Energy can never be destroyed
It is converted from one form to another. One of the most familiar forms of energy is heat.
When a current (I) is forced through a resistor (R) by applying a potential (V), the electrical energy is converted to heat energy as observed by the rise in temperature of the resistor. Remember that power is the rate at which energy is consumed.
Note: Heat is only produced by a resistive load (electrical friction) and not in an inductive or capacitive load.
The value of the real power (P) consumed by the resistor is:
Power = Voltage x Current (P = V x I)
Since we already know that an applied Voltage (V) is required to force Current (I) through a resistor (R), another way of defining Power is:
Power = (Current)2 x Resistance (P = I2 x R)
Note: Real power only occurs when the magnitude of the voltage and current increase and decrease at exactly the same rate as illustrated below.
At any time in an AC circuit the instantaneous power flow in the product of the voltage and the current. In AC circuits the power flow is not constant, but fluctuates with the voltage and current.
The integral of the power with respect to time is the energy delivered to the load. In a purely resistive circuit the product of voltage and current is always positive. In the course of one complete AC cycle energy is delivered to the load.
Reference: Science and Reactor Fundamentals – Electrical CNSC Technical Training Group