How do you convert joules per second to watts?

1 joule per second (J/s) is exactly equal to 1 watt (W). To convert, use the simple formula: P(W) = E(J) / t(s). If your value is already in joules per second, the numerical value in watts is the same.

Why are joules per second and watts equivalent?

By definition, power is the rate of energy transfer: P = E / t. The SI unit of energy is the joule (J) and the SI unit of time is the second (s). The watt is defined as 1 joule per second, so 1 W = 1 J/s.

How do I convert joules and time to watts?

First divide energy in joules by time in seconds to get joules per second, then recognize that this value is numerically equal to watts. For example, 600 J used over 30 s gives 600 / 30 = 20 J/s = 20 W.

Can I convert watts back to joules per second?

Yes. Since 1 W = 1 J/s, any power value in watts has the same numerical value in joules per second. For example, 75 W = 75 J/s.

What is the difference between watts and watt-hours?

Watts (W) measure power, which is energy per unit time (J/s). Watt-hours (Wh) measure total energy used over time. To get watt-hours, multiply power in watts by time in hours.

Joules per Second to Watts Converter (J/s to W)

How to convert joules per second to watts

Power is defined as the rate at which energy is transferred or converted:

\( P = \dfrac{E}{t} \)

where:
\( P \) = power in watts (W)
\( E \) = energy in joules (J)
\( t \) = time in seconds (s)

Because of this definition, the watt is exactly equal to one joule per second:

\( 1 \text{ W} = 1 \dfrac{\text{J}}{\text{s}} \)

So if you already have a value in joules per second, the conversion to watts is simply:

\( P(\text{W}) = P\left(\dfrac{\text{J}}{\text{s}}\right) \)

From joules and time to watts

If you know total energy and the time interval, first compute joules per second, then recognize that this is numerically equal to watts:

\( P(\text{W}) = \dfrac{E(\text{J})}{t(\text{s})} \)

Example: A device uses 900 J of energy in 60 s.

Compute joules per second: \( 900 \text{ J} / 60 \text{ s} = 15 \text{ J/s} \)
Convert to watts: \( 15 \text{ J/s} = 15 \text{ W} \)

Worked examples

Example 1 – 120 J/s to watts

Convert 120 joules per second to watts.

Since 1 J/s = 1 W:

\( 120 \dfrac{\text{J}}{\text{s}} = 120 \text{ W} \)

Example 2 – 75 W to joules per second

Convert 75 watts to joules per second.

\( 75 \text{ W} = 75 \dfrac{\text{J}}{\text{s}} \)

Example 3 – From joules and seconds

A small motor does 2,400 J of work in 3 minutes. What is its average power in watts?

Convert time to seconds: 3 min × 60 = 180 s
Compute power: \( P = \dfrac{2400 \text{ J}}{180 \text{ s}} = 13.33\overline{3} \text{ J/s} = 13.33\overline{3} \text{ W} \)

Joules, watts, and watt-hours

It’s easy to mix up power (watts) and energy (joules, watt-hours). Here’s the difference:

Joule (J) – unit of energy.
Watt (W) – unit of power, equal to J/s.
Watt-hour (Wh) – unit of energy, equal to power × time.

To go from watts to joules over a time period:

\( E(\text{J}) = P(\text{W}) \times t(\text{s}) \)

Example: A 60 W bulb running for 10 seconds uses:

\( E = 60 \text{ W} \times 10 \text{ s} = 600 \text{ J} \)

Frequently asked questions

Is there ever a conversion factor other than 1?

No. In SI units, the definition of the watt is exactly 1 joule per second. There is no scaling factor; the numerical value is always the same.

Why use J/s instead of W?

In physics and engineering, expressions like J/s make the relationship between energy and time explicit, which can be helpful in derivations and dimensional analysis. In everyday use, watts are more common.

How do I convert J/s to kilowatts?

First convert J/s to W (same number), then divide by 1,000:

\( P(\text{kW}) = \dfrac{P(\text{J/s})}{1000} \)

Example: 5,000 J/s = 5,000 W = 5 kW.

Joules per Second to Watts Converter (J/s ⇄ W)