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

Fill in the hours-per-day and rate fields to get this worked out for you. Both start blank, and with no rate entered the calculator shows the energy in kilowatt-hours and stops there rather than guessing at a price. Rates vary enormously by country and by season, so the number on your own bill is the only one worth using. US residential rates sat somewhere around $0.12 to $0.18 per kWh in 2024, but that is a starting point, not a default.

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 is above the 12A continuous limit, so no. Use a 20A dedicated circuit.
• Daily energy use (8 hours): 1,500W × 8h = 12,000 Wh = 12 kWh/day
• At $0.15/kWh that is 12 × 30 × $0.15 = $54/month

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

Resistance is not allowed to be zero

A short circuit has effectively no resistance, and V = IR then demands infinite current. Real hardware answers that by melting something: the wire, the fuse, or the battery. The calculator refuses a resistance of zero rather than printing “Infinity A”, which it used to do, because an infinite figure is not a result. If you are modelling a short, give the path its real resistance instead. Even a length of thick copper has milliohms, and battery internal resistance is usually what actually sets the fault current.