Energy Calculator Online: Convert kWh, Calories, Joules and BTU (With Real Examples)

Published 1 November 2026 · By Sardar Zain, Founder of Orixiva · 12-minute read

Your electricity bill is in kWh, your gas bill in therms, a food label in kcal or kJ, an air conditioner in BTU and a physics worksheet in joules. They all measure the same thing: energy. An energy calculator online puts them on one scale, so you can compare a meal with a phone charge, or a gas bill with an electric one.

This guide explains each unit in simple English, shows the difference between energy and power, and works through real examples from food, home bills, fuel and science. You can check every result with the free Orixiva energy converter, which is free online, needs no signup or login, and shows all 11 energy units at once.

Energy units explained

The US Energy Information Administration describes a BTU as the heat needed to raise one pound of water by 1 °F, and lists 1 kWh as 3,412 BTU, the same factor the converter uses.

Energy vs power: kWh is not kW

Power is how fast energy is used, measured in watts (W) or kilowatts (kW). Energy is the total amount used, measured in kWh or joules. A 2,000 W (2 kW) kettle running for 3 minutes (0.05 h) uses 2 × 0.05 = 0.1 kWh, which is 360 kJ or 86 kcal.

Air conditioners are a common source of confusion: a “12,000 BTU” unit really means 12,000 BTU per hour, which is power (about 3.5 kW of cooling). In the energy converter, 12,000 BTU of energy is 3.52 kWh.

Flowchart: pick the right energy unit

1. What are you measuring?Food, electricity, gas, fuel or physics
2. Is it food or exercise?
   YesUse kcal or kJNoGo to the next question
3. Is it electricity or a battery?
   YesUse Wh or kWhNo, heating or gasUse BTU, therm or kWh
4. Is it a physics or chemistry problem?Use joules; eV for atoms and photons
5. Convert to comparePut everything in kWh or MJ to compare sources fairly

Food energy: kcal and kJ

Food labels in the US use Calories (kcal); labels in Australia, the EU and many other places show kJ as well. Examples:

• A banana (about 105 kcal) = 439 kJ.
• A 250 kcal snack bar = 1,046 kJ.
• A 2,000 kcal day = 8,368 kJ = 8.37 MJ = 2.32 kWh.

That last line surprises many people: a whole day of food is about the same energy as running a 2 kW heater for just over an hour. For daily energy needs, see BMI vs body fat vs TDEE and the health and fitness calculators.

Home energy: kWh, therms and BTU

Comparing gas and electricity bills

Gas may be billed in therms, cubic metres or kWh; electricity in kWh. Convert gas to kWh to compare:

• 1 therm = 29.31 kWh.
• A 50-therm month = 1,465 kWh of heat energy.
• 100 cubic feet of natural gas (about 103,600 BTU, using EIA’s average) ≈ 30.4 kWh.

Comparing appliances in BTU and kWh

Heaters in the US are sometimes sold by BTU output. A 1,500 W electric heater running for 4 hours uses 6 kWh, which is about 20,500 BTU of heat. Converting both sides to kWh makes a gas heater and an electric heater easy to compare.

Small everyday amounts

• Boiling 1 liter of water from 20 °C takes about 335 kJ (0.093 kWh) of heat.
• A phone battery of about 15 Wh holds 54 kJ, roughly 13 kcal.
• A 60 kWh electric car battery holds 216 MJ, about 51,600 kcal.

Turning energy into cost

Convert everything to the unit your tariff uses, then multiply. A heater using 3 kWh a day at $0.15 per kWh costs $0.45 a day, or about $13.50 a month. A home using 60 therms of gas in a winter month at $1.50 per therm pays $90 for 1,758 kWh of heat.

Heat pumps and efficiency

A heat pump moves heat instead of making it, so it can deliver more heat energy than the electricity it uses. Its coefficient of performance (COP) describes this: with a COP of 3, 1 kWh of electricity gives about 3 kWh of heat. Compare heating options by the cost per kWh of heat delivered, not per kWh bought.

Which units appear on bills?

• Electricity: kWh almost everywhere.
• Gas: therms or hundreds of cubic feet in the US; kWh on many UK and European bills, converted from the cubic metres the meter records.
• Food: kcal in the US; kJ and kcal together in the EU, Australia and many other countries.

Fuels compared in kWh

A US gallon of gasoline holds about 35.2 kWh, about 9.3 kWh per liter. For running costs on the road, see EV vs petrol running cost.

Batteries and solar panels

From mAh to Wh

Phone and power bank batteries are labelled in milliamp-hours (mAh), which measure charge, not energy. To get energy, multiply by the battery voltage:

• A 4,000 mAh phone battery at 3.85 V: 4,000 × 3.85 ÷ 1,000 = 15.4 Wh.
• A 20,000 mAh power bank at 3.7 V: 74 Wh. Airlines usually state carry-on limits in Wh, so this conversion is useful before a flight.

Once you have Wh, the energy converter turns it into kJ, kcal or BTU.

Solar panel output

A panel’s rating in watts is power. Energy depends on sunshine: a 400 W panel that gets the equivalent of 4.5 hours of full sun produces about 0.4 × 4.5 = 1.8 kWh in a day, or about 54 kWh in a 30-day month. Real output varies with location, season, shading and panel angle.

Worked example: which heating fuel is cheapest?

Convert each fuel to cost per kWh of heat, using EIA heat contents. Prices below are examples only; use your own.

Before boiler or heat-pump efficiency. A boiler at 90% efficiency raises the effective cost by about 11%; a heat pump with a COP of 3 cuts the electricity figure to about $0.05.

Science: joules, foot-pounds and electronvolts

• Lifting work: raising 10 kg by 2 m takes mass × g × height = 10 × 9.81 × 2 = 196 J, only 0.047 kcal.
• Imperial work: 1 J = 0.7376 ft·lbf, and 1 ft·lbf = 1.356 J.
• Atomic scale: 1 eV = 1.602 × 10⁻¹⁹ J. 1 kWh is about 2.25 × 10²⁵ eV, so the converter switches to scientific notation automatically.

For work, power and other physics formulas, the electrical and physics calculator includes an Energy, Work & Power tool.

Energy units from smallest to largest

Energy conversion chart

How to convert energy in Excel or Google Sheets

The CONVERT function covers several energy units. In Microsoft’s CONVERT reference, “cal” is the International Table calorie (4.1868 J), while the Orixiva converter uses the thermochemical calorie (4.184 J), the value used for food energy. The difference is under 0.07%.

How to use the free energy converter (step by step)

1. Open the energy converter. Go to the Orixiva energy converter. It opens on 1 kWh to kcal.
2. Type a value or E = P × t. Enter an amount such as 2000, or a power-times-time sum with kWh selected, such as 2*3/60 for a 2 kW kettle running 3 minutes.
3. Choose From and To units. Joule, kilojoule, megajoule, calorie, kilocalorie, watt-hour, kilowatt-hour, BTU, therm, foot-pound force or electronvolt.
4. Read the result. It updates as you type, with both rates (1 kWh = 860.4 kcal and 1 kcal = 0.001162 kWh), the scientific form and full precision.
5. Check the working. “How it was calculated” converts your value into joules first, then into the target unit.
6. Compare all 11 units. The table shows one amount of energy in every unit at once.
7. Save or share. Swap, copy, download TXT or print. Press Convert to keep it in Recent conversions and export as CSV.

The energy calculator is free online, without signup, sign-in or login, and runs entirely in your browser on PC or mobile.

Mistakes to avoid when converting energy

Confusing kW and kWh

kW is power; kWh is energy. A 1 kW heater uses 1 kWh only if it runs for an hour.

Mixing calories and kilocalories

Food “Calories” are kilocalories. A 200-Calorie snack is 200 kcal, or 200,000 small calories.

Treating BTU/h as BTU

Air conditioner and heater ratings in BTU are usually BTU per hour (power). Multiply by hours of use to get energy.

Mixing up the calorie definitions

Thermochemical calories (4.184 J) and International Table calories (4.1868 J) differ slightly. For food and everyday use the difference does not matter, but check which one a scientific source uses if you need many decimal places.

Comparing heat content with useful energy

A gallon of gasoline holds 35 kWh of heat, but a car engine turns only part of it into motion. Compare sources fairly by including efficiency.

Related guides: pressure calculator online, speed calculator online, Celsius to Fahrenheit and how to convert units.