Power and Your Bill

Lesson 7 of 10 · 7 min · Free, no account needed

What you will learn

Electric power is how fast energy is moved: P = V × I, in watts.
Energy = power × time. Bills count it in kilowatt-hours: 1 kWh is 1000 W for 1 hour.
Big heaters (kettles, showers, dryers) use the most. Small things left on all the time add up slowly.

What costs more over a year: a charger left plugged in, or one hot shower?
One 10-minute electric shower uses about 1.5 kWh. Drag the marker to your guess for a whole year of an idle charger.
Set the heater so the wattmeter shows exactly 24 W.
Watch V, I and P = V × I as you turn the dial.
Voltage is joules per coulomb, current is coulombs per second, so V × I is joules per second: watts. 12 × 2 = 24 W.
A 2000 W kettle runs for an hour of fast time. Watch the meter.
Watts and kilowatt-hours.
Electric power is energy per second: P = V × I, in watts (W). 1000 W = 1 kW.
Energy = power × time. Bills use the kilowatt-hour: 1 kWh = 1 kW for 1 hour (3.6 million joules).
A 2 kW kettle for 3 minutes (0.05 h) uses 2 × 0.05 = 0.1 kWh.
Put each device on the power scale.
Game: Energy budget. Live one day in this home on 5 kWh or less.
Pick one option in every row.
Code: build a loop that adds up a day's energy. Tap the lines in order.
Each device: watts × hours. Then divide by 1000 for kWh.
A 2000 W kettle runs for 3 minutes. How many kWh is that?
Energy = power × time = 2 kW × 0.05 h = 0.1 kWh.
Your day against a neighbour who never switches anything off.

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