Energy, power and time
Energy is power multiplied by time: E = P × t. One watt for one second is one joule, and one kilowatt for one hour is one kilowatt-hour: 1 kWh = 3,600,000 J.
kW and kWh are not interchangeable. kW is a rate; kWh is an amount. A 2 kW heater running for 3 hours uses 6 kWh. To go back from energy to power you need the time: P = E ÷ t.
Energy, charge and voltage
Voltage is energy per unit charge: V = E ÷ Q, so E = Q × V. Moving one coulomb through one volt takes one joule.
The electronvolt is the energy one elementary charge gains across one volt. Since the 2019 SI redefinition, the elementary charge is exactly 1.602176634 × 10⁻¹⁹ C, so 1 eV = 1.602176634 × 10⁻¹⁹ J exactly. For a single electron or proton, the energy in eV equals the voltage in volts.
Worked examples
Power to energy: 1,500 W for 2 hours is 1,500 × 7,200 s = 10.8 MJ, or 3 kWh.
Charge to energy: a 3,000 mAh battery holds 3,000 × 3.6 = 10,800 C. At 3.7 V that charge carries 10,800 × 3.7 = 39,960 J, about 11.1 Wh.