Ohm's Law and Electrical Power Calculator

Give any two of voltage, current, resistance and power and get the other two, plus the energy used and what it costs.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Ohm's law says voltage equals current times resistance. Power is voltage times current, which also equals current squared times resistance, or voltage squared over resistance. Any two of the four quantities fix the other two.

Choose which two you know and enter them. The calculator gives all four, the energy in kilowatt hours over your running time, and the cost at the price you enter.

Formulas used

  • V = I R
  • P = V I = I^2 R = \dfrac{V^2}{R}

Worked example

With the default numbers in the calculator above:

  1. Voltage: 24 V
  2. Current: 2 A
  3. Resistance: 12 Ω
  4. Power: 48 W
  5. Energy over 8 hours: 0.384 kWh
  6. Cost at the assumed price: $0.1152

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Does it work for alternating current?

For resistive loads such as heaters, yes. For motors and anything with a power factor the current is higher than power over voltage. Use the three-phase calculator for those.

Why does cable heat so much at high current?

The heat in a wire goes with the square of the current. Doubling the current gives four times the heat.

Related calculators

Further reading: Standard electrical engineering texts (Ohm's law and electrical power). Electrical work must be done by a licensed person.

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