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Free calculator

Use this calculator to

  • Turn the energy per cycle on a machine datasheet (J) into average electrical demand (W)
  • Convert pulse energy from welders, lasers or capacitor banks into the power the supply must deliver
  • Check the average power of any process when you know the total energy released and how long it took
  • Cross-check joule figures against kW ratings: 1 kWh is 3.6 million joules

Joules to Watts Calculator

Convert energy (J) to power (W) over a time period.

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Formulas

  • W = J / t (seconds)

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Watts to Joules

Convert power in watts to energy in joules

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Joules to Volts

Convert energy (J) and charge (C) to voltage

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Volts to Joules

Convert voltage (V) and charge (C) to energy in joules

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Common scenarios

Select one to run it in the calculator above.

For business

Why this matters for businesses

Pulse loads are the part of an industrial load profile that the headline kW figure usually hides. A spot-welding line, an induction furnace charge, or a hydraulic press cycle delivers energy in joules over a fraction of a second, and the peak watts during that window can be ten times the average draw the meter records over a half-hour. That matters for cable sizing, for breaker selection, and for whether the site is paying capacity charges sized for the peak it occasionally hits rather than the load it usually runs.

Demand profiling on this kind of plant is where the engineering and the commercial work meet. A facility that smooths pulse loads with a flywheel UPS or a small behind-the-meter battery can shave the peak the network sees, drop a band on its MIC, and avoid the capacity uplift the DNO would otherwise quote at six-figure connection-charge level. The maths starts with knowing the joules per pulse, the cycle rate, and the watts that the network and the LV switchgear actually have to deliver.

Purely Energy has worked through these conversations on sites where the peak demand profile drives the procurement choice as much as the annual kWh does. Time-of-use tariff selection, capacity charge optimisation, and DSR participation all hang off an accurate load profile, and that profile only holds up if the underlying energy and power numbers are honest. The free tools here are step one of that audit, and the half-hourly data review is step two. By the time the procurement strategy is signed off, the joules-per-pulse number sits in a spreadsheet alongside the capacity charge model and the contract recommendation.

Common questions

What is the difference between joules and watts?

A joule measures energy, the total amount of work done. A watt measures power, the rate at which that energy is delivered: one watt is one joule per second. The two are linked by time, so the same 1,000 joules can mean 1,000 W for one second or 10 W for 100 seconds. This calculator divides energy by time to give average power, W = J / t.

How do I convert joules to watts?

Divide the energy in joules by the time in seconds: W = J / t. For example, a heating element that releases 540,000 joules over a 180 second cycle is delivering 540,000 / 180 = 3,000 W, or 3 kW. The conversion gives an average power across the period; it says nothing about peaks within it.

How many joules are in a kilowatt-hour?

One kilowatt-hour is 3.6 million joules (3.6 MJ). The maths: 1 kW is 1,000 joules per second, and an hour has 3,600 seconds, so 1 kWh = 1,000 x 3,600 = 3,600,000 J. This is why electricity is billed in kWh rather than joules: the joule is far too small a unit for metering whole buildings.

Is a watt-second the same as a joule?

Yes. One watt sustained for one second delivers exactly one joule, so a watt-second and a joule are the same quantity. The pattern scales: a watt-hour is 3,600 joules and a kilowatt-hour is 3.6 million joules. Equipment datasheets sometimes quote stored energy in watt-seconds (flash units, capacitor banks); you can read those figures directly as joules.

When would a business need to convert joules to watts?

Mostly when working from equipment datasheets that quote energy per cycle or per pulse. Examples: converting the joule rating of a welding pulse, laser pulse or capacitor discharge into the average power the supply must deliver, or turning the energy used per machine cycle into a continuous load figure for sizing circuits. Once you have watts, the kW to amps calculator takes you on to current draw and supply capacity.

Related calculators

The conversions that procurement and engineering use alongside this one.

Joules to Watts Calculator | Free UK Tool | Purely Energy