Calculators · Electrician
Ohm's Law and Power Calculator
Short answer
Ohm's law says voltage equals current times resistance (E = I × R), so resistance is volts divided by amps. Power in a resistive DC circuit is volts times amps (P = E × I). For 120 V at 10 A, resistance is 12 Ω and power is 1,200 W.
- Resistance (R)
- 12 Ω
- Power (P)
- 1,200 W
Formula
R = E ÷ I · P = E × I
Worked example
A 120 V heater draws 10 A. R = 120 ÷ 10 = 12 Ω, and P = 120 × 10 = 1,200 W.
Before you rely on it
- P = E × I holds for resistive loads. For AC motor and inductive loads, real power also depends on power factor (P = E × I × PF).
- Exam calculations usually let you use a basic calculator; practise rearranging E = I × R without one.
Studying for the Red Seal Electrician (309A) exam? Try 30 free practice questions or read the exam guide.