What is the relationship between volts, amps, and watts?

Watts = Volts × Amps. This is the Power Law. Knowing any two values lets you calculate the third. Combined with Ohm's Law (Volts = Amps × Resistance), you can solve for all four electrical quantities.

How do I calculate watts from volts and amps?

Multiply volts by amps: Watts = Volts × Amps. Example: 120 volts × 15 amps = 1,800 watts (1.8 kW).

Volts Amps Watts Calculator

Enter any two electrical values — instantly solve for watts, amps, volts, and resistance. Uses Ohm's Law and the Power Law.

Solve for

Voltage?

Current?

Power?

Resistance?

Calculated power

1,800.0 W (1.800 kW)

Voltage120.00 V

Current15.000 A

Power1,800.0 W

Resistance8.000 Ω

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How to Use This Calculator

Select what to solve for

Click one of the four buttons — Watts, Amps, Volts, or Resistance — to choose the unknown value you want to calculate. The corresponding field will be grayed out (calculated automatically), and you fill in the other two known values.

Enter the known values

Fill in any two of the four electrical quantities. The calculator instantly solves for all four based on Ohm's Law and the Power Law. Leave the target field empty — it updates automatically as you type.

Use the presets

Click a scenario button — 120V outlet, solar panel, 48V battery bank, or find resistance — to load pre-filled values for common solar and electrical scenarios. These get you to a working calculation immediately.

The Formula (Electrical Triangle)

Power Law: W = V × A (Watts = Volts × Amps) V = W ÷ A (Volts = Watts ÷ Amps) A = W ÷ V (Amps = Watts ÷ Volts) Ohm's Law: V = A × R (Volts = Amps × Ohms) A = V ÷ R (Amps = Volts ÷ Ohms) R = V ÷ A (Ohms = Volts ÷ Amps) Combined: W = V² ÷ R or W = A² × R

These relationships form the "electrical triangle" — if you know any two values, you can calculate all the others. In solar systems, these formulas are used constantly: sizing charge controllers (amps), calculating panel output (watts), selecting battery voltage (volts), and choosing wire gauge based on resistance and current.

Example

Sizing a charge controller for a solar array

You have four 400W solar panels wired in parallel at 24V. What charge controller current rating do you need?

Solving forAmps

Total power1,600 W (4 × 400W)

System voltage24 V

Result

Current (amps)66.7 A

With 25% safety margin83.3 A

Minimum charge controller80A MPPT controller

A 60A MPPT charge controller would be undersized. The calculation shows you need at least an 80A controller — or consider rewiring to 48V (which halves the current to 33.3A, allowing a smaller, cheaper 40A controller).

FAQ

What is Ohm's Law and why does it matter for solar? ▼

Ohm's Law (V = I × R) describes the relationship between voltage, current, and resistance in electrical circuits. In solar systems, it's used to: size charge controllers (max current from panels), calculate voltage drop in wiring, size fuses and breakers, and verify inverter capacity. Understanding these relationships prevents undersized components and potential fire hazards.

Why is 48V better than 12V for large solar systems? ▼

For the same power output, 48V requires 1/4 the current of 12V (since W = V × A). Lower current means: thinner (cheaper) wire, less heat generated, less voltage drop over long cable runs, and cheaper charge controllers. A 3,000W system at 12V requires 250A of current — requiring 4/0 AWG cable. The same system at 48V only needs 62.5A, manageable with 4 AWG cable. Most residential solar installations today use 48V battery banks.

What is the difference between VA (volt-amps) and watts? ▼

VA (volt-amps) is apparent power — the product of voltage and current. Watts is real power — the actual work done. For DC circuits and purely resistive AC loads, VA = Watts. For inductive loads (motors, transformers), they differ by the power factor (typically 0.8-0.95). Inverter ratings are often in VA to indicate capacity regardless of load type. When sizing inverters, check both the watt rating (for your actual loads) and VA rating (for inductive motor starts).