How this instrument works
Electrical power is no mystery, only one product. Each coulomb crossing your component surrenders joules, many coulombs cross every second, and multiplying those two facts gives P = V·I joules per second. Rearranging that identity is all this sheet does. Watch units cancel and division stops feeling arbitrary — one watt is one joule per second, one volt is one joule per coulomb, so watts divided by volts leaves coulombs per second, which is precisely what an ampere counts. Since 20 May 2019 SI has fixed elementary charge at 1.602176634 × 10⁻¹⁹ C exactly, making one amp mean about 6.241509074 × 10¹⁸ charges streaming past any point each second.
James Prescott Joule pinned down how much heat current makes, reporting to London's Royal Society in 1841 that dissipation follows I²R — measurements taken by sinking coils in water and watching thermometers, patient work suiting one brewer's son unusually well. Naming arrived four decades later. Carl Wilhelm Siemens proposed honouring James Watt in his 1882 presidential address to that year's British Association meeting at Southampton, and that watt sat among coherent derived units when SI was formally named in 1960. Volta and Ampère already had units bearing their names; Watt's belonged to steam engines until electricians borrowed it for their own rate of doing work.
Straight division holds exactly for DC and for purely resistive AC loads — heaters, kettles, toasters, incandescent lamps. Add motors, transformers, or inexpensive switching supplies and current stops peaking in step with voltage, so real power drops below apparent power by that ratio called power factor: I = P ⁄ (V · PF). An induction motor at PF 0.7 pulls roughly 43% more current than watts alone predict, and three-phase supplies add √3 on top. Motor nameplates compound this by stating mechanical output, not electrical intake, so divide by efficiency first or your breaker will disagree with your arithmetic.
- Enter Power in watts, or switch that unit to kW, MW, or horsepower to match however your load is labelled.
- Enter Voltage: 120 or 240 for North American mains, 230 across most of Europe, 12 or 48 for battery and solar work.
- Read Current drawn, returned to four figures, with mA and kA on hand for signal-level or utility-scale results.
- For AC motors and switching supplies, divide Current drawn by power factor before sizing any cable or breaker.
- Compare against your circuit rating; anything running three hours or more should stay below 80% of it.
Worked example — 1800 W on a 120 V outlet
A hair dryer stamped 1800 W goes into a North American 120 V outlet. Put 1800 into Power and 120 into Voltage; Current drawn comes back as 15, because 1800 ⁄ 120 = 15 amperes exactly, with no rounding hiding anywhere.
Fifteen amperes is not a comfortable figure. It matches a standard 15 A branch breaker exactly, and North American rules cap that same circuit at 12 A — 1440 W — for loads running three hours or more. Hair dryers escape by being brief; portable space heaters are held to 1500 W, or 12.5 A, for this very reason. Run one alongside a kettle on one kitchen circuit and a breaker will settle matters for you.
Carry that 1800 W abroad and current nearly halves: 1800 ⁄ 230 = 7.83 A, sitting comfortably inside a 13 A British plug fuse. Carry it into a 12 V camper instead and 1800 ⁄ 12 = 150 A, which demands battery cable thicker than your thumb and lugs to match. Watts stay watts; amperes are what wiring actually feels.
Questions
Why does one appliance draw fewer amps in Europe than in America?
Because current is power divided by voltage, and European mains sits near 230 V against 120 V across North America. An 1800 W kettle pulls 15 A at 120 V but only 7.8 A at 230 V, under half as much. That gap explains why 3000 W kettles are ordinary in Britain on ordinary flex, yet essentially impossible on a 15 A American circuit — 3000 W at 120 V would want 25 A.
Do I need power factor for this?
Only for AC loads that are not purely resistive. Heaters, kettles, toasters and incandescent lamps sit at power factor 1, so plain division is exact. Motors, transformers, fluorescent ballasts and cheap switching supplies run anywhere from 0.6 to 0.95, meaning true draw is your answer divided by that number. At 0.7, actual current runs about 43% above what watts alone suggest — quite enough to matter when a breaker gets chosen.
How do watts differ from volt-amperes?
Watts measure real power, genuinely converted into heat, light or motion. Volt-amperes measure apparent power — RMS volts times RMS amps, indifferent to whether those waveforms move in step. Their ratio is power factor. Conductors, breakers and generators must be sized on VA, since copper heats according to current whatever its phase, while domestic meters bill watt-hours. Uninterruptible supplies carry VA ratings for exactly that reason.
Can I size a breaker or a cable straight from this answer?
Only as a starting point. Take Current drawn, divide by power factor where it applies, then bring in local rules. North American practice limits continuous loads — three hours or more — to 80% of a breaker's rating, so 15 A of computed draw belongs on a 20 A circuit rather than a 15 A one. Ambient heat, bundling, insulation rating and voltage drop along a long run each shave real capacity further.
My 750 W motor draws far more current than this predicts. Why?
Motor nameplates state mechanical output, not electrical intake. A 750 W (1 hp) motor running at 80% efficiency and power factor 0.8 consumes 938 W and draws 938 ⁄ (230 × 0.8) = 5.1 A, not 3.3 A. Starting is rougher still: locked-rotor current commonly reaches five to seven times full-load current for a second or two, which is why motor circuits get time-delay protection instead of plain fast fuses.
What happens if I enter zero volts?
Nothing useful, so this sheet refuses. Division by zero has no answer, and physically a source at zero volts drives no current through any finite resistance, meaning power would have to vanish as well. If you are working with genuinely small voltages — millivolts across a shunt, say — switch Voltage to mV rather than typing a long decimal. Prefix handling beats counting zeros.