Watts to Joules: What the Formula Means and How to Use It
Watts measure power, joules measure energy. Here is the formula J = W × t, worked examples for common appliances, and how kWh and joules relate.
Read articleWatts to Joules Converter
We have all been there: staring at a spec sheet trying to figure out what watts and joules actually mean for your battery. Stop stressing. Drop your numbers below, and we'll handle the math.
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- kWh to JoulesTurn the kWh figure on your electricity bill into joules — 1 kWh = 3,600,000 J.Energy
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- Watts to AmpsCalculate amps from watts and volts for DC and AC circuits — I = P ÷ V.Power
- Joules to CaloriesConvert joules into calories and food Calories (kcal) — 1 cal = 4.184 J.Energy
How it works
Tell us what you want to find
Pick your target: Joules, Watts, or Time. We automatically rearrange the formula so you do not have to.
Drop in your numbers
Enter the two values you already know. Switch between units like kW, MJ, or kWh without breaking a sweat.
Get your answer instantly
Boom. Your result is calculated in real time, complete with every unit conversion you could possibly need.
The Math: Watts to Joules
Here is the reality: a watt is just a rate. It is literally one joule per second (1 W = 1 J/s). To figure out the total energy in joules, you just multiply the power (watts) by how long it is running (seconds). The math is actually pretty simple. Total Energy (Joules) = Power (Watts) × Time (Seconds). But human brains do not naturally process massive numbers of joules. That is why your power company bills you in kilowatt-hours (kWh). One kWh is just a massive chunk of joules—3,600,000 of them, to be exact. Need to reverse it? No problem. To find watts, divide joules by seconds. To find time, divide joules by watts.
J = W × t (Joules = Watts × Seconds)
W = J ÷ t (Watts = Joules ÷ Seconds)
t = J ÷ W (Seconds = Joules ÷ Watts)
Cheat Sheet:
1 kWh = 3,600,000 J
1 Wh = 3,600 J
1 kJ = 1,000 J
1 MJ = 1,000,000 JExample
Let's look at the numbers. You have a 1,500 W space heater running for 90 seconds. 1,500 × 90 = 135,000 Joules (or 135 kJ). Not too bad for a cold morning.
Worked examples
| Scenario | Calculation | Result |
|---|---|---|
| LED bulb (10 W for 1 hour) | 10 W × 3,600 s | 36,000 J (36 kJ or 0.01 kWh) |
| Electric kettle (1,500 W for 90 seconds) | 1,500 W × 90 s | 135,000 J (135 kJ or 0.0375 kWh) |
| Solar panel (300 W for 6 hours) | 300 W × 21,600 s | 6,480,000 J (6.48 MJ or 1.8 kWh) |
| EV charger (7.2 kW for 8 hours) | 7,200 W × 28,800 s | 207,360,000 J (207.36 MJ or 57.6 kWh) |
| Laptop charger (45 W for 30 mins) | 45 W × 1,800 s | 81,000 J (81 kJ or 0.0225 kWh) |
Conversion reference
| Power | 1 Second | 1 Minute | 1 Hour | 1 Day |
|---|---|---|---|---|
| 1 W | 1 J | 60 J | 3,600 J | 86,400 J |
| 5 W | 5 J | 300 J | 18,000 J | 432,000 J |
| 10 W | 10 J | 600 J | 36,000 J | 864,000 J |
| 50 W | 50 J | 3,000 J | 180,000 J | 4,320,000 J |
| 100 W | 100 J | 6,000 J | 360,000 J | 8,640,000 J |
| 500 W | 500 J | 30,000 J | 1,800,000 J | 43,200,000 J |
| 1,000 W | 1,000 J | 60,000 J | 3,600,000 J | 86,400,000 J |
| 2,000 W | 2,000 J | 120,000 J | 7,200,000 J | 172,800,000 J |
| 5,000 W | 5,000 J | 300,000 J | 18,000,000 J | 432,000,000 J |
| 10,000 W | 10,000 J | 600,000 J | 36,000,000 J | 864,000,000 J |
Quick facts
- A watt is exactly one joule per second (1 W = 1 J/s).
- Your power company bills in kilowatt-hours (kWh) because joules are too tiny. 1 kWh = 3,600,000 J.
- Running a 60 W lightbulb for just one hour burns through 216,000 joules.
- A single AA battery holds roughly 10,000 joules of energy.
- The joule is the standard SI unit of energy, named after English physicist James Prescott Joule.