Electrical Power Calculator (Ohm's Law + Power Wheel)

Ohm’s Law and electrical power are the two fundamental equation sets for analyzing any DC (and many AC) electrical circuits. Together they let you calculate voltage, current, resistance, and power from any two known quantities.

Ohm’s Law: V = I × R

Power formulas (derived from Ohm’s Law): P = V × I P = I² × R P = V² ÷ R

Variable definitions:

• V: voltage (volts, V), electrical “pressure” driving current through a circuit
• I: current (amperes, A), rate of electron flow; from the French “intensité”
• R: resistance (ohms, Ω), opposition to current flow
• P: power (watts, W), rate of energy conversion (heat, light, motion)

The Ohm’s Law wheel (all derived relationships): From V, I, R, P, any two known values give you the other two:

• V = IR = P/I = √(P×R)
• I = V/R = P/V = √(P/R)
• R = V/I = V²/P = P/I²
• P = VI = I²R = V²/R

Energy consumption: Energy (Wh) = Power (W) × Time (hours) Energy (kWh) = Wh ÷ 1,000 Electricity cost = kWh × Cost per kWh

Typical US residential electricity: $0.12–$0.18/kWh

Safety reference — fuse/breaker sizing: Never exceed 80% of a circuit’s rated capacity continuously:

• 15A circuit: max continuous load = 12A (1,440W at 120V)
• 20A circuit: max = 16A (1,920W at 120V)
• 30A circuit: max = 24A (5,760W at 240V)

Worked example: An electric heater is rated 1,500W and operates on a 120V household circuit.

• Current draw: I = P ÷ V = 1,500 ÷ 120 = 12.5A
• Resistance: R = V ÷ I = 120 ÷ 12.5 = 9.6Ω
• Can a 15A circuit handle it? 12.5A < 12A continuous limit → borderline: use a 20A dedicated circuit
• Daily energy use (8 hours): 1,500W × 8h = 12,000 Wh = 12 kWh/day
• Monthly cost: 12 × 30 × $0.15 = $54/month

This is why space heaters are among the costliest appliances to run continuously.