Wattage Calculator

Wattage calculator for volts, amps, and resistance

Wattage is electrical power. It tells you how quickly electrical energy is being used or delivered. In real life, wattage helps you size power supplies, choose cables, estimate load on a circuit, compare appliances, and sanity-check electrical measurements. If you have ever looked at a device label that says something like “230 V, 2 A” or “12 V, 60 W”, you have already seen the variables this calculator uses.

This wattage calculator supports the three most common ways to calculate power (P) depending on what you know. If you know voltage (V) and current (I), use P = V × I. If you know voltage (V) and resistance (R), use P = V² ÷ R. If you know current (I) and resistance (R), use P = I² × R. The calculator also converts the result to kilowatts (kW) so you can compare against circuit ratings and appliance specs.

Pick the method that matches your situation. For household appliances and basic circuit load checks, volts and amps are often easiest because those values are commonly listed on labels or measured with a meter. For resistive components (heaters, resistors, some simple loads), volts and resistance can be more natural. For cases where you measure current through a component and know its resistance, amps and resistance can be the fastest way to estimate power.

After you enter your values and calculate, you will see wattage in W and kW, plus the formula used. Use this output to compare loads, check whether a power supply is undersized, or estimate whether a circuit may be overloaded. Keep in mind that electrical systems are not always ideal. Power factor, efficiency losses, and real-world operating conditions can make actual power different from the ideal math result. This calculator is meant for practical estimates and quick checks, not a replacement for design standards or professional inspection.

Assumptions and how to use this calculator

• This calculator uses ideal DC or purely resistive AC relationships (no power factor or reactive components).
• Enter positive values only. If your measured values vary, use a typical operating value rather than a brief peak.
• Units are volts (V), amps (A), ohms (Ω), watts (W), and kilowatts (kW). kW is shown as W ÷ 1000.
• If you are working on mains electricity, treat calculations as estimates and follow local electrical codes and safety practices.
• For motor loads, switch-mode power supplies, and other non-resistive loads, real power can differ due to efficiency and power factor.

Common questions

What is the difference between watts and kilowatts?

A watt (W) is a unit of power. A kilowatt (kW) is 1,000 watts. Utility bills and larger equipment often use kW because the numbers are easier to read. For example, 2,000 W is the same as 2.0 kW.

Can I calculate watts from volts and amps for AC power?

You can estimate power using P = V × I, but for many AC loads the real power depends on power factor. If the load is mostly resistive (like a simple heater), the estimate is close. For motors and many electronics, the real power can be lower than V × I.

Why does the calculator offer three different formulas?

Power can be expressed using different known values. If you know voltage and current, use V × I. If you know resistance instead of current, you can use V² ÷ R. If you know current and resistance, you can use I² × R. They are all consistent forms of the same relationships.

How do I use this to size a power supply?

Calculate the expected wattage of your load, then choose a power supply with a higher continuous rating than the calculated value. Leave margin for startup surges, efficiency losses, and temperature. A conservative approach is to add headroom rather than aiming for an exact match.

What if my resistance changes with temperature?

Many real components change resistance as they heat up. If you use V² ÷ R or I² × R, your wattage estimate will only be as accurate as the resistance value you entered. If you expect temperature change, use typical operating resistance or measure under normal conditions.