Power, Current, and Voltage
Understanding electrical quantities and calculating power in circuits
Electrical Power
The rate of energy transfer in circuits
Current (I) is the rate of flow of electric charge. It's measured in Amperes (A) and calculated using I = Q/t (charge divided by time). One ampere equals one coulomb of charge flowing per second.
Voltage (V), also called potential difference, is the energy transferred per unit charge. It's measured in Volts (V). One volt equals one joule per coulomb (J/C).
Resistance (R) is the opposition to current flow in a circuit. It's measured in Ohms (Ω) and causes a voltage drop across components.
Power (P) is the rate of energy transfer, measured in Watts (W). There are three equivalent formulas:
P = V × I
Voltage × Current
P = I² × R
Current² × Resistance
P = V² / R
Voltage² ÷ Resistance
Use the formula that matches the values you're given. All three give the same result when applied correctly.
Energy (E) is power multiplied by time: E = P × t = V × I × t. Energy is measured in Joules (J) or kilowatt-hours (kWh) for electricity billing.
Efficiency measures how much input energy becomes useful output:
Efficiency = (Useful Output ÷ Total Input) × 100%
No device is 100% efficient—some energy is always "wasted" as heat, sound, or light.
P = V × I
Power
2300.0 W
2.300 kW
Efficiency Calculator
Efficiency = (Useful Output / Total Input) × 100%
Efficiency
75.0%
Wasted: 25.0 W
Current (I)
Question:
A hairdryer has a power rating of 1800 W and operates at 230 V mains voltage. Calculate the current drawn and suggest an appropriate fuse.
Answer:
Using P = V × I, rearrange to find I:
I = P ÷ V = 1800 W ÷ 230 V = 7.83 A
The fuse should be rated just above normal current.
A 13 A fuse would be appropriate (next standard size above 7.83 A).
What is the formula for electrical power?