Watts to Joules Calculator
Convert power in watts (W) to energy in joules (J). Enter the power in watts and the time period in seconds, then press Calculate — the energy result appears in joules, multiplied straight from E = P × t. Everything runs locally in your browser.
Enter power in watts
Type the power P in watts (W) — the rate at which energy is used or converted. Zero watts is fine and gives 0 J.
Enter time in seconds
Type the time period t in seconds (s). One hour is 3600 s. Any finite number works — zero included — because the energy simply multiplies power by time.
Press Calculate, read joules
The energy E appears in joules (J), shown to at most ten significant digits. Copy the result to your clipboard, or press Reset to clear everything and start over.
How watts become joules
- What is the watts-to-joules formula?
- The energy E in joules (J) is equal to the power P in watts (W), times the time period t in seconds (s): E(J) = P(W) × t(s). One watt is one joule per second, so energy in joules is power multiplied by how long it flows.
- Worked examples
- A 60 W device running for 120 s uses E = 60 W × 120 s = 7200 J. A 100 W bulb left on for one hour uses E = 100 W × 3600 s = 360000 J. Half a watt for 10 s is 0.5 × 10 = 5 J.
- Is zero allowed?
- Yes. Because E = P × t is a multiplication, zero on either operand is perfectly valid: 0 W × 7 s = 0 J and 60 W × 0 s = 0 J. There is no division, so there is no undefined case — zero power or zero time just means no energy is transferred.
- What about negative values?
- The arithmetic is defined for negative power or negative time (E = −60 W × 120 s = −7200 J), which you can read as energy flowing in the opposite direction of the assumed convention. Two negatives multiply to a positive. Both compute here rather than being rejected.
- How accurate is the result?
- Results are computed in standard double-precision arithmetic and displayed with at most ten significant digits with floating-point noise trimmed, so 1.1 × 3 shows as exactly 3.3. If a product overflows the number range (e.g. 1e308 × 10), the calculator reports that the result is too large to display instead of printing Infinity.
Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Copy result uses your clipboard only. Results are ideal-theory figures from the entered numbers; real metered energy depends on actual load behaviour.