What is the difference between power and energy? The simple answer is that energy is the ability to do work and power is the rate at which energy is created or consumed.
Both terms appear quite frequently whenever talk about electricity or environmental sustainability crops up, but are often incorrectly used interchangeably. When I pay my electric bill to Ameren, I pay for every kilowatthour (kWh, an energy measurement) that I’ve consumed over the last month. But the lightbulb I put into my fixture reads 13 watts (W, a power measurement) and my gas bill from Laclede makes me pay for every therm (another energy measure, but in different units) I use for heating. It can quickly get confusing when trying to figure out how to lower your utility bills without turning off every device in your home.
Don’t worry if it seems overwhelming, even the utility experts can have difficulty balancing both the energy and power needs of consumers. Let’s take a simple example to show the difficulties that an electric utility might face when providing electricity to its customers. ABC Electricity has only one customer, Ella Tricity (so corny, I know), and Ella only has five 20W CFL lightbulbs and no other devices (we’ll keep it simple for now). Most of the day Ella is at work or asleep and usually only has one or two of the lightbulbs on at once. She measures how long she wants each bulb on each day and determines that each bulb should be on for 4 hours per day.
Let’s figure out how much energy Ella must buy from the utility. She has 5 lightbulbs with a power rating of 20W. A watt (power) is equal to one Joule (energy) per second, so for every second the lightbulb is on, it is drawing 20 Joules (J) of energy from the electric grid. Since Ella wants to run each lightbulb for 4 hours per day, each bulb will use 80 watthours (Wh) (4hx20W) daily. So Ella will use 400Wh (80Whx5bulbs) per day. We’ll report this as 0.4kWh. That sounds like a lot of energy (it is 1.44 million Joules!), but it isn’t that much for a home. In Missouri, using 0.4kWh will cost you a little less than 4 cents.
ABC Electricity sees it needs to provide 0.4kWh of energy per day so it builds a power plant that can run 24 hours a day and produces a constant amount of energy, which works out to 17W of power (0.4kWh/24hr plus a little extra) at all times. In this scenario ABC will produce 0.408kWh of energy a day and Ella will consume 0.4kWh of energy per day, but in reality none of Ella lightbulbs will be able to work. Each bulb requires 20W in order to run and ABC’s power plant only provides 17W at any given time per day.
While a customer pays for total energy usage, a utility must match instantaneous power demands throughout the entire day. ABC now determines that if Ella runs all her lights they would require 100W of power, so they built an 83W addition to their 17W power plant. But Ella rarely uses all her lightbulbs at the same time and most of the day is using none of them. Luckily ABC’s new plant can ramp up and down, but it needs to know when Ella is using her lights so it can ramp the plant up ahead of time to meet the new demand. However, for most of the day the expensive plant sits idle.
Now expand that example to a million utility customers. While the example of Ella is simplistic, it mirrors reality. Utilities must ramp up production to meet peak demand on a daily basis. The demand in midafternoon can often be double the demand in the middle of the night. Seasonal variation adds another level of complexity. Power demand in the summer with lots of AC running is much higher than in the winter when many customers heat their homes with gas. Utilities need to build up power capacity to meet the peak demand on the hottest day of the year (when AC can account for 75 percent of residential demand in in St. Louis), but often run well below that. In fact, some power plants might only be run for a few hours each year. In 2013 Ameren had natural gas plants that could produce about 3 gigawatts (GW) of power (28 percent of its total power capacity if all power plants were turned on). But Ameren obtained less than 1 percent of the total energy provided to customers from these plants. Essentially the natural gas power plants were turned off almost the entire year, and only used during the extreme peak demands of the summer. Other power plants like Ameren’s coal and nuclear plants are cheaper and harder to ramp up and down than the gas turbines.
Investing capital costs in power plants that are infrequently used seems wasteful, but that is the cost of maintaining a stable grid. The main difficulty is that the peak power (capacity) demanded by consumers creates inefficiencies in the energy capacities of those plants. How can we close the gap and not have so many idle plants? 1) We can lower the peak demand by using more power-efficient devices. Though most marketing calls these devices “energy” efficient, they usually lower power demand more than energy demand because the device will run for a longer period of time at the low power rating. 2) Implement energy storage in the grid to allow plants to run at a constant level, charging large battery modules overnight during low demand and releasing that energy to cover peak demand in the afternoon. Both solutions are being rapidly pursued by the Department of Energy. An intimate understanding of the relationship between power and energy is integral to the implementation of either plan.