the pumping of your heart to the movement of nutrients across your cells.
There are many ways to measure energy. For the energy contained in food, we often use calories. (Here we are using food calories, which is equal to 1000 of the calories used in chemistry and physics.) Meanwhile, for the electricity, we often use kilowatt-hours.
There are about 860 food calories in 1 kilowatt-hour. That is comparable to the amount of energy in a meal composed of one quarter pounder, one small order of fries, and 1 bottle of soda.
This means that we can say there is 1 kilowatt-hour of energy in the meal. It would be weird because we are not used to using kilowatt-hours to measure the energy in foods, but that would not be wrong.
There is actually an advantage in using kilowatt-hours to measure the electrical energy—it makes calculating energy from power easier.
This brings us to power. You can compare power to how fast water is coming out of a hose and energy to the total amount of water that comes out after a given time.
A certain hose can release water slowly, but if it stays open for a long time, the total water coming out can fill an entire pool. Meanwhile, a certain hose can release water quickly, but if it is open for only a short time, the total water coming out can be small.
The same goes with power and energy. For example, a 25-watt light bulb uses a lot of energy per second compared to a 5-watt light bulb. This is why if both light bulbs are the same type, say both are LEDs, the 25-watt bulb will shine brighter.
However, if the 25-watt bulb is turned on for only 1 hour but the 5-watt bulb is turned on for 5 hours, they will use the same amount of energy.
To see this better, we can now connect wattage and energy consumed. How much energy does a 25-watt bulb consume in 1 hour? Simple! Just multiply the number of hours with the power: 25 watts times 1 hour equals 25 watt-hours. That’s also how much energy a 5-watt bulb uses in 5 hours.
What if we turn on a 25-watt bulb for 5 days straight? Well, we just multiply 25 watts with the number of hours in 5 days, which is 120 hours. That gives us 3000 watt-hours or 3 kilowatt-hours. (1 kilowatt-hour is 1000 watt-hours.)
Notice that power plants are often measured in terms of how much power they can produce. (Most big power plants can produce megawatts of power. 1 megawatt equals 1000 kilowatts.) That is because when consumers are using electricity, a certain amount of energy needs to be released at that very moment.
Power demands fluctuate throughout the day, so if you want to know the total consumption over a period of time, you must think in terms of energy. Power is instantaneous while energy is cumulative.
The production of energy is one of the most important processes in modern civilization. It can also be one of the most harmful to the environment. Thus, understanding the basic science involved is important in engaging with the possible solutions to the crises related to energy.