Which relation correctly defines energy as power times time, expressed in kilowatt-hours?

Which relation correctly defines energy as power times time, expressed in kilowatt-hours?

• A. Energy equals power times time
• B. Energy equals current times resistance
• C. Energy equals voltage squared over resistance
• D. Energy equals charge times voltage

Answer: (A)

Energy is the amount of work done or energy transferred when power is used over a period. If power is constant, the energy used equals power multiplied by time. Using kilowatts for power and hours for time gives energy in kilowatt-hours, which is a common unit for electricity. For example, running a 1 kW heater for 2 hours uses 2 kWh of energy. Why the other expressions don’t fit: current times resistance gives voltage, not energy. Voltage squared over resistance is a way to compute power, not energy. Charge times voltage represents energy in particular situations where a certain amount of charge moves through a potential difference, but it isn’t the general definition of energy in terms of power multiplied by time.

Energy is the amount of work done or energy transferred when power is used over a period. If power is constant, the energy used equals power multiplied by time. Using kilowatts for power and hours for time gives energy in kilowatt-hours, which is a common unit for electricity. For example, running a 1 kW heater for 2 hours uses 2 kWh of energy.

Why the other expressions don’t fit: current times resistance gives voltage, not energy. Voltage squared over resistance is a way to compute power, not energy. Charge times voltage represents energy in particular situations where a certain amount of charge moves through a potential difference, but it isn’t the general definition of energy in terms of power multiplied by time.