Free tool
Watts to Volts Calculator
Volts are watts divided by amps — and by the power factor on AC. Watts alone never give you a voltage, and a voltage that does not land near 120, 208, 240 or 480 is telling you one of the other figures is wrong.
Volts = watts ÷ amps
volts = watts ÷ amps on DC, ÷ (amps × power factor) on single-phase AC, and ÷ (amps × 1.732 × power factor) on three-phase, which gives the line-to-line voltage. 1,800 W at 15 A is 120 V; 4,500 W at 18.75 A is 240 V.
From resistance, volts = √(watts × ohms). Watts on their own cannot be converted — and because a real supply sits within about 5 percent of its nominal voltage, an answer far from one is a sign the inputs do not belong together.
What you know
Real power. Enter 4.5 kW as 4,500.
The current the load draws at those watts.
On AC, watts are volts × amps × power factor. A nameplate figure beats any preset.
The answer
Voltage
120.0 V
1,800 W ÷ (15.00 A × 1.00 PF)
Consistent with a 120 V supply
120.0 V is 0.0 percent above 120 V, the nearest nominal voltage. That is inside the ±5 percent service-voltage band of ANSI C84.1 Range A, so the figures describe a load on an ordinary supply.
What this does not tell you
What voltage an appliance needs. That is on its rating plate, and it is the one figure never to derive — a 240 V appliance on a 120 V receptacle is not a calculation problem.
What a circuit measures. A meter across the terminals reads the actual voltage, which moves with load and time of day. The calculation gives the voltage the stated watts and amps imply, nothing more.
Informational use only, please verify before you rely on it
This reproduces the standard arithmetic relating power, current, resistance and voltage for DC, single-phase and three-phase systems, with power factor applied on AC, and reads the result against common nominal voltages using the ±5 percent ANSI C84.1 Range A service band. It is a preparation aid, not a measurement and not a determination. It does not state what voltage an appliance requires — the rating plate does — and does not size conductors or overcurrent protection. Confirm with the equipment nameplate and a meter.
This tool is provided for general informational and educational purposes only. Its output is an illustrative estimate generated from the values you enter and from general assumptions that will not match every deal, market, lender, or homeowner. It is not tax, legal, accounting, financial, or professional advice, and it is not a quote, an offer, a credit decision, or a guarantee of pricing, approval, timing, savings, or eligibility.
You are solely responsible for independently confirming all information presented here including any figures, rates, fees, margins, timelines, tax treatment, and federal, state, local, or utility incentives, with the applicable lender, authority having jurisdiction, and your own qualified tax, legal, and financial advisors before acting on it, relying on it, or presenting it to a homeowner or any third party. Incentive programs, lender terms, and permitting requirements change frequently and vary by jurisdiction.
Seamless Home is not a tax advisor, law firm, lender, or licensed installing contractor, and makes no representation or warranty as to the accuracy, completeness, or currency of the information produced by this tool. To the fullest extent permitted by law, Seamless Home accepts no liability for any decision made or action taken in reliance on it.
When a watts-to-volts answer looks wrong
Most of the time someone converts watts to volts, they are trying to work out what a device runs on from the figures they can see. The calculation will always return a number; the useful part is whether that number is a voltage a house actually has. A US home is supplied at 120/240 V, and many apartments at 120/208 V, which is why a 240 V appliance delivers less on 208 V — the volts to watts calculator works out how much less for a heating element.
The most common reason for a strange answer is power factor. On AC, watts and volt-amps differ, and dividing watts by amps without it understates the voltage. What power factor is explains the gap. The second is an output rating read as an input: a microwave's cooking power is not what it draws. To go the other way — current from a wattage — use the watts to amps calculator, which carries the answer through to a breaker size.
Separately, the watts to kWh calculator answers how much energy that wattage uses over a day or a month once runtime and duty cycle are known.
Frequently Asked Questions
How do you convert watts to volts?+
Divide the watts by the amps: volts = watts ÷ amps. 1,800 W at 15 A is 120 V, and 4,500 W at 18.75 A is 240 V. On AC, divide by the power factor as well, and on three-phase by the square root of three too, which gives the line-to-line voltage. Watts on their own do not convert to volts — without the current or the resistance there is no answer.
How many volts is 1,500 watts?+
It depends on the current. 1,500 W at 12.5 A is 120 V; 1,500 W at 6.25 A is 240 V. The same power can be delivered at any voltage, which is why a wattage alone never tells you what a device plugs into. The voltage an appliance needs is on its rating plate.
Why does power factor change the answer?+
Because on AC, watts are volts × amps × power factor, not just volts × amps. A motor drawing 5 A and using 510 W at a power factor of 0.85 is on a 120 V circuit: 510 ÷ (5 × 0.85) = 120 V. Leaving the power factor out gives 102 V, which is not a voltage any ordinary supply delivers — the tool flags it for that reason.
How do you get volts from watts and ohms?+
Multiply watts by ohms and take the square root: volts = √(watts × ohms). A heating element that produces 4,500 W with a resistance of 12.8 Ω is running at √57,600 = 240 V, and carries 18.75 A. This holds only for a resistive load such as a heater element; a motor or electronic supply does not have a fixed resistance.
What voltages should the answer come out near?+
In a US house, 120 V or 240 V; in an apartment or commercial building, often 120 V and 208 V; on commercial three-phase, 208 V or 480 V line-to-line, with 277 V line-to-neutral for lighting. ANSI C84.1 Range A holds service voltage within ±5 percent of nominal — 114 to 126 V on a 120 V system. A result well outside those bands usually means the watts and amps are from different loads or moments, or the watts are an output rating.
Why does my microwave's wattage give a strange voltage?+
Because the wattage on the box is cooking power delivered to the food, not the electrical input. A microwave sold as 1,100 W typically draws considerably more from the outlet, and dividing its cooking power by its label current gives a voltage well below 120. Use the input watts from the rating label, which is the figure that pairs with the label current.
Is this a determination?+
No. It reproduces the standard arithmetic relating power, current, resistance and voltage as a preparation aid. It does not measure a circuit, does not say what voltage an appliance needs — the rating plate does — and does not size conductors or protection. Confirm against the equipment nameplate and with a meter.
Related resources
Volts to Watts Calculator
Watts from volts and current, resistance, or a rating at another voltage.
Learn more →Watts to kWh Calculator
Energy from the same wattage over a runtime, with duty cycle.
Learn more →Watts to Amps Calculator
Current from a wattage, carried through to a standard breaker size.
Learn more →Single-Phase vs Three-Phase Power
Why a house has 120/240 V and where 208 V comes from.
Learn more →What Is Power Factor?
Why watts and volt-amps differ on AC.
Learn more →Electrical work, planned before it starts
On the home projects Seamless Home runs — battery backup, HVAC, water treatment — the supply voltage and the circuit each piece of equipment needs are settled during design rather than discovered on installation day. Coverage is confirmed per service area rather than promised as blanket availability.
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