What a learner can do afterwards
- Calculates the power of an appliance from its voltage and current, or from its current and resistance
- Works out energy transferred in joules and in kilowatt-hours over a stated time
- Costs a week of using an appliance given a price per kilowatt-hour
1 · Read
Power is how fast energy moves, measured in watts. In a circuit it equals voltage times current, and for a resistor that also gives current squared times resistance. A 230 V kettle pulling 10 A runs at 2300 W, all ending up heating the water.
Energy is power stretched over time. Multiply watts by seconds to get joules. Bills use a bigger unit, the kilowatt hour: 1000 W running for one hour, which is 3.6 million joules.
A 2 kW heater running 3 hours a day uses 6 units a day. Over a week that is 42 units. Multiply 42 by the price per unit on the bill to cost the week.
Read watts off the label, then divide by 1000 to get kilowatts before multiplying by hours. Forgetting that step makes the answer 1000 times too big. High power devices need thick wiring, because big currents heat thin wires.
Volts times amps gives watts, watts times hours gives units, and units times price gives the bill.
More to read
20.4 Electric Power and Energy
Power is how fast energy moves, measured in watts. In a circuit it equals voltage times current, which also gives current squared times resistance. A 230 V kettle pulling 10 A runs at 2300 W, and that energy all ends up heating the water.
Energy is power stretched over time. Multiply watts by seconds to get joules, the physics unit of energy. Bills use a bigger unit instead, the kilowatt hour, which is simply 1000 W running for one hour, or 3.6 million joules.
9.5 Electrical Energy and Power
Resistors turn electrical energy into heat, and the rate of that heating is the power. High current through a high resistance means fierce heating, which is why thin extension leads can get warm under a heavy load. Fuses and breakers exist to cut the circuit before that heating turns dangerous.
To cost an appliance, find its power in kilowatts and multiply by the hours it runs to get kilowatt hours. Then multiply by the price per unit on the bill. A 2 kW heater running 3 hours a day uses 6 units a day, so a week costs 42 times the unit price.
4 Ways to Calculate Kilowatt Hours
The kilowatt hour looks scary but it is just power times time. Read the wattage off the appliance label, convert to kilowatts by dividing by 1000, and multiply by the hours it runs. A 100 W bulb glowing for 10 hours uses 0.1 times 10, which is 1 kilowatt hour.
The common slip is mixing up watts and kilowatts. If you forget the divide by 1000 step, your answer comes out 1000 times too big. Always check: a household appliance running for hours should give a handful of units, not thousands.
4 Ways to Calculate Wattage
Wattage tells you how hungry an appliance is. If the label is missing, multiply its voltage by its current in amps to get watts. A 12 V lamp drawing 2 A is a 24 W lamp. For resistive heaters you can also use current squared times resistance.
Knowing the wattage lets you predict bills and pick safe sockets. High wattage devices like kettles and heaters need sturdy wiring and should never share one thin extension lead. Low wattage gadgets like phone chargers sip energy by comparison.
Explore more
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.