Volts to Watts Calculator — Free Online Calculator
Convert volts to watts using current or resistance. Free online voltage to power calculator.
How to Use This Calculator
Enter voltage and either current (amps) or resistance (ohms) to calculate power in watts.
The Formula Explained
Power equals voltage times current (P = V × I), or voltage squared divided by resistance (P = V²/R).
Voltage and Power: The Foundation
The voltage-to-watts conversion is the most basic power calculation in electrical work, rooted directly in Ohm's Law. For a given voltage applied to a load, the power depends on how much current that load draws — which depends on the load's resistance (or impedance for AC). Three equivalent formulas describe this: P = V × I, P = I² × R, and P = V² / R. All three give the same answer for the same circuit; you pick whichever uses the variables you already know.
This calculation appears everywhere: sizing resistance heaters (where you know voltage and want to know power for a given wattage), calculating bulb brightness on non-standard voltage (V² / R relationship), finding the power rating of a motor starter coil, determining how much heat a power resistor dissipates. The underlying physics is simple but the applications are endless. Understanding it deeply — especially the V² relationship — separates electricians who can troubleshoot mysterious problems from those who just follow recipes.
Worked Example: 240V Heating Element on 208V Circuit
A common mistake: installing a 240V 5,000W water heater element on a 208V commercial circuit. The element resistance is R = V² / P = 240² / 5000 = 11.52 ohms. This is fixed by the physical element and does not change with applied voltage.
At 208V, actual power = V² / R = 208² / 11.52 = 3,754 watts. That is 75% of rated power. Water heats 25% slower. Not dangerous, but the water heater takes much longer to recover hot water after heavy use. The fix is to buy a 208V-rated element (which has lower resistance to compensate) rather than using the 240V element.
Conversely, installing a 208V 5,000W element on 240V: R = 208² / 5000 = 8.65 ohms. Power at 240V = 240² / 8.65 = 6,659 watts — 33% over rated. The element burns out much faster because it operates at higher temperature than designed.
Worked Example: Calculating Line Losses
A 100A load supplied by 500 feet of 2 AWG copper wire. 2 AWG has about 0.156 ohms per 1000 feet. Total round-trip resistance: 500 × 2 × 0.156 / 1000 = 0.156 ohms. Power dissipated in the wire: P = I² × R = 100² × 0.156 = 1,560 watts. That is 1.56 kilowatts wasted as heat in the wire at full load.
Over a year at 80% duty cycle: 1.56 × 0.80 × 8760 hours = 10,931 kWh wasted. At 12 cents commercial rate, that is 1,312 USD per year in pure losses. Upgrading to 1/0 AWG (half the resistance at 0.098 ohm/1000ft): losses drop to 980 watts, saving 580 watts, or 4,065 kWh, or 488 USD per year. Payback on the wire upgrade is typically 2-4 years for continuous-duty feeders.
This is why data centers and continuous industrial operations use larger wire than code minimum — the energy savings over the installation life dwarf the incremental copper cost.
Common Volts-to-Watts Errors
1. Using peak voltage instead of RMS. AC voltages are quoted as RMS (root-mean-square) values. 120V AC has a peak of 170V. Using peak in the power formula overstates power by 41%. Always use RMS for power calculations.
2. Forgetting that resistance is fixed. A heater element has fixed resistance. Power changes with applied voltage squared. A 1000W 120V element at 110V produces only 840W, not 920W.
3. Applying DC formulas to reactive AC loads. For motors and transformers, V × I gives VA not watts. You need power factor to convert. V × I calculations are only valid for resistive loads in AC.
4. Ignoring the three-phase factor. On three-phase systems, total power = sqrt(3) × V_LL × I × PF, not just V × I. Miss the 1.732 factor and you are off by 73%.
5. Using line voltage instead of phase voltage on wye systems. In a 208Y/120V system, phase-to-neutral is 120V and phase-to-phase is 208V. Choose correctly based on which you measured. A 120V load uses 120V in the formula; a 208V load uses 208V.
Common Voltage Standards and Power Examples
North American residential: 120V single-phase for outlets and lighting (15-20 amp circuits, 1,800-2,400W). 240V split-phase for large appliances (30-50 amp circuits, 7,200-12,000W). Typical household incandescent: 60W at 120V = 0.5 amp. Microwave: 1,200W at 120V = 10 amps. Electric dryer: 5,400W at 240V = 22.5 amps.
Commercial three-phase: 208Y/120V or 480Y/277V. 277V used for fluorescent/HID lighting in commercial (smaller wire than 120V for same wattage). 480V used for motors and machinery (even smaller wire than 277V).
European residential: 230V single-phase. Same 3,500W water heater draws only 15 amps vs 30 amps at 120V. This is why European kitchens can use smaller wire and why European-style 230V systems are more efficient for heating applications.
Historical and Standards Context
The 120/240V split-phase system in North America originated with Edison's DC distribution at 110V, which was the highest voltage considered safe for indoor use in the 1880s. As AC replaced DC, the voltage rose to 120V to compensate for line losses in wider distribution, then to split-phase 240V for high-power appliances without running separate wires.
IEC 60038 defines international voltage standards. ANSI C84.1 defines acceptable voltage ranges in North America: 114-126V for "Range A" (normal operation) and 110-127V for "Range B" (extreme conditions). Always design within Range A with margin for equipment variations.
Volts to watts: when voltage and resistance give you power
If you know the voltage across a load and its resistance (or current), you can compute power. This is Ohms law plus the power equation combined. Common use: figuring out how much heat an element generates, sizing a resistor, or backing out load when you have a clamp on a circuit and a voltage reading.
The formula and what it does
Three forms of the same relationship. Pick the one matching what you know. V x I when you have voltage and current. V squared over R when you have voltage and resistance. I squared times R for current and resistance.
Worked example
Scenario: 12 ohm resistive heater on 240 V supply. Find the power and current.
P = 240 squared / 12 = 57,600 / 12 = 4,800 W. I = V/R = 240/12 = 20 A. Verify: P = V x I = 240 x 20 = 4,800 W. Matches. This is a standard residential electric water heater.
Common mistakes to avoid
undefinedFrequently asked questions
Does this work for AC and DC?
For pure resistive loads (heaters, incandescent), yes both. For AC with inductive or capacitive loads, you need to add power factor: P = V x I x PF.
Why does power scale with voltage squared?
Because at fixed resistance, doubling voltage doubles current, and power is V x I. So 2V x 2I = 4P. This is why a 240 V heater puts out 4x the power of the same element at 120 V.
How does this apply to LEDs?
It does not directly. LEDs are non-linear and use a driver that converts AC to constant-current DC. The LED wattage is set by the driver, not by simple V/R math.
What if I know wattage and voltage but need current?
I = P / V for DC and resistive AC. So 1500 W on 120 V = 12.5 A. Inverse of the calculator above.
Does my battery follow these rules?
For instantaneous power output yes. But battery voltage sags under load (internal resistance), so the actual delivered V at the terminals is lower than nominal under heavy current.
Why is high-voltage transmission more efficient?
Losses in a wire = I squared x R. Doubling voltage halves the current for the same power, which cuts losses by 4. This is why long-distance transmission runs at 138 kV or higher.