Energy Converter

What this calculates

Enter an amount of energy in joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours or British thermal units, and this shows it in all seven at once, with each conversion worked through. It also places the amount on a scale running from 1 J to 1 PJ, in bands a thousand times apart, each shown in kilowatt-hours as well, so the size of an amount can be read at a glance.

It converts an amount of energy as it is. It does not convert power, such as kilowatts, or a change in energy.

The formula

Every conversion goes through joules: into joules with one unit's size, then out again with another's.

Each unit's size comes from its definition.

The calorie here is the thermochemical calorie. The BTU is the International Table BTU, built on the International Table calorie of 4.1868 J, the grams in a pound and the 1.8 degrees Fahrenheit in a kelvin. Every factor is an exact decimal; where a joule figure runs past ten significant figures — 1 BTU is 1,055.05585262 J — the working shows it to ten, marked ≈, while every result is worked out from the exact value.

Worked example

Some conversions come out exact. 1 kcal is exactly 4.184 kJ, so 100 kcal is exactly 418.4 kJ. 1 Wh is exactly 3.6 kJ, and 1 kWh is exactly 3,600,000 J.

When to use it, and the mistakes to avoid

Use it when an amount of energy is given in a unit you do not think in, when comparing a figure in kilowatt-hours with one in BTU or joules, and when moving between kilocalories and kilojoules.

The mistakes that cost the most:

• Mixing up kW and kWh. A kilowatt is power, a rate; a kilowatt-hour is energy, an amount. A steady 1 kW for one hour is 1 kWh, but a kilowatt figure on its own cannot be converted into kilowatt-hours without knowing for how long. This converter takes amounts of energy only.
• Mixing up cal and kcal. A kilocalorie is 1,000 cal. Food labels usually give energy in kilocalories, sometimes written as Calories with a capital C, so reading 250 kcal as 250 cal makes it a thousand times too small: 250 kcal is 1,046 kJ, 250 cal is 1.046 kJ.
• Mixing up Wh and kWh. A kilowatt-hour is 1,000 Wh, so 500 Wh is 0.5 kWh, not 500 kWh. Check which of the two a figure is in before comparing it with another.
• Expecting every digit of every result. Results are shown to seven significant figures — 1 kWh shows as 860.4207 kcal — while each is worked out from the exact value. An amount far too small for a unit, such as 0.000000000000001 cal in kilojoules, kilocalories, watt-hours, kilowatt-hours or BTU, reads as 0, and a note under the result says so rather than leaving a silent zero.
• Entering a negative amount. This version converts an amount of energy as it is and does not convert a change in energy, where a figure below zero would make sense, so it refuses a negative value with a message instead of converting it.

FAQ