Energy Converter — Joules, kWh, Calories & BTU Explained

Energy units are everywhere — and rarely the same one twice

A food label shows calories. Your electricity bill is in kilowatt-hours. A physics textbook uses joules. A heating engineer quotes BTUs. A car's fuel efficiency is measured in kilojoules per kilometre in some markets and miles per gallon in others. These are all measuring the same underlying concept — energy — but in units that don't convert intuitively between each other.

An energy converter handles the translation. Whether you're comparing appliance efficiency, understanding your electricity consumption, analysing nutritional content, or working through an engineering problem, the converter gives you the equivalent in whatever unit you need.

The Main Energy Units

Joule (J)

The SI unit of energy. One joule is the energy transferred when a force of one newton acts through one metre. It's the foundation unit — all others are defined relative to it.

• 1 kilojoule (kJ) = 1,000 joules
• 1 megajoule (MJ) = 1,000,000 joules

Calorie (cal) and Kilocalorie (kcal)

The calorie used in food science is actually the kilocalorie — what food labels call a "Calorie" (capital C). 1 food Calorie = 1 kcal = 4,184 joules. The small calorie (lowercase c) = 4.184 joules. This distinction trips people up constantly.

1 kcal = 4,184 J = 4.184 kJ

Kilowatt-hour (kWh)

The unit on your electricity bill. One kWh is the energy used by a 1,000-watt device running for 1 hour. 1 kWh = 3,600,000 J = 3.6 MJ

BTU (British Thermal Unit)

Common in HVAC, heating, and air conditioning. 1 BTU is the energy to raise 1 pound of water by 1°F. 1 BTU = 1,055 J ≈ 1.055 kJ

Electronvolt (eV)

Used in physics and chemistry for atomic-scale energy. 1 eV = 1.602 × 10⁻¹⁹ J — extremely small, useful for describing energy of particles and photons.

Watt-hour (Wh)

1 Wh = 3,600 J = 3.6 kJ. Used for battery capacities (phone batteries are typically 10–20 Wh; electric car batteries are 40–100 kWh).

How to Use the Energy Converter on sadiqbd.com

1. Enter the energy value — the number you want to convert.
2. Select the source unit — joules, kilocalories, kWh, BTU, etc.
3. Select the target unit — the unit you need.
4. Read the result — instant conversion.

Real-World Examples

Understanding your electricity bill

Your electricity bill shows you used 250 kWh this month. How much energy is that in megajoules?

250 kWh × 3.6 MJ/kWh = 900 MJ

Or in BTU: 250 × 3,412 = 853,000 BTU. These numbers give scale — 900 megajoules is roughly equivalent to burning about 22 litres of diesel fuel.

Food and exercise energy

A meal contains 650 kcal. A 30-minute jog burns approximately 300 kcal. In joules:

• Meal: 650 × 4,184 = 2,719,600 J = 2.72 MJ
• Jog: 300 × 4,184 = 1,255,200 J = 1.26 MJ

The meal provides about twice the energy the jog burns. In kWh terms, the meal = 0.755 kWh — roughly the same energy as running a 100W light bulb for 7.5 hours.

Air conditioner energy consumption

A 1.5-tonne air conditioner is rated at 18,000 BTU/hour. In watts: 18,000 BTU/hr ÷ 3.412 BTU/Wh = 5,275 W = 5.275 kW

Running for 8 hours: 5.275 × 8 = 42.2 kWh

At a Bangladesh electricity rate of approximately ৳8/kWh for residential: 42.2 × 8 = ৳337.6 per day in AC cost. Over 30 days: approximately ৳10,128.

Battery capacity comparison

A laptop battery is rated at 72 Wh. A power bank is rated at 20,000 mAh at 3.7V.

Power bank energy: 20,000 mAh × 3.7V = 74,000 mWh = 74 Wh

The power bank has slightly more energy capacity than the laptop battery. In joules: 74 Wh × 3,600 = 266,400 J = 266.4 kJ.

Energy in Everyday Context

Building intuition for energy scales helps make conversions meaningful:

Tips for Working With Energy Units

Remember: food Calories are kilocalories. The "100 calories" on a snack bar label is 100 kcal = 418,400 joules. If you see a recipe showing energy in kJ (common on Australian and European food labels), divide by 4.184 to get kcal.

Use kWh for anything electrical. Joules are inconveniently small for household electricity — a fridge running for a month uses around 30–50 kWh, which is 108–180 million joules. Always use kWh for appliance and bill calculations.

BTU is for HVAC; kWh is for electricity. When comparing air conditioner specs, BTU/hour is used for cooling capacity. For running cost, convert to kW (watts ÷ 1,000) and multiply by hours to get kWh.

For solar panel output, use kWh. A 300W solar panel in full sun for 5 hours produces 300W × 5h = 1.5 kWh. Annual production estimates are in kWh/year.

Frequently Asked Questions

What's the difference between energy and power? Energy is the total amount of work done or heat transferred. Power is the rate at which energy is used — energy per unit time. A 1,000W (1 kW) appliance running for 1 hour uses 1 kWh of energy. Watts measure power; joules and kWh measure energy.

Why do food labels use kcal but some use kJ? Both are correct — different countries use different conventions. The UK, Australia, and EU often show kJ alongside kcal. The US typically shows only kcal (called "Calories"). 1 kcal = 4.184 kJ.

What is 1 BTU in watts? 1 BTU/hour = 0.293 watts. So a 12,000 BTU/hour air conditioner draws about 3,516 watts = 3.516 kW.

How do I calculate my monthly electricity cost from appliance wattage? Watts × hours per day × 30 ÷ 1,000 = kWh per month. Then multiply by your electricity rate per kWh.

Is the energy converter free? Yes — completely free, no sign-up required.

Energy appears in so many different units across so many different fields that having a quick converter is genuinely useful — not just for engineers, but for anyone comparing electricity bills, evaluating appliance efficiency, or making sense of food nutrition labels.

Try the Energy Converter free at sadiqbd.com — instant conversion between joules, kilocalories, kWh, BTU, and more.