Ohm’s law and power calculator
Ohm’s law and the power formula are the calculations electricians do most often: what current will flow through a resistor, how much power it dissipates, what resistance a coil should read. Enter any two values and the calculator works out the other two.
The formulas
| You want | Formulas |
|---|---|
| Voltage | V = I × R · V = P / I · V = √(P × R) |
| Current | I = V / R · I = P / V · I = √(P / R) |
| Resistance | R = V / I · R = V² / P · R = P / I² |
| Power | P = V × I · P = I² × R · P = V² / R |
Control panel examples
- A 24 V relay coil that reads 480 Ω on the multimeter draws 24 / 480 = 0.05 A, that is 50 mA, and 1.2 W. If a PLC output is rated for 0.5 A, it can drive that relay easily.
- A 2 kW, 230 V heating element: I = 2000 / 230 ≈ 8.7 A and R = 230² / 2000 ≈ 26.5 Ω. If it reads OL when measured dead, it is open-circuit; if it reads almost 0, it is shorted.
- A 24 V, 10 A power supply can deliver at most 240 W. Adding up the coils, sensors and the PLC tells you whether it has plenty of margin or is running close to its limit.
Careful with AC
These formulas apply as they are to DC and to purely resistive loads. With motors and coils on AC, power factor and impedance come into play: the resistance you measure with the multimeter on an AC coil is much lower than its real impedance, so the current you would calculate with V / R comes out far higher than the real one.
Typical mistakeMeasuring 30 Ω on the coil of a 230 V AC contactor and concluding that
it draws 230 / 30 ≈ 7.7 A. In operation it draws a fraction of that, because what limits the current is its impedance,
not just its resistance.