Chapter 5: Electric Power and Energy

 

Defining Power in Electrical Circuits

Electric power is the rate at which electrical energy is converted into another form of energy (heat, light, mechanical work, etc.). The unit of power is the Watt (W), defined as one joule per second: P = W / t

In electrical terms, power is the product of voltage and current: P = V × I. Using Ohm's Law substitutions, power can also be expressed as: P = I² × R (power dissipated as heat) and P = V² / R.

Every electronic component has a power rating — the maximum power it can continuously dissipate without damage. Exceeding this rating causes overheating and eventual failure. A standard ¼ watt resistor, for example, should never be asked to dissipate more than 250 mW.

 

Electrical Energy and the Kilowatt-Hour

Energy is power multiplied by time: W = P × t. In electronics, energy is often measured in joules (J). In domestic electricity billing, the practical unit is the kilowatt-hour (kWh) — the energy consumed by a 1 kW device running for 1 hour.

1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J = 3.6 MJ

Understanding power and energy is critical when designing battery-powered systems (where energy determines runtime), when selecting components (to avoid thermal damage), and when designing power supplies and regulators.

 

Power Dissipation and Thermal Management

When current flows through a resistance, power is dissipated as heat. This is known as Joule heating or resistive heating. The power dissipated equals I²R watts, and must be managed to prevent component temperatures from exceeding their safe limits.

In high-power applications, heat sinks, thermal paste, fans, and in extreme cases, liquid cooling systems are used to remove heat from power components. Transistors, voltage regulators, and power resistors all require careful thermal design. The thermal resistance (θ_JA, junction-to-ambient) is a key parameter found on component datasheets.

 

Efficiency in Electronic Systems

No electronic system is 100% efficient. Power lost to heat represents waste. Efficiency (η) is defined as: η = (P_out / P_in) × 100%. A power supply delivering 45W while drawing 50W from the mains has an efficiency of 90%.

Maximizing efficiency is critical in battery-powered devices, renewable energy systems, and high-power converters. Modern switching power supplies achieve efficiencies above 90% compared to the 50-70% efficiency of traditional linear regulators.