If you have ever found the energy on a spec sheet in kilojoules while your battery, charger or electricity bill talks in watt hours, you already know the small annoyance this page solves. Our free kJ to Wh converter turns kilojoules into watt hours, and watt hours back into kilojoules, in one tap, so you can convert energy from kJ to Wh without touching a formula. It then shows what that energy means in practice: how long a device will run, how many mAh a battery would need and what the electricity costs.

Under the calculator you will find the formula, worked examples, ready-made tables, a graph and a plain walkthrough of every button. Use the contents list above if you only need one part.

kJ to Wh in one line

A watt is one joule per second and an hour has 3,600 seconds. So one watt hour is 3,600 joules, which is exactly 3.6 kilojoules. That single fact sits behind every kilojoule to watt hour conversion on this page.

A kilojoule is the smaller unit, so the answer in Wh is always the smaller number: 1,000 kJ is only 277.78 Wh. Going the other way, from watt hours to kilojoules, the number grows: 100 Wh is 360 kJ. Whichever direction you need, this kilojoules to watt-hours converter handles it, and it works as a general energy unit converter too.

How to use this kJ to Wh converter

This kJ to Wh calculator is one card that switches between an input screen and a result screen. That keeps it short on a phone, so you never have to scroll to find your answer. Here is what every part does.

1. Choose a direction. The two buttons at the top read “kJ to Wh” and “Wh to kJ”. Tap the one you need and the highlight slides across. The label above the box and the unit badge beside it change as well, so you always know which unit you are typing.
2. Type your number. The box takes whole numbers, decimals such as 2.5, commas such as 1,500 and scientific notation such as 1e3. A leading plus sign is fine. Negative energy makes no sense here, so you get a short red message instead. The upper limit is 1,000,000,000,000,000.
3. Or tap a preset. The row of buttons under the box fills it for you: 1, 3.6, 100, 500, 1,000 and 3,600 in kJ mode, and 1, 10, 50, 100, 500 and 1,000 in Wh mode.
4. Press Convert. The yellow button, or the Enter key, swaps the input screen for the result screen. Your number shrinks into a thin “You entered” strip with an Edit button, which takes you straight back if you made a typo.
5. Read the result card. The dark panel shows the converted value in large type, a battery graphic that fills according to how much energy you entered, the formula used and a small badge that says Exact or Rounded.
6. Explore the tabs. Under the card, Units, Runtime, Battery and Cost turn the same energy into other useful figures. They are explained below.

Decimal places and the Exact or Rounded badge

The yellow menu in the header sets how many decimals you see: Auto, 0, 2, 4 or 6. Auto shows up to seven significant digits for numbers below a million and two decimals above that, which suits most jobs. Pick 4 dp and 1,000 kJ reads 277.7778 Wh.

The badge is the honest part. Dividing by 3.6 rarely ends neatly, so most kJ to Wh answers are rounded for display and the badge says so. It reads Exact only when the number on screen equals the true value, for example 36 kJ = 10 Wh. Wh to kJ multiplies by 3.6, so those results are usually exact.

The Units tab

Six tiles show the same energy in joules, megajoules, kilowatt hours, milliwatt hours, kilocalories and BTU. Tap a tile to copy that one value with its unit, ready to paste into a document or spreadsheet. It doubles as a quick kJ to kWh converter and kJ to mWh converter.

The Runtime tab

Pick a device from the list (LED bulb 9 W, phone charger 20 W, laptop 65 W, TV 100 W, fridge 150 W, microwave 1,000 W, electric heater 2,000 W or kettle 3,000 W) or type your own wattage. The tool divides your watt hours by the device power and writes the answer as plain time, such as 4 h 16 min. It is an ideal figure for a steady load with no losses, so treat it as an upper limit.

The Battery tab

Choose a nominal voltage (3.7 V cell, 5 V USB, 12 V car or leisure battery, 24 V, 48 V e-bike or solar, or your own) and the tab shows the matching capacity in mAh and Ah. The sum is Wh ÷ V × 1,000. This is what turns the page into a kJ to Wh battery capacity calculator: enter the energy in kJ, add the voltage and read off the capacity.

The Cost tab

Choose a currency (£, $, €, ₹, Rs, AED, C$ or A$), type the price you pay per kWh and the tab shows what that energy costs. The starting value of 0.25 is only a placeholder, so take your real rate from your bill. To see what whole appliances cost to run, try our electricity cost calculator.

Recent history

The last four conversions sit above the Convert button as small chips. Tap one to run it again. They are kept only in your own browser and nothing is sent to us. The small red button at the end of the row clears the lot.

Copy, Swap, Reset and the i button

• Copy puts a line such as 1,000 kJ ≈ 277.7778 Wh on your clipboard.
• Swap flips the direction and feeds the full-precision result back in as the new input, so you can check any answer by running it in reverse.
• Reset clears the box and takes you back to the start. Your recent chips and decimal setting stay as they were.
• The i button opens a short panel with the formulas and the exact factors used. Press Escape or Got it to close it.
• Every button scrolls the page back to the top of the card, which saves you hunting for it on a small screen.

kJ to Wh formula and worked examples

Wondering how to convert kJ to Wh by hand? The kJ to Wh formula is short enough to remember:

Dividing by 3.6 is the same as multiplying by 0.277778, the figure most converter pages quote. The exact value is 1 ÷ 3.6 = 0.2777… with the sevens repeating forever, which is why this calculator keeps full precision and only rounds what it shows you. That is the whole kilojoules to watt hours formula. For kJ to kWh divide by 3,600 instead, and for kJ to mWh multiply by 277.7778.

Worked examples

• 1,000 kJ to Wh: 1,000 ÷ 3.6 = 277.78 Wh, or 0.2778 kWh.
• 1 kJ to Wh: 1 ÷ 3.6 = 0.2778 Wh. So how many watt hours are in a kilojoule? A little over a quarter of one.
• 18 kJ to Wh: 18 ÷ 3.6 = 5 Wh exactly.
• 65 Wh to kJ: for a laptop battery rated at 65 Wh, 65 × 3.6 = 234 kJ.
• A phone battery in kJ: 4,500 mAh at 3.7 V is 4.5 × 3.7 = 16.65 Wh, and 16.65 × 3.6 = 59.94 kJ.
• 7,200 kJ to kWh: 7,200 ÷ 3,600 = 2 kWh.

kJ to Wh conversion table

Use this table when you just need a quick answer for a common value. Figures are rounded to five significant digits. Rows such as 3.6, 18, 36 and 3,600 kJ come out as whole numbers because they are exact multiples of 3.6.

Wh to kJ conversion table

Going from watt hours to kilojoules is plain multiplication by 3.6, so this Wh to kJ converter table is exact except for the kcal column, which is rounded.

A quick graph of kJ against Wh

Because 1 Wh is always 3.6 kJ, the relationship is a perfectly straight line. Double the kilojoules and you double the watt hours. The three marked points are handy landmarks: 1,000 kJ, 1,800 kJ (exactly half a kilowatt hour) and 3,600 kJ (exactly one kWh).

Straight-line conversion from 0 to 3,600 kJ. Read across from any kJ value to find its Wh.

What 1,000 kJ can actually run

1,000 kJ equals 277.78 Wh. That sounds like a lot until you see how fast a heating element eats it: a kettle gets through the lot in under six minutes, while an LED bulb would sip it for more than a day. Bars use a log scale so the short runtimes stay visible, so read the times rather than comparing bar lengths.

Ideal runtimes at constant power with no conversion losses. Real devices cycle on and off and lose some energy as heat, so expect a little less.

Phone, laptop and car batteries in kJ

Battery labels use mAh, Wh or, now and then, kJ. To compare them, get everything into watt hours with Wh = mAh × V ÷ 1,000, then multiply by 3.6 for kilojoules. The table below does that for some common ratings.

Voltages assumed: 3.7 V for the watch, phone and power bank, 1.2 V for the AA cell, 36 V for the e-bike and 12 V for the car. Check the label on your own battery.

If the battery in question sits on your driveway, our EV charging cost calculator turns that energy into pounds.

kJ, kcal and food labels

UK food labels show energy twice, once in kJ and once in kcal. The two are linked by 1 kcal = 4.184 kJ, and since 1 Wh is 3.6 kJ, one kilocalorie is also about 1.1622 Wh. A 2,000 kcal day works out at 8,368 kJ, roughly 2,324 Wh or 2.32 kWh, which is what a 100 W bulb would use in about 23 hours.

Rounded to five significant digits. Kilocalories here are the thermochemical kind used on food labels.

Wh, kWh, MJ and BTU compared

Energy has a surprising number of units, and most of the confusion comes from mixing them up. This table puts the common ones side by side in joules and watt hours.

BTU here is the International Table BTU, exactly 1,055.05585262 J.

For very small or very large scales such as electronvolts, take a look at our joule, eV, MeV, keV and GeV converter.

Mistakes that give the wrong answer

• Mixing up Wh and kWh. One kWh is 1,000 Wh, so a slip here is a thousand-fold error. Check which one your source uses.
• Treating watts as energy. A watt is power, a watt hour is energy. A 65 W charger tells you how fast it works, while 65 Wh is how much it delivers in an hour.
• Comparing kJ with kcal directly. They are different sizes. 1,000 kJ is only about 239 kcal.
• Ignoring voltage when you see mAh. 5,000 mAh means little until you know the voltage: at 3.7 V it is 18.5 Wh, at 12 V it is 60 Wh.
• Using a rounded factor on a big number. Multiplying 1,000,000 kJ by 0.2778 gives 277,800 Wh, but the true figure is 277,777.78 Wh, so you are 22 Wh out. Dividing by 3.6 avoids the problem.
• Expecting lab numbers from real kit. Runtime and battery figures assume perfect efficiency. Chargers, inverters and ageing cells all take a cut.

Frequently asked questions

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