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Volts to Watts Calculator

Convert voltage to power using current (amps) or resistance (ohms). Supports millivolts to kilovolts with unit dropdowns, common appliance presets, and dual-unit output in watts and kilowatts.

Power
1,200 W
1.20 kW

How to Convert Volts to Watts

Converting volts to watts calculates the electrical power consumed or produced by a device. Power determines your electricity bill, heat generation, and circuit loading—making this one of the most important calculations in electrical work. This volts to watts calculator features unit dropdowns (mV to kV), appliance presets, and dual-unit output that most online calculators lack.

1

Select Method

Choose "Using Amps" for the standard P = V × I formula, or "Using Resistance" for P = V²/R when you know the load impedance.

2

Enter Values with Units

Input voltage in mV, V, or kV. Enter current in mA, A, or kA. Use the unit dropdowns for automatic conversion.

3

Get Power in Multiple Units

Results show watts plus milliwatts or kilowatts automatically. Try preset appliance values for instant verification.

Volts to Watts Formula

Two formulas allow converting voltage to power depending on whether you know the circuit current or the load resistance. Both are fundamental to electrical engineering and are derived from Ohm's Law and Watt's Law.

Method 1: Using Current (Watt's Law)
P = V × I
P = Power in watts (W)
V = Voltage in volts (V)
I = Current in amps (A)
Method 2: Using Resistance
P = V² ÷ R
P = Power in watts (W)
V = Voltage in volts (V)
R = Resistance in ohms (Ω)

Method 1 comes directly from the definition of electrical power. Method 2 is derived by substituting I = V/R (Ohm's Law) into P = VI, giving P = V × (V/R) = V²/R. This is particularly useful when working with resistive heating elements, where the resistance is stamped on the component but the current draw must be calculated.

Example Calculations

LED Bulb: 120V at 0.1A
P = 120 × 0.1 = 12W
Result: 12 Watts
Equivalent to a 60W incandescent bulb in brightness, but using 80% less energy.
Microwave: 120V at 10A
P = 120 × 10 = 1,200W
Result: 1,200 Watts (1.2 kW)
A standard 1200W microwave. Needs a 20A dedicated circuit (draws 83% of a 15A circuit).
Clothes Dryer: 240V at 24A
P = 240 × 24 = 5,760W
Result: 5,760 Watts (5.76 kW)
240V circuit with 30A breaker and 10 AWG wiring. NEMA 14-30 outlet.
Heating Element: 240V across 12Ω
P = 240² ÷ 12 = 57,600 ÷ 12 = 4,800W
Result: 4,800 Watts (4.8 kW)
Using resistance method. Common in water heater elements where resistance is known from nameplate.
USB-C PD: 20V at 5A
P = 20 × 5 = 100W
Result: 100 Watts
Maximum USB Power Delivery standard. Enough to charge a high-end laptop at full speed.

Volts to Watts Conversion Table

Quick reference showing wattage at standard voltages and current levels. Formula: P = V × I.

Voltage1A2A5A10A15A20A30A
5V5W10W25W50W75W100W150W
12V12W24W60W120W180W240W360W
24V24W48W120W240W360W480W720W
120V120W240W600W1,200W1,800W2,400W3,600W
208V208W416W1,040W2,080W3,120W4,160W6,240W
240V240W480W1,200W2,400W3,600W4,800W7,200W
480V480W960W2,400W4,800W7,200W9,600W14,400W

Common Appliance Power Reference

Verify your calculations against real-world appliance specifications. This table covers the most common residential and commercial devices with their typical voltage, current, and resulting wattage.

ApplianceVoltageCurrentPower (Watts)Annual Cost*
LED Bulb120V0.1A12W$5.26
Laptop Charger120V0.5A60W$14.02
Desktop Computer120V2.5A300W$42.05
Microwave120V10A1,200W$14.02
Space Heater120V12.5A1,500W$175.20
Central AC (3-ton)240V16A3,840W$276.48
Clothes Dryer240V24A5,760W$100.91
Electric Range240V40A9,600W$168.19

*Estimated annual energy cost at $0.16/kWh with assumed daily usage hours.

⚠️ Voltage & Power Safety Guidelines

Working with voltage and power requires understanding safety limits and code requirements. Incorrect calculations can lead to overloaded circuits, fire hazards, and electrocution risks. Always follow the National Electrical Code (NEC) or your local equivalent.

  • NEC 80% Rule: Continuous loads must not exceed 80% of the circuit breaker rating. A 120V/20A circuit can safely handle 1,920W continuously (not the full 2,400W). This is NEC Article 210.20(A).
  • Wire gauge for current: Higher wattage at lower voltage means higher current, which requires thicker wire. 14 AWG for ≤15A, 12 AWG for ≤20A, 10 AWG for ≤30A, 8 AWG for ≤40A, 6 AWG for ≤55A.
  • Voltage mismatch danger: Connecting a 120V-rated device to 240V will typically destroy it instantly. The power delivered would be 4× the rating (since P = V²/R). A 1,200W device would receive 4,800W, causing catastrophic failure.
  • Shared circuit limits: Multiple devices on the same circuit share the total capacity. Two 1,000W appliances on a single 15A/120V circuit (1,800W limit) will trip the breaker. Use dedicated circuits for high-wattage appliances.

Understanding Voltage and Power

Voltage and power are two sides of the same coin in electrical engineering. Voltage is the "pressure" driving electrons through a circuit, while power is the rate at which those electrons deliver energy to a load. Every electrical device is designed for a specific voltage and draws a specific amount of power—understanding their relationship is fundamental to safe and efficient electrical work.

The relationship P = V × I tells us that power is the product of voltage and current. This means a device's power consumption depends on both how much voltage pushes through it and how much current actually flows. A high-voltage, low-current device (like a plasma TV at 120V/1.5A = 180W) uses less power than a low-voltage, high-current device (like a car starter motor at 12V/200A = 2,400W).

Why do utilities transmit electricity at high voltage (115kV–765kV)? Because for the same power delivery, higher voltage means lower current, and lower current means dramatically less I²R heat loss in the transmission wires. A 1MW load at 100V would need 10,000A (massive cables, huge losses), but at 100kV needs only 10A (small cables, minimal losses). Transformers at substations step the voltage down to usable levels—which you can calculate using our Transformer Calculator.

For the reverse conversion, use our Watts to Volts Calculator to find voltage from power. When the load is AC-powered, our AC Wattage Calculator calculates real power, apparent power (VA), and reactive power (VAR) from voltage and current — useful for sizing generators, UPS units, and circuit breakers. To find current from voltage and power, try our Volts to Amps Calculator which uses both Watt's Law and Ohm's Law methods.

Frequently Asked Questions

Multiply volts by amps to get watts: P = V × I (Watt's Law). If you know resistance instead of amps, use: P = V² ÷ R. For example, 120 volts at 10 amps equals 1,200 watts. Or 120V across a 10Ω resistor gives 120² ÷ 10 = 1,440 watts. You always need a second value (current or resistance) because volts alone cannot be converted to watts.
Two formulas: With current: P = V × I (power equals voltage times current, from Watt's Law). With resistance: P = V² / R (power equals voltage squared divided by resistance, derived from combining P = VI and V = IR). Both yield power in watts. The second formula is useful when you know the component's resistance rating.
Yes, if you know resistance. Use P = V² ÷ R. For example, 240V across a 12Ω heating element produces 240² ÷ 12 = 57,600 ÷ 12 = 4,800 watts. This method is especially useful for heating elements and resistive loads where the resistance value is printed on the component nameplate.
It depends on the current or resistance in the circuit. At 1A, 120V is 120W. At 10A, 120V is 1,200W. At 15A, 120V is 1,800W. Without knowing the current draw or load resistance, you cannot determine watts from volts alone. Use our calculator to compute exact values for your specific circuit.
At 10A: 240V × 10A = 2,400W. At 20A: 4,800W. At 30A: 7,200W. At 50A: 12,000W. 240V circuits are used for heavy appliances in the US (dryers, ranges, water heaters, EV chargers) and are the standard household voltage in Europe, UK, Australia, and most of Asia.
The power triangle is a visual tool connecting three equations: P = VI (top), V = IR (left), P = I²R (right). From the center point, any two known values lead to the third. The extended "Ohm's Law wheel" includes 12 formulas: P = VI, P = I²R, P = V²/R, V = IR, V = P/I, V = √(PR), I = V/R, I = P/V, I = √(P/R), R = V/I, R = V²/P, R = P/I².
Watts measure real power (the power that does useful work). VA measures apparent power (total power drawn from source). For DC and purely resistive AC loads, watts equals VA. For inductive/capacitive loads (motors, transformers), VA is higher than watts because some current flows back and forth without doing work. The ratio is the power factor: PF = W / VA.
Because P = V²/R. If resistance stays constant, doubling voltage quadruples power—not doubles it. This is why a 240V heating element produces 4× more heat than a 120V element with the same resistance. It's also why high-voltage transmission (115kV–765kV) reduces power loss: the same power can be transmitted at lower current, and losses (I²R) decrease dramatically.
Most of the world uses 220–240V at 50Hz (Europe, UK, Asia, Africa, Australia, South America). The US, Canada, Japan, and a few other countries use 110–120V at 60Hz (with 240V available for heavy appliances). Japan uses 100V. Understanding your local voltage is critical for sizing appliances and calculating power correctly.
Your electricity bill is based on watts (actually kilowatt-hours, kWh), not voltage. A 1,000W device running for 1 hour uses 1 kWh regardless of whether it runs at 120V or 240V. However, at higher voltage, the device draws less current (1000W ÷ 240V = 4.17A vs. 1000W ÷ 120V = 8.33A), which can reduce I²R losses in wiring—saving a small amount.

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