Energy Conversion
BTU and joules
Required Parameters
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Quick Answer
1 Joule = 1 Watt-second. 1 kWh = 3,600,000 J = 3.6 MJ. 1 BTU = 1,055 J. 1 calorie = 4.184 J. Energy conversion is fundamental for comparing electrical, thermal, and mechanical power sources.
Energy Conversion Calculator
Convert between energy units: Joules, kilowatt-hours, calories, BTU, and electron-volts.
Key Conversions
- 1 kWh = 3,600,000 J
- 1 BTU = 1,055.06 J
- 1 cal = 4.184 J
Design Notes
In electronics, energy is most commonly expressed in watt-hours (Wh) for batteries and kWh for mains power. Joules appear in capacitor energy calculations (E = ½CV²) and ESD specifications.
Common Mistakes
- 1
Confusing energy (Joules, kWh) with power (Watts) — power is the RATE of energy transfer.
- 2
Mixing up calories (cal, 4.184 J) with food Calories (Cal = kcal = 4,184 J).
- 3
Comparing battery capacity in mAh across different voltages without converting to Wh first.
Knowledge Base
What is 1 Joule in watt-seconds?
1 Joule = 1 Watt-second exactly. They are the same unit (J = W·s = kg·m²/s²). A watt-hour (Wh) = 3,600 joules. A kilowatt-hour (kWh) = 3,600,000 joules = 3.6 MJ. Your electricity bill is measured in kWh.
How do I convert between kWh and Joules?
1 kWh = 3,600,000 J = 3.6 MJ. To convert: kWh × 3,600,000 = joules. Reverse: joules / 3,600,000 = kWh. Example: a 100Wh battery stores 360,000 J = 360 kJ. A 1MWh grid battery stores 3.6 GJ.
What is a calorie vs a Calorie?
Lowercase calorie (cal) = energy to heat 1g of water by 1°C = 4.184 J. Uppercase Calorie (Cal or kcal) = 1000 calories = 4,184 J = energy unit used for food. A 2000 Cal daily diet = 8.37 MJ. The distinction matters: a candy bar with 250 Cal = 250,000 cal = 1.046 MJ.
How much energy does a capacitor store?
E = ½CV² (in joules). A 1000µF capacitor at 400V stores 80 joules. A supercapacitor (100F at 2.7V) stores 365 joules. For comparison, a AA battery (1.5V, 2500mAh) stores about 13,500 joules — vastly more than any capacitor of similar size.
What is an electron-volt (eV)?
1 eV = 1.602 × 10⁻¹⁹ joules — the energy gained by one electron accelerated through 1 volt. Used in physics, semiconductors, and X-ray/particle physics. Silicon bandgap = 1.12 eV. Visible light photons = 1.65-3.26 eV. X-ray photons = 100 eV to 100 keV.
What is a BTU (British Thermal Unit)?
1 BTU = 1,055.06 joules = energy to heat 1 pound of water by 1°F. Used in HVAC and US heating/cooling specifications. A typical furnace is rated at 80,000-100,000 BTU/hr. Air conditioners: 1 ton of cooling = 12,000 BTU/hr. 1 therm (natural gas) = 100,000 BTU.
How do I compare battery capacity in different units?
Batteries are rated in mAh or Wh. To convert: Wh = mAh × V / 1000. Example: a 3000mAh phone battery at 3.7V nominal = 11.1 Wh = 39,960 J. A 100Ah 12V car battery = 1200 Wh = 4.32 MJ. Always use Wh (not mAh) to compare batteries at different voltages.
What is the difference between energy and power?
Energy (Joules, kWh) is the total work done or stored. Power (Watts) is the rate of energy transfer. Power × Time = Energy. A 100W light bulb running for 10 hours consumes 1 kWh. A 1000W microwave running for 1 minute uses 0.0167 kWh = 60,000 J.
How much energy is in common fuels?
Gasoline: 34.2 MJ/liter (9.5 kWh/L). Diesel: 38.6 MJ/L. Natural gas: 37.3 MJ/m³. Li-ion battery: 0.36-0.95 MJ/kg. Hydrogen: 120 MJ/kg (by mass) but only 8.5 MJ/liter (by volume). Gasoline has roughly 100x the energy density of the best batteries — which is why EVs need large battery packs.
What is a foot-pound (ft-lb)?
1 ft-lb = 1.35582 joules. Used in US mechanical engineering for torque (ft-lbf) and energy (ft-lb). Torque on a bolt: 25 ft-lbs. Muzzle energy of a bullet: 300-1500 ft-lbs. Not to be confused with pound-feet (lb-ft), which is technically the same unit but conventionally used for torque.
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