Skip to content
RFrftools.io
Convert

Energy Unit Converter

Convert energy between joules, millijoules, electron-volts, kilowatt-hours, calories, and BTU.

Loading calculator...

Formula

1kWh=3.6MJ,1eV=1.602×10−19J1 kWh = 3.6 MJ, 1 eV = 1.602 × 10⁻¹⁹ J

How It Works

This calculator converts between joules, electron-volts, watt-hours, kilowatt-hours, calories, and BTU for electronics engineers, power systems designers, and physicists. Per SI Brochure (BIPM), the joule is the SI unit of energy: 1 J = 1 W·s = 1 kg·m^2/s^2. Key conversion factors: 1 eV = 1.602176634 × 10^-19 J exactly (2019 SI redefinition), 1 kWh = 3.6 MJ exactly, 1 cal = 4.184 J exactly (thermochemical), 1 BTU = 1055.06 J. Energy spans 25 orders of magnitude: electron-volts for bandgap (Si = 1.12 eV, GaN = 3.4 eV), millijoules for ESD pulses (0.5-3 mJ per JEDEC), joules for capacitor discharge, and kilowatt-hours for battery capacity (smartphone 10-15 Wh, EV 50-100 kWh).

Worked Example

Problem

A 600 nm red LED has a 1.8 V forward voltage drop. Verify the photon energy matches the electrical input and calculate wall-plug efficiency at 20 mA.

Solution
  1. Photon energy: E = hc/lambda = (6.626e-34 × 3e8) / 600e-9 = 3.31e-19 J
  2. Convert to eV: 3.31e-19 / 1.602e-19 = 2.07 eV (per NIST: E_eV = 1240/lambda_nm)
  3. Electrical energy per electron: E = qV = 1.602e-19 × 1.8 = 2.88e-19 J = 1.8 eV
  4. Quantum deficit: 2.07 - 1.8 = 0.27 eV (photon needs more energy than electrical input - thermal assist)
  5. Optical power (if 100% IQE): P_opt = I × E_photon/q = 0.02 × 2.07 = 41.4 mW
  6. Electrical power: P_elec = I × V = 0.02 × 1.8 = 36 mW
  7. Wait - photon energy > electrical input means thermal energy assists emission (real efficiency ~20-40%)

Practical Tips

  • ✓Battery capacity conversion per IEEE 1725: Wh = mAh × V / 1000. A 3000 mAh battery at 3.7 V = 11.1 Wh = 39.96 kJ. Self-discharge: Li-ion 2-3%/month, NiMH 15-30%/month per IEC 61960
  • ✓ESD energy per JEDEC JESD22-A114: HBM at 2 kV = 0.2 mJ, at 8 kV = 3.2 mJ (E = 0.5CV^2 with C = 100 pF). Low energy, but 7.5 A peak current causes localized damage. CDM (charged device model) has even higher current, lower energy
  • ✓Capacitor energy: E = 0.5CV^2. A 100 uF cap at 50 V stores 125 mJ - enough to cause injury. Per OSHA guidelines: > 1 J stored energy requires safety interlocks. Supercaps (3000 F at 2.7 V) store 10.9 kJ = 3 Wh

Common Mistakes

  • ✗Confusing energy (joules) with power (watts) - power is energy per unit time: P = E/t. A 100 W device running for 1 hour consumes 100 Wh = 360 kJ = 0.1 kWh. Always track both quantities in power budgets
  • ✗Treating eV as a voltage - eV is energy (1.602e-19 J), not voltage. A 1.12 eV bandgap means 1.12 eV energy per electron-hole pair, requiring minimum 1.12 V to forward-bias a silicon LED
  • ✗Using wrong calorie definition - thermochemical calorie = 4.184 J exactly (per NIST), but 15 C calorie = 4.1855 J, and food Calorie (kcal) = 4184 J. Always verify which calorie definition applies

Frequently Asked Questions

Per NIST SP 811: thermochemical calorie (cal) = 4.184 J exactly, raises 1 g water by 1 C. Food Calorie (Cal, kcal) = 1000 cal = 4184 J. Electronics thermal calculations use joules directly: 1 W dissipated for 1 s = 1 J of heat. To convert thermal ratings: 1 C/W = 1 K/W = 0.239 cal/(s·W).
Per NIST: 1 eV = energy gained by 1 electron accelerated through 1 V = 1.602176634e-19 J (exact). Semiconductor bandgaps are eV-scale: Si = 1.12 eV, GaAs = 1.42 eV, GaN = 3.4 eV. Photon energy E = hc/lambda = 1240/lambda_nm eV. At 1.12 eV, Si absorbs light < 1107 nm (infrared cutoff).
Per JEDEC JESD22-A114: HBM (100 pF, 1.5 kohm) at 8 kV contains E = 0.5 × 100e-12 × 8000^2 = 3.2 mJ. CDM (4-30 pF, < 10 ohm) at 1 kV contains 2-15 uJ but delivers 5-15 A peak in < 1 ns. Damage is from current density (10^6 A/cm^2 in gate oxide), not total energy.
Energy (J) = mAh × 3.6 × V_nominal. Example: 3000 mAh at 3.7 V = 3000 × 3.6 × 3.7 / 1000 = 39.96 kJ = 11.1 Wh. Derivation: 1 mAh = 0.001 A × 3600 s = 3.6 C (coulombs), and E = Q × V. For Li-ion, V varies 3.0-4.2 V during discharge - use 3.7 V nominal per IEC 61960.

Related Calculators

General

Capacitor Energy

Calculate capacitor energy (E = 1/2·CV²), stored charge (Q = CV), and average charging current. Enter capacitance in μF and voltage — get energy in mJ, charge in mC, and power instantly.

General

Inductor Energy

Calculate energy stored in an inductor, L/R time constant, and current rise time

Power

Battery Life

Estimate battery runtime for IoT and portable devices given average current draw, duty cycle, self-discharge rate, and depth-of-discharge cutoff. Suitable for LiPo, alkaline, NiMH, and coin-cell batteries.

Convert

Frequency ↔ Wavelength

Convert frequency to wavelength instantly: 150 MHz = 2.0 m, 2.4 GHz = 12.5 cm, 5.8 GHz = 5.2 cm. Calculates full, half, and quarter wavelengths for any medium — antenna design, coax, and RF planning.