In a label such as "220 V 60 W" on an appliance, 60 W is the electric power: how much electrical energy it uses each second. For bulbs of the same type, the one with more watts is brighter.
Power is voltage times current, P = VI, so raising either the voltage or the current in the diagram above makes the bulb brighter.
What sets the monthly electricity bill is not power but electrical energy, which is power multiplied by the time of use.
In the graph below, with power as the height and time as the width, the area of the rectangle is the energy used.
Electrical Energy Used per Second
💡 Power Is How Fast Energy Is Used
①Power is the electrical energy used each second, measured in watts (W); 1 W means using 1 J every second
②For bulbs of the same type, a 100 W bulb is brighter than a 60 W bulb
③At the same voltage, more current means more power; at the same current, more voltage means more power
④An LED bulb gives the same light with far less power, so watts alone do not compare brightness across bulb types
Power and the Brightness of a Bulb
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How power and bulb brightness change as voltage and current change
10V
3A
Finding power
P = V × I
Power (W) = voltage (V) × current (A). At 220 V with 0.5 A, the power is 110 W
Using Ohm's law too
P = I²R = V²/R
Substituting V = IR lets you find power from current and resistance, or from voltage and resistance
Energy Is the Total Used
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Graph showing that energy is the area of a rectangle with power as its height and time as its width
60W
5h
Finding energy
W = P × t = V × I × t
Energy (Wh) = power (W) × time (h). With time in seconds, the unit is joules (J)
💡 kWh, the Unit on the Bill
①1 kWh = 1,000 Wh, the basic unit for charging electricity
②A 1,000 W heater on for 1 hour and a 100 W device on for 10 hours both use 1 kWh
③The "usage" on an electricity bill is the kWh used that month
④Even a low-power device uses a lot of energy if it stays on long enough
Electrical Energy and Heat
Converting energy units
1 Wh = 3,600 J, 1 cal ≈ 4.2 J
Using 1 W for 1 hour (3,600 s) is 3,600 J. When electrical energy turns into heat, it is sometimes expressed in calories
Heat from a current
Q = 0.24 × I² × R × t
Current through a resistor turns electrical energy into heat; this gives the heat in calories, the principle behind electric heaters and rice cookers
Summary
Power and Energy
P = VI, W = Pt = VIt
Power is the energy used each second; energy is the total used over the whole time
🎯 Common Test Points
①Power P = VI in watts; with Ohm's law, P = I²R = V²/R
②Energy W = Pt in Wh, kWh or J (1 Wh = 3,600 J)
③Bulbs of the same type are brighter at higher power
④Electricity is billed by energy (kWh), not by power
⑤At the same power, a longer time of use means more energy
Examples and Unit Review
Example 1
A device connected to a 220 V supply draws a current of 2 A. What is its power in watts?
1
Power is voltage times current (P = V × I).
2
P = 220 V × 2 A = 440 W.
▸ 440 W
Used for 1 hour, this device uses 440 Wh = 0.44 kWh.
Example 2
A 1,000 W heater is left on for 2 hours. How many kWh of energy does it use?
1
Energy is power times the time of use (W = P × t).
2
1,000 W × 2 h = 2,000 Wh, and since 1 kWh = 1,000 Wh, that is 2 kWh.
▸ 2 kWh
Changing power and time in the graph above, you can build rectangles with different heights and widths but the same area, that is, the same energy.
Unit-test style
Which statement about electric power and energy is correct?
①Power is found with P = V ÷ I
②Electrical energy is the product of power and time used
③The smaller the wattage (W), the brighter the bulb
④1 kWh is 100 Wh
⑤The unit of power is V (volt)
▸ ② Electrical energy is the product of power and time used
1
Electrical energy is W = P × t, found by multiplying power by the time used.
2
Power is P = V × I (①), a larger wattage is brighter (③), 1 kWh = 1,000 Wh (④), and the unit of power is W (watt) (⑤).