Chapter 2: What Is the Difference Between Power and Energy?

Quick Recap of Basic 2. This chapter revisits ideas from Green Energy — Basic 2 and takes them one step further.

The Big Idea

Power is how fast energy is used or made, measured in watts (W). Energy is the total amount used over time, measured in kilowatt-hours (kWh) — the “unit” on your bill. Energy = Power × Time. A 1,000-watt heater running for 1 hour and a 10-watt LED running for 100 hours both use exactly 1 kWh.

Did You Know?

Appliances in India carry a star label from the Bureau of Energy Efficiency (BEE). More stars mean the appliance uses less electricity for the same work. A 5-star fridge can save many units a year compared with a 1-star one.

Story Time — Siddharth and the Talking Meter

Siddharth, age 12 · Pune, Maharashtra

Siddharth’s building in Pune got new smart meters. His Aai showed him the app: a live number in watts, and a total in kWh for the day. “Why are there two different numbers?” Siddharth asked.

He decided to experiment. With only the fridge running, the app showed about 100 W. When his Aai switched on the induction cooktop, it jumped to about 2,000 W. “The watts tell me how fast we are using electricity right now,” he wrote in his notebook, “like the speedometer of a car.”

The kWh number crept up more slowly. After the cooktop ran for 30 minutes, the day’s total had grown by about 1 kWh. “And the kWh is like the odometer — the total distance travelled!”

He made a chart of the family’s appliances: power, hours used, and energy per day. The geyser surprised him most. At 2,000 W for 45 minutes every morning, it used 1.5 units a day — more than the fridge used all day.

At dinner, Siddharth presented his findings. “If everyone takes shorter hot showers, we save about 15 units a month.” His Baba raised an eyebrow. “Is this a science project or a campaign against my long showers?” Everybody laughed — but the next month’s bill was lower.

Story Time — The Night the Cyclone Came

Bijoya, age 13 · Kolkata, West Bengal

When a cyclone warning was issued for Kolkata, Bijoya’s family charged every phone and power bank. That night, fierce winds brought down trees and power lines, and the whole neighbourhood went dark.

Across the road, however, the windows of the city hospital still glowed. The next morning, Bijoya’s aunt, a nurse there, explained why. “The moment the grid failed, big battery systems took over in less than a second, so the life-support machines never stopped. Then the backup generators started.”

“How long can batteries run a hospital?” Bijoya asked. “That depends on two numbers,” said her aunt. “How much energy the batteries hold, in kilowatt-hours, and how much power the hospital draws, in kilowatts. Divide the first by the second, and you get the hours.”

Bijoya did the maths for her home inverter: a 1.2 kWh battery, and a fan plus two LED lights drawing 100 W, or 0.1 kW. “That’s 12 hours!” she said. “But only if we don’t switch on anything else.”

When power returned two days later, Bijoya wrote in her diary: “The grid shares power across the country. Batteries keep it for the moments when sharing fails. We need both.”

🔢 Numbers Corner

Battery backup time = battery energy (kWh) ÷ load (kW). Example: 1.2 kWh ÷ 0.1 kW = 12 hours.

Myth or Fact?

Myth: A bigger number of watts always means a bigger electricity bill.

Fact: Not by itself. The bill depends on energy, which is power × time. A 2,000 W hair-dryer used for 5 minutes uses less energy than a 60 W fan running all night.

🔬 Try it in the Green Energy Lab

Experiment with these ideas in Lab 7: Home Energy Audit.

Brain Buster

Find the power rating (in watts) printed on three appliances at home. Which one would use the most units if all ran for one hour?