Electrical Power: P = VI
Electrical power is the rate of energy transfer, calculated as P = VI, where P is power (watts), V is voltage (volts), and I is current (amperes). One watt is one joule per second. Power can also be calculated using Ohm's law: P = I^2R or P = V^2/R. Power represents how quickly electrical energy is converted to other forms (heat, light, motion). A 100W light bulb consumes 100 joules per second. Understanding power is essential for electrical safety, energy efficiency, and cost calculation.
Power, Energy, and Cost
Power is energy per time (watts), while energy is power multiplied by time (watt-hours or kilowatt-hours). A 1000W (1kW) appliance running for 1 hour uses 1kWh of energy. Electricity bills charge for energy, typically in kWh. At $0.12/kWh, running a 1kW heater for 10 hours costs 10 x 0.12 = $1.20. Home appliances have power ratings: microwave 1000W, refrigerator 150W, LED bulb 10W, laptop 60W. Understanding power helps estimate energy consumption and electricity costs.
Applications and Safety
Power calculations are crucial for electrical safety and design. Circuit breakers limit current to prevent excessive power dissipation causing fires. A 15A, 120V circuit can safely handle 1800W. Exceeding this trips the breaker. Electrical engineers size wires and components for expected power levels. Power ratings indicate heat dissipation-high-power resistors need heat sinks. Renewable energy systems are sized by power needs. Understanding electrical power is essential for household electricity use, solar panel installation, electric vehicle charging, and industrial electrical systems.
Quick Tips
- Always verify units are consistent
- Use scientific notation for very large/small numbers
- Results are approximations — real conditions may vary
Frequently Asked Questions
Power is energy per time (watts = joules/second), measuring the rate of energy use. Energy is power multiplied by time (watt-hours), measuring total consumption. Your utility bill charges for energy (kWh), not power.
A kilowatt-hour (kWh) is a unit of energy: the energy consumed by a 1kW device running for 1 hour, equal to 3.6 million joules. It's the standard unit for electricity billing. A typical home uses 10,000-30,000 kWh/year.
Cost = Power (kW) x Time (hours) x Rate ($/kWh). Example: A 1500W heater running 8 hours daily at $0.12/kWh costs (1.5 kW x 8 hr x 0.12) = $1.44 per day or $43.20/month.
Circuit breakers trip when current exceeds their rating, protecting from fire hazards. Excessive current (from too many devices or a short circuit) causes overheating. A 15A breaker on 120V allows 1800W max; exceeding this trips the breaker.
Heating/cooling (HVAC) typically uses the most, followed by water heaters, major appliances (refrigerator, washer, dryer), and lighting. A central AC might use 3000W, while LED bulbs use only 10W each.