Ohm's Law Calculator

Enter any two electrical values — voltage, current, resistance, or power — and all four are calculated instantly using Ohm's Law and the power formula.

Enter any two values — all four are calculated

Voltage (V)?

Current (I)?

Resistance (R)?

calculated

Power (P)?

calculated

All four electrical quantities

Voltage120 V

Current10 A

Resistance12 Ω

Power1,200 W

V = 120 V | I = 10 A | R = 12 Ω | P = 1,200 W Check: V × I = 120 × 10 = 1,200 W ✓

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How to Use This Calculator

Enter any two values

Type any two values into the Voltage (V), Current (I), Resistance (R), and Power (P) fields. The calculator instantly solves for all four quantities. The calculated fields turn blue to indicate they were derived, not entered.

The most common use cases

Know voltage and current: Enter V and I to find resistance and power — useful when measuring a circuit with a voltmeter and clamp meter. Know voltage and resistance: Enter V and R to find current and power — useful for understanding how much a load will draw. Know current and resistance: Enter I and R to find voltage drop across a component and its heat dissipation. Know power and voltage: Enter P and V to find current and resistance — the most common way to size solar system components from appliance wattages.

Use for solar system design

Enter your inverter or appliance wattage (P) and system voltage (V) to find the DC current (I) your battery cables and fuses must handle. Enter solar panel Voc (V) and Isc (I) to find panel resistance. Enter charge controller output amps (I) and battery voltage (V) to find charging power (P).

The Ohm's Law Triangle

The triangle diagram: cover the quantity you want to find. If you cover V, you see I × R. If you cover I, you see V ÷ R. If you cover R, you see V ÷ I. The power wheel extends this: P = V × I, and substituting Ohm's Law gives P = I² × R and P = V² ÷ R — allowing power to be calculated from any two of the four quantities.

Example Calculations

Solar system examples

5,000W inverter at 48VI = 5,000 ÷ 48 = 104A DC

100A at 48VP = 48 × 100 = 4,800W

10Ω resistor at 12VI = 12 ÷ 10 = 1.2A; P = 12 × 1.2 = 14.4W

500W load at 2AV = 500 ÷ 2 = 250V; R = 250 ÷ 2 = 125Ω

5Ω resistance at 25WV = √(25×5) = 11.18V; I = 11.18 ÷ 5 = 2.24A

FAQ

What is Ohm's Law and who discovered it? ▼

Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = I × R. It was formulated by German physicist Georg Simon Ohm and published in 1827. Combined with Joule's Law (P = V × I), it forms the complete set of relationships for DC circuit analysis. These four quantities — voltage, current, resistance, and power — are the foundation of all electrical engineering.

Does Ohm's Law apply to AC circuits? ▼

Ohm's Law applies to resistive AC circuits (heaters, incandescent lights, resistors) in the same way as DC. For reactive loads (capacitors, inductors, motors, transformers), AC impedance replaces resistance — impedance includes both resistance and reactance. The relationship V = I × Z (where Z is impedance) still holds in AC, but the current and voltage are no longer in phase, creating a concept called power factor. For solar system DC wiring and resistive loads, Ohm's Law is directly applicable.

What does resistance mean in a solar system? ▼

In solar systems, resistance appears in wiring, connections, fuses, and breakers. Low resistance in these components means less voltage drop and less power wasted as heat. High resistance at connections (from loose terminals, corroded contacts, or undersized wires) creates hot spots that can start fires. Resistance in the load itself (a heater, motor winding, or inverter) determines how much current it draws. The goal in solar wiring design is to minimize parasitic resistance (wiring resistance) while matching load resistance to system voltage for desired current.

Why does increasing voltage reduce current for the same power? ▼

From P = V × I, if P is constant and V increases, I must decrease proportionally. This is why high-power equipment uses higher voltages — a 240V appliance draws half the current of a 120V appliance at the same wattage, allowing smaller wire, smaller breakers, and lower I²R losses. In solar systems, 48V systems have 4× less current than equivalent 12V systems at the same power — wire costs, connection losses, and heat generation are all dramatically reduced. This is why large off-grid systems use 48V and utility solar uses even higher DC bus voltages (600-1,500V).

What is I²R and why does it matter? ▼

I²R is the power dissipated as heat in a resistance. If a wire has 0.01Ω resistance and carries 100A, it dissipates 100² × 0.01 = 100W as heat. Notice the square relationship — doubling current quadruples heat generation. This is why high-current DC wiring in solar systems (battery cables, inverter connections) must be oversized: the heat generated increases as the square of current, while voltage drop only increases linearly. An undersized battery cable doesn't just cause voltage drop — it generates heat proportional to I², potentially causing fires.