Power: watts

Watts measure how fast energy is used

A 60-watt bulb and a 1,500-watt heater both use energy, but at very different rates. A watt is a rate: one watt is one joule of energy every second. So a 60 W bulb uses 60 joules each second, and a 1,500 W heater uses 1,500 joules each second.

More watts doesn't mean "stronger electricity". It means more energy used every second.

The formula: watts = volts × amps

For electricity there's a simple formula:

Power (W) = Voltage (V) × Current (A)

You'll also see it written P = V × I, where I stands for current. One volt pushing one amp gives one watt. Voltage is the push and current is the flow (from earlier in the series); power is what you get when you multiply them.

A bulb and a heater on the same outlet

Plug both into a standard North American outlet at 120 V.

• The bulb draws half an amp: 120 V × 0.5 A = 60 W.
• The heater draws twelve and a half amps: 120 V × 12.5 A = 1,500 W.

Same voltage, but the heater draws 25 times the current, so it uses 25 times the power. That's 25 of those bulbs' worth of energy every second.

LEDs: same light, fewer watts

An old incandescent bulb turns most of its power into heat, not light. An LED bulb that's as bright as a 60 W incandescent (about 800 lumens) uses only about 10 W. Same light, about a sixth of the power. That's why "60 W equivalent" is printed on LED boxes: it tells you the brightness, not the power it uses.

Try it yourself

Find the label on a hair dryer, kettle, phone charger or space heater. Look for the watts (W), or for volts (V) and amps (A). If it only lists volts and amps, multiply them to get the watts. Which device in your home has the biggest number?

Good to know

• One watt is one joule of energy per second, and 1 V × 1 A = 1 W.
• The numbers in the video are round examples: a 60 W incandescent bulb at 120 V draws 0.5 A, and a 1,500 W heater draws 12.5 A.
• Most plug-in space heaters in North America top out near 1,500 W, because a standard 15 A, 120 V circuit should carry no more than about 1,440-1,800 W continuously.
• For AC, the volts and amps here are RMS values, and the simple formula fits resistive loads like heaters and filament bulbs. Motors and electronics have a power factor below 1; that comes later.
• A "60 W equivalent" LED gives about 800 lumens using roughly 8 to 10 W, depending on the bulb.
• The wattage on a label is usually the maximum draw, not what the device always uses.
• Plugging several 1,500 W devices into one circuit can trip the breaker. Leave wall wiring to a licensed electrician.