How this instrument works
This instrument strips electrical power down to its single multiplication: enter Voltage and Current, and Power comes back as V × I, in watts, with no unit menu to second-guess. That narrowness is deliberate. A quiz question about electrical power is rarely testing whether you can convert milliamps to amps or apply a power factor — it is testing whether you remember that power is a product of potential difference and current, full stop, and can carry that arithmetic through cleanly. Strip away unit conversions and what remains is one fact worth having automatic: joules per coulomb times coulombs per second is joules per second, a watt.
Because this tool only runs forward — voltage and current in, power out — it will not solve a rearranged version of that equation for you. If a problem instead gives power and voltage and asks for current, that is I = P ÷ V, and you do that division yourself before checking your final number here. This restriction mirrors how these questions actually appear on trade and licensing exams: you are handed two known quantities and asked to produce a third, and multiplying itself is rarely hard — misreading which quantity was given, or dropping a factor of a thousand between milliamps and amps, usually is.
One boundary case is worth knowing cold: current of zero always returns zero power, no matter how large the voltage. A charged capacitor sitting at 400 V with no current path draws no power at all, because nothing is moving — power measures a rate of energy transfer, and a rate needs something in motion. That single fact catches out anyone who treats voltage alone as a proxy for how strong a circuit is; a live wire with an open switch has plenty of voltage and delivers exactly zero watts until current is allowed to flow.
- Enter Voltage — the value given in your problem or read from a supply, in volts.
- Enter Current — the value given or measured, in amperes.
- Work out V × I yourself first, then read Power and compare it to your own answer.
- If the question gives Power and asks for Voltage or Current instead, rearrange the formula by hand — this tool only checks the forward direction.
Worked example — 12 volts at 3 amperes
Enter 12 into Voltage and 3 into Current. Power returns 36 — because P = V × I gives 12 × 3 = 36 watts, with no rounding and no unit conversion involved, since every field on this instrument already sits in volts, amperes, and watts. It is the same pairing a car's 12-volt accessory socket presents to a 3-amp device: the socket delivers exactly 36 watts to whatever is plugged in, no more and no less, for as long as the current holds at 3 A.
Check it by hand before trusting the readout: twelve threes are thirty-six, so the answer should look obvious the moment you see it, and that is rather the point of a quiz problem this small. Change either input and the answer scales with it directly — 12 V at 6 A would double to 72 W, while 6 V at 3 A would halve to 18 W — because power is a straight product of the two, not a curve.
Questions
Why doesn't this calculator let me solve for voltage or current instead of power?
Because it is built as a one-direction check, not a full solver. Feed it the same Voltage and Current your problem gives you, and it multiplies them the way you should have; if instead you need current from power and voltage, divide by hand — I = P ÷ V — and only compare that final figure against your own working. Restricting the direction keeps you doing the algebra, which is the actual skill a quiz is testing.
What's the most common mistake on a P = V × I quiz question?
Reaching for the wrong formula entirely. Under pressure, plenty of people substitute P = I²R or P = V² ÷ R from memory, both of which need a resistance value the question never supplied. If a problem hands you voltage and current directly, the answer is one multiplication — V times I — and pulling in a resistance-based version just invents a number that was never asked for.
Does zero current always mean zero power, even at high voltage?
Yes. Power is V × I, and multiplying any voltage by zero current returns zero — a fully charged 400 V capacitor with its terminals open draws no power at all, because nothing is actually moving. Voltage alone describes a potential difference; it takes current, an actual flow of charge, for any energy to be converted. That is also why an open switch on a live circuit reads full supply voltage but delivers nothing.
Why are there no unit options for kilowatts or milliamps here?
Because this page is built as a bare arithmetic check, not a conversion tool. Every field is fixed to its base SI unit — volts, amperes, watts — so the only thing being tested is whether you multiplied the two numbers correctly. The site's full electrical power calculator handles kW, MW, and unit conversions if that is what your problem actually needs; this one is deliberately narrower.
Is this the same relationship used in Ohm's law?
It's related but not identical. Ohm's law, V = I × R, connects voltage, current, and resistance; this page's P = V × I connects voltage, current, and power instead, and needs no resistance value at all. The two combine when you substitute one into the other — P = I²R or P = V² ÷ R — but this instrument deliberately stops at the plain multiplication, since that is the form most quiz and exam questions actually ask for.