How this calculator works
A volt is defined as one joule per coulomb. Moving one coulomb of charge through a potential difference of one volt therefore transfers one joule of energy. That single relation, E = QV, links the three quantities, so any two determine the third. The same relation describes a battery delivering charge to a circuit, a capacitor storing energy and a particle accelerated through a voltage. Multiplying by time gives power, because one coulomb per second is one ampere.
Formula
E = Q x V, where E is energy in joules, Q is charge in coulombs and V is voltage in volts. V = E / Q. Q = E / V. The charge on one electron is 1.602176634 x 10^-19 C, so an electron accelerated through 1 V gains 1 eV, or 1.602176634 x 10^-19 J.
Worked example
10 J transferred to 2 C is 5 V. 3 C through 12 V transfers 36 J. 50 J at 5 V moves 10 C. A 12 V, 60 Ah battery stores 12 x 60 x 3,600 = 2,592,000 J, because 60 Ah is 216,000 C.
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Frequently asked questions
Why is charge in coulombs and not amperes?
A coulomb is a quantity of charge; an ampere is a rate of charge flow. One ampere for one second carries one coulomb. To use amperes, multiply by the time in seconds first.
Does this work for alternating current?
The relation holds at each instant. With AC you usually work in RMS values and include a power factor, because voltage and current may not peak together. This page assumes a steady direct-current case.
Can voltage be negative?
Yes, a potential difference has a sign. This page asks for magnitudes and rejects negatives so that division stays unambiguous; use the magnitude and record the direction separately.