The three quantities
| Quantity | Unit | What it measures | Water analogy |
|---|---|---|---|
| Voltage | Volt (V) | The electrical pressure between two points; set by the supply (a 1.5 V battery, a 120 V or 230 V outlet) | Water pressure |
| Current | Ampere, amp (A) | The flow of charge; what a wire, a fuse or a breaker is rated for | Flow rate through the pipe |
| Power | Watt (W) | The rate of energy delivery; what a device is rated for | Pressure × flow: how much work the water does |
| Energy | Watt-hour, kilowatt-hour (kWh) | Power over time; what a bill counts | The total water that passed |
The formulas
The first line is the one to remember. A device's power is fixed by what it does; the voltage is fixed by the outlet; the current is whatever makes the multiplication work. The power converter moves between watts, kilowatts, horsepower and BTU per hour when a rating is given in another unit.
Real ratings, worked
Nominal figures from the kind of label found on the device. The last column is the watts those volts and amps produce.
| Device | Volts | Amps | Watts (V × A) |
|---|---|---|---|
| Phone charger (USB) | 5 | 2 | 10 |
| Laptop charger | 20 | 3.25 | 65 |
| LED bulb on a 120 V circuit | 120 | 0.07 | 9 |
| Kettle on a 230 V circuit | 230 | 8.7 | 2001 |
| Space heater on a 120 V circuit | 120 | 12.5 | 1500 |
Read the table either way. A charger label that says 20 V, 3.25 A is a 65 W charger. A heater sold as 1,500 W on a 120 V outlet draws 12.5 A, whether or not the label says so.
What a breaker's amps mean in watts
Circuit breakers and fuses are rated in amps because the wire's safe limit is a current limit: too much current heats the wire. Multiply by the voltage to see how much power the circuit can carry before it trips.
| Circuit | Maximum continuous power | Example |
|---|---|---|
| 15 A at 120 V | 1,800 W | One 1,500 W heater plus almost nothing else |
| 20 A at 120 V | 2,400 W | Two 1,000 W appliances |
| 16 A at 230 V | 3,680 W | A 2,000 W kettle and a 1,500 W heater together |
This is why two heaters on one 120 V, 15 A circuit trip the breaker: 12.5 A each, 25 A together, against a limit of 15. It is also why the same kettle is faster in a 230 V country: at the higher voltage a socket of similar current rating delivers nearly twice the power. Electrical codes add their own limits for continuous loads; the table shows the arithmetic, not the code.
From watts to the bill
Watts say how fast energy is used; the bill counts how much. Multiply the watts by the hours and divide by 1,000 to get kilowatt-hours, then by the price per kilowatt-hour. The same 1500 W heater for 3 hours is 4.5 kWh, which at 17 cents per kilowatt-hour costs about 77 cents; a 9 W LED bulb for the same 3 hours is 0.03 kWh, well under a cent. The electricity cost calculator does this for any device and your own rate, the EV charging cost calculator applies it to a car battery, and the kW vs kWh guide explains the power-versus-energy distinction with a bill in hand.
Where resistance comes in
Ohm's law, volts = amps × ohms, is the relation inside a single component. A 10 ohm heating element on 120 V passes 12 A and dissipates 120 × 12 = 1,440 W; the same element on 230 V would pass 23 A and dissipate 5,290 W, which is why appliances are built for one voltage. The power lost in a wire is amps squared times its resistance, so the current, not the voltage, decides how hot a cable gets: a 12 A load through a thin extension cord is a fire risk at any voltage, and the reason transmission lines run at very high voltage is to move the same power with a small current.
Reading a label
- Input: 100-240 V, 1.5 A. A power supply that accepts any mains voltage; the amps are the maximum at the low end. Its power is roughly 100 × 1.5 = 150 W at most, less at 240 V.
- Output: 5 V, 3 A or 9 V, 2.22 A. A USB charger offering two combinations; both are about 15 to 20 W.
- 1,500 W, 120 V. A heater or kettle; divide to get the 12.5 A it needs, and check what else shares the circuit.
- 60 W equivalent, 9 W. An LED bulb: the first number is the brightness of the old bulb it replaces, the second is what it draws. Only the second one goes on the bill.
Frequently asked questions
What is the difference between volts, amps and watts?
Volts measure electrical pressure, the push behind the current. Amps measure the current, how much charge flows per second. Watts measure power, the rate at which energy is delivered, and they are the product of the two: watts = volts × amps. A device is rated by the watts it draws; the volts are set by the outlet; the amps follow.
How do I calculate watts from volts and amps?
Multiply them. A charger that supplies 20 volts at 3.25 amps delivers 20 × 3.25 = 65 watts. To go the other way, divide: a 1,500 watt heater on a 120 volt outlet draws 1,500 ÷ 120 = 12.5 amps, which is most of a 15 amp circuit.
What is a kilowatt-hour?
Energy: one kilowatt of power running for one hour, which is what electricity bills count. Watts × hours ÷ 1,000 = kilowatt-hours. A 1,500 watt heater on for 3 hours uses 4.5 kilowatt-hours; at 17 cents per kilowatt-hour that is about 77 cents. The kW vs kWh guide goes into the difference between power and energy in detail.
Why does the same device draw fewer amps in Europe?
Because the voltage is higher. Power is what the device needs, so at 230 volts the current is about half of what it is at 120 volts for the same watts: a 1,500 watt kettle draws 12.5 amps at 120 volts and 6.5 amps at 230 volts. That is why European kettles can be more powerful on an ordinary socket.
What do ohms have to do with it?
Ohms measure resistance, how hard a material pushes back against current. Ohm's law ties the three together: volts = amps × ohms. For a fixed resistance, more voltage means proportionally more current, and the power dissipated is volts × amps, or amps squared × ohms, which is why a thin wire carrying a large current gets hot.