โก Wattage Calculator: Find Watts from Volts and Amps
By Shihab Mia ยท Updated 2026-07-11
Helper: amps = watts / volts = 600 / 120 = 5 A.
| Appliance | Typical wattage | Amps at 120 V |
|---|
This wattage calculator finds electrical power in watts from the values you already know. The core rule is watts = volts x amps (P = V x I), so 120 volts times 5 amps equals 600 watts. Pick which two of voltage, current, and resistance you have, and the tool returns the power plus the missing electrical quantity, so you can size circuits, chargers, and appliances with confidence.
What is the Wattage Calculator?
Wattage measures how fast electrical energy is used or delivered. One watt equals one joule per second, and for direct current or resistive loads the power is simply the voltage across a device multiplied by the current flowing through it: P = V x I. That single equation answers most everyday questions, such as how many watts a 120 volt appliance draws at 5 amps (600 watts) or how much current a 1500 watt heater pulls from a 120 volt outlet (12.5 amps). Because the three quantities are linked, knowing any two lets you solve for the rest.
When you know the current and the resistance instead of the voltage, use P = I squared x R. This comes from combining P = V x I with Ohm law (V = I x R): substitute the voltage and the current term squares. It is the natural form for heating elements and resistors, where the resistance is fixed and the current is what you measure. Doubling the current through a fixed resistor quadruples the power, which is why a small rise in current can cause a large rise in heat.
When you know the voltage and the resistance but not the current, use P = V squared / R. Again this follows from Ohm law, replacing the current with V / R. This form is handy for a device rated at a known voltage with a known resistance, and it shows that halving the resistance at a fixed voltage doubles the power. All three formulas describe the same physical relationship; you simply choose the one that matches the two values in front of you.
A few practical points keep results accurate. These formulas give real power in watts for DC and for purely resistive AC loads such as heaters, kettles, and incandescent bulbs. For motors, transformers, and other reactive AC loads the true power is volts x amps x power factor, where the power factor is a number between 0 and 1, so the plain V x I result (called apparent power, measured in volt-amps) can be higher than the watts actually consumed. Always match voltage to your local supply too: most of North America uses about 120 volts, while much of Europe, Asia, and Africa uses 220 to 240 volts, which changes the current for the same wattage.
When to use it
- Working out the wattage of an appliance from its voltage and current rating.
- Checking how many amps a heater, kettle, or dryer draws so you do not overload a circuit.
- Sizing a power supply, inverter, or battery bank for a known load.
- Converting volts and amps to watts for a solar panel, charger, or LED strip.
- Estimating running cost by turning wattage into kilowatt-hours over time.
- Verifying that several devices on one circuit stay under the breaker rating.
How to use the Wattage Calculator
- Choose which two values you know: volts and amps, amps and ohms, or volts and ohms.
- Type your numbers into the two fields that appear (decimals are fine).
- Read the power in watts at the top, shown large and formatted.
- Check the voltage, current, resistance, and kilowatts breakdown below.
- Use the amps = watts / volts helper line to confirm the current draw.
Formula & method
Worked examples
Find the wattage of a device running at 120 volts and drawing 5 amps.
- Use the primary formula P = V x I
- Substitute the values: P = 120 x 5
- Multiply: P = 600
- Convert to kilowatts if needed: 600 / 1000 = 0.6 kW
- Cross-check current: amps = watts / volts = 600 / 120 = 5 A
Result: The device uses 600 watts (0.6 kW), drawing 5 amps.
Find the power in a heating element carrying 10 amps through 15 ohms of resistance.
- Use P = I squared x R because you know current and resistance
- Square the current: 10 x 10 = 100
- Multiply by resistance: 100 x 15 = 1500
- So the element dissipates 1500 watts
- Its voltage would be V = I x R = 10 x 15 = 150 volts
Result: The element produces 1500 watts (1.5 kW) at 150 volts.
Typical appliance wattages and their current at 120 volts
| Appliance | Typical wattage | Amps at 120 V |
|---|---|---|
| LED light bulb | 10 W | 0.08 A |
| Laptop computer | 65 W | 0.54 A |
| LED television (50 inch) | 100 W | 0.83 A |
| Refrigerator (running) | 150 W | 1.25 A |
| Microwave oven | 1000 W | 8.3 A |
| Space heater | 1500 W | 12.5 A |
| Clothes dryer | 3000 W | 25 A |
The three power formulas and when to use each
| You know | Formula | Also solves |
|---|---|---|
| Volts and amps | P = V x I | Resistance R = V / I |
| Amps and ohms | P = I squared x R | Voltage V = I x R |
| Volts and ohms | P = V squared / R | Current I = V / R |
Common mistakes to avoid
- Confusing watts with volt-amps on AC loads. For motors and other reactive AC devices, real power is volts x amps x power factor. The plain V x I result is apparent power in volt-amps and can be higher than the watts actually used.
- Using the wrong supply voltage. The same wattage draws very different current at 120 V versus 230 V. Always plug in your local mains voltage, not a default from another country, or the amps will be wrong.
- Forgetting to square the current in P = I squared x R. A common slip is multiplying I x R once. The current must be squared first, so 10 amps through 15 ohms is 100 x 15 = 1500 W, not 150 W.
- Mixing up milliamps and amps. A 500 mA current is 0.5 A, not 500 A. Convert milliamps to amps by dividing by 1000 before multiplying by voltage, or the wattage will be off by a factor of a thousand.
Glossary
- Watt (W)
- The unit of power, equal to one joule of energy per second. In a circuit it is the rate at which electrical energy is used or delivered.
- Volt (V)
- The unit of electrical potential difference, the push that drives current through a circuit.
- Ampere (A)
- The unit of electric current, the rate at which charge flows. One amp is one coulomb per second.
- Ohm
- The unit of electrical resistance. Higher resistance means less current flows for a given voltage.
- Power factor
- A number from 0 to 1 for AC loads that relates real power (watts) to apparent power (volt-amps). Resistive loads have a power factor of 1.
- Kilowatt-hour (kWh)
- A unit of energy equal to one kilowatt used for one hour. It is the unit shown on electricity bills.
Frequently asked questions
How do I calculate wattage from volts and amps?
Multiply the voltage by the current: watts = volts x amps. For example, 120 volts times 5 amps equals 600 watts. This works for DC and for purely resistive AC loads such as heaters and incandescent bulbs.
How many watts is 120 volts at 5 amps?
It is 600 watts, because P = V x I = 120 x 5 = 600. That is 0.6 kilowatts, and the same device would draw 5 amps of current, since amps = watts / volts.
How do I convert watts to amps?
Divide the wattage by the voltage: amps = watts / volts. A 1500 watt heater on a 120 volt supply draws 1500 / 120 = 12.5 amps. On a 230 volt supply the same heater draws only about 6.5 amps.
What is the formula for power using resistance?
If you know current and resistance, use P = I squared x R. If you know voltage and resistance, use P = V squared / R. Both come from combining P = V x I with Ohm law, V = I x R.
Is wattage the same as apparent power?
Not always. For resistive loads, volts x amps equals the real power in watts. For reactive AC loads like motors, volts x amps gives apparent power in volt-amps, and the true watts are that value multiplied by the power factor.
Why does the same appliance draw fewer amps on 230 volts than on 120 volts?
Because power stays the same but current is power divided by voltage. A higher voltage means less current is needed to deliver the same wattage, which is why 230 volt regions can use thinner wiring for the same load.