Article — Joules to Volts Converter
Joules to Volts: Converting Energy to Potential Difference
Volts equal joules divided by coulombs. V = J / Q is the SI definition of the volt: one volt is the potential difference that does one joule of work per coulomb of charge moved through it. Energy alone does not determine voltage; you need to know how much charge carried that energy.
The conversion appears in any circuit calculation that mixes energy and voltage. Battery capacity in milliamp-hours converts to coulombs, then to joules at the nominal voltage. Capacitor design balances stored energy against rated voltage. Defibrillators specify shock energy in joules and rely on the body's impedance to set the resulting voltage and current. In every case, the bridge between energy and voltage is the amount of charge that moves.
What is joules to volts conversion?
The joules to volts conversion takes a known energy in joules and a known charge in coulombs, and returns voltage in volts via V = J / Q. The relationship is symmetric: knowing any two of the three (energy, charge, voltage) lets you compute the third. This calculator solves for any of the three when you provide the other two.
Joules and volts measure different physical quantities. Joules are an SI unit of energy or work; volts are an SI unit of electric potential difference. They become related the moment electric charge flows. Moving one coulomb of positive charge from a low potential to a higher potential of 1 V requires 1 J of work. That sentence is the entire conversion, restated.
The volt is named after Alessandro Volta, who built the first chemical battery in 1800 (the voltaic pile). The unit was officially adopted by the International Electrical Congress in 1881, before either the joule or the coulomb had been precisely defined; their definitions converged later.
The joules to volts formula
V = J / Q. Voltage in volts equals energy in joules divided by charge in coulombs. The formula reduces to three identities depending on which quantity is unknown. To find voltage, divide energy by charge. To find energy, multiply voltage by charge. To find charge, divide energy by voltage.
V = J / Q J = V × QQ = J / V 1 mAh = 3.6 C1 V = 1 J / 1 C 1 kWh = 3.6 MJJoules to volts in batteries
Batteries store energy and deliver it at a nominal voltage. The capacity rating in milliamp-hours converts to charge in coulombs (multiply mAh by 3.6), and the total energy follows from energy equals voltage times charge. A 2700 mAh AA alkaline cell at 1.5 V holds 2700 times 3.6 = 9720 C, delivering 1.5 times 9720 = 14,580 J or about 4.05 Wh.
The joules to volts relationship explains why high-voltage battery packs are more efficient at delivering large amounts of energy. A 360 V Tesla Model 3 pack and a 12 V car battery can both store 60 kWh of energy, but the Tesla pack does it with one thirtieth the current at any given power level. Lower current means smaller cables, less ohmic loss, and shorter charging time.
- AA alkaline: 1.5 V, 2700 mAh = 9720 C = 14,580 J
- 9 V block: 9 V, 550 mAh = 1980 C = 17,820 J
- 18650 Li-ion: 3.7 V, 3000 mAh = 10,800 C = 39,960 J
- Car SLA: 12 V, 60 Ah = 216,000 C = 2.59 MJ
- EV pack (medium): 360 V, 60 kWh = 600 kC = 216 MJ
Joules to volts in capacitors
Capacitors store energy in an electric field. The relationship there is slightly different: E = 0.5 C V squared, where C is capacitance in farads (not coulombs) and V is voltage across the plates. Rearranging gives V = square root of (2 E divided by C). A 1 F capacitor storing 50 J would charge to sqrt(100) = 10 V.
The factor of one-half matters. The total work done by the source charging the capacitor is Q times V, but only half that energy ends up stored in the capacitor; the other half is dissipated as heat in the charging path, regardless of resistance. This is why capacitor banks are inherently lossy when switched between voltage levels, and why DC-DC converters use inductors rather than capacitor stacking for high-efficiency conversion.
Charge, energy, and voltage
The three quantities are linked through definitions in the 2019 SI system. The coulomb is defined from the elementary charge (1 e = 1.602176634 times 10 to the minus 19 C, exact). The joule descends from the kilogram, meter, and second. The volt is derived as joule per coulomb, or equivalently watt per ampere. None of the definitions involve laboratory artifacts anymore; they all anchor on fixed values of physical constants.
This means joules to volts conversions are exact within the limits of your input precision. A computed voltage carries no SI uncertainty of its own. The only sources of error are measurement precision in the energy and charge values and any rounding you apply along the way.
Joules to volts in everyday electronics
A USB port delivers 5 V at up to 3 A in its standard mode. Over one second of charging, that moves 3 C of charge and delivers 15 J of energy. A USB-C Power Delivery profile at 20 V and 5 A moves the same 5 C per second but delivers 100 J per second, which is why fast charging is feasible only at higher voltages.
Defibrillators highlight the energy-voltage tradeoff. A biphasic external defibrillator delivers 200 J across a patient's chest at about 2000 V peak. The charge transferred is 200 / 2000 = 0.1 C, which flows in roughly 10 milliseconds. The same energy at a household 230 V would require 0.87 C, an order of magnitude more charge, and much longer to deliver.
A common misconception is that joules and volts are interchangeable. They are not. A 9 V battery and a 12 V battery can hold the same energy if the lower-voltage cell has more charge. Sizing a circuit by voltage alone (without considering capacity) is the most common reason a battery dies before expected.
Common joules to volts mistakes
To sanity check a joules-to-volts result, work backwards. Multiply the computed voltage by the charge you entered. The product should equal the energy you started with. If it does not, you have a sign, decimal, or unit error somewhere.
The biggest pitfall is mixing capacitance (farads) with charge (coulombs). Both use C as a symbol but represent different quantities. A 1 farad capacitor at 12 V holds 12 C of charge and 72 J of energy. Plugging 1 F into V equals J over Q gives the wrong answer; plug 12 C instead. Another common slip is forgetting that battery mAh times voltage gives energy in joules only if you also multiply the mAh by 3.6 to convert to coulombs.