What this tool does
This tool converts energy values between watt-seconds and eleven other energy units: joules, kilojoules, megajoules, kilowatt-hours, watt-hours, BTUs, calories, kilocalories, foot-pounds, electronvolts, and ergs. You enter a numeric value, select the unit you are converting from, and the tool instantly displays the equivalent value in every supported unit. The conversion works in any direction. If you have a value in BTUs, you can see it expressed in joules, kilowatt-hours, or any other unit, not just watt-seconds.
A watt-second is a derived unit of energy in the International System of Units (SI). It represents the amount of energy transferred when one watt of power is sustained for one second. One watt-second is exactly equal to one joule, making it a convenient bridge between electrical power measurements and the standard SI energy unit.
How it works
The converter uses a single formula applied through a base-unit normalization approach:
E_out = E_in × (factor_from / factor_to)
Where factor_from is the number of watt-seconds per unit of the input, and factor_to is the number of watt-seconds per unit of the output. The tool first converts the input value to watt-seconds, then divides by the target unit's conversion factor.
The conversion factors used are:
1 W·s = 1 J (joule, exact by definition) 1 kJ = 1,000 W·s 1 MJ = 1,000,000 W·s 1 kWh = 3,600,000 W·s 1 Wh = 3,600 W·s 1 BTU = 1,055.06 W·s 1 cal = 4.184 W·s (thermochemical calorie) 1 kcal = 4,184 W·s 1 ft·lbf = 1.35582 W·s 1 eV = 1.602 × 10⁻¹⁹ W·s 1 erg = 1 × 10⁻⁷ W·s
For example, to convert 500 watt-seconds to kilowatt-hours: 500 ÷ 3,600,000 = 0.0001389 kWh. To convert 2 BTUs to calories: first convert to watt-seconds (2 × 1,055.06 = 2,110.12 W·s), then to calories (2,110.12 ÷ 4.184 = 504.31 cal).
Common conversions
Here are some frequently needed watt-second conversions for quick reference:
100 W·s = 100 J = 0.1 kJ = 0.0000278 kWh = 0.0948 BTU = 23.88 cal 1,000 W·s = 1 kJ = 0.000278 kWh = 0.948 BTU = 238.85 cal = 737.56 ft·lbf 3,600 W·s = 1 Wh = 3.6 kJ = 3.412 BTU = 860.4 cal 3,600,000 W·s = 1 kWh = 3,600 kJ = 3.6 MJ = 3,412 BTU = 860,421 cal 1,055.06 W·s = 1 BTU = 1.055 kJ = 252.16 cal = 778.17 ft·lbf
These values come up regularly in electrical engineering, HVAC sizing, nutrition calculations, and physics coursework.
Who should use this
Electrical engineers working with power systems who need to convert between watt-based units and thermal units like BTUs. HVAC technicians converting between electrical energy consumption and heat output. Physics students solving problems that require converting between SI, CGS, and imperial energy units. Nutrition professionals converting between calories and joules for energy content labeling. Photographers and electronics hobbyists calculating flash energy in watt-seconds and comparing to other energy references. Scientists in particle physics converting between electronvolts and macroscopic energy units.
FAQs
Q: Is a watt-second the same as a joule? A: Yes, exactly. One watt-second equals one joule by definition. The watt-second emphasizes the electrical origin of the measurement (power multiplied by time), while the joule is the general SI unit of energy. They are fully interchangeable in all calculations.
Q: Why would I use watt-seconds instead of joules? A: Watt-seconds are commonly used in contexts where power and duration are the natural way to describe energy. Photography flash units are often rated in watt-seconds. Electrical engineers may prefer watt-seconds when discussing energy delivery over specific time intervals. In all other respects, watt-seconds and joules are identical.
Q: What is the difference between a calorie and a kilocalorie? A: A calorie (cal, sometimes called a "small calorie" or "gram calorie") is the energy needed to raise one gram of water by one degree Celsius, equal to about 4.184 joules. A kilocalorie (kcal, sometimes called a "large calorie" or "food Calorie" with a capital C) is 1,000 calories. Nutritional labels in the United States use "Calories" (capital C) to mean kilocalories.
Q: How do I convert watt-seconds to kilowatt-hours? A: Divide the watt-second value by 3,600,000. For example, 7,200 W·s ÷ 3,600,000 = 0.002 kWh. This conversion is useful when comparing small energy events to the kilowatt-hour scale used on electricity bills.
Q: What are electronvolts used for? A: Electronvolts are used in atomic and particle physics. One electronvolt is the kinetic energy gained by an electron accelerating through a potential difference of one volt, equal to about 1.602 × 10⁻¹⁹ joules. Because atomic-scale energies are tiny compared to everyday units, the electronvolt provides a practical scale for nuclear reactions, photon energies, and semiconductor band gaps.
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