Wattage Calculator

What Wattage Really Means

Wattage is a measure of power. Power is the rate at which energy is used, delivered, or converted. In electrical systems, wattage tells you how quickly electrical energy is turning into something else: heat in a heater or toaster, light in a lamp, motion in a motor, sound in a speaker, or stored chemical energy in a battery charger. When people say a device “uses 100 watts,” they mean it is consuming energy at a rate of 100 joules per second, because one watt equals one joule per second.

The reason wattage matters is practical. Power connects directly to heat, supply sizing, and cost. Higher wattage generally means more heat to manage, larger wire sizes or breakers to consider, and a higher electricity bill for the same operating time. Even if you are not building a circuit, wattage helps you compare devices and understand what “1000 W” on a label implies about current draw and energy usage.

Watts vs Watt-Hours and kWh

A common source of confusion is mixing power with energy. Watts (W) are power. Watt-hours (Wh) and kilowatt-hours (kWh) are energy over time. Energy is what gets billed by electricity providers. If a device uses 1000 W continuously for one hour, it uses 1000 Wh, which is the same as 1 kWh. If it runs for half an hour, it uses 0.5 kWh. If it runs for 10 hours, it uses 10 kWh.

This difference explains why a high-wattage device can still be cheap to run if it runs rarely, and why a low-wattage device can become expensive if it runs all day. A 2000 W kettle might run for only a few minutes per day, while a 150 W fan might run for many hours. The Energy & Cost tab in this tool converts your wattage into daily and monthly kWh so you can compare real-world usage patterns instead of relying only on the nameplate number.

The Core Electrical Relationships

Most wattage calculations start from a simple relationship between voltage and current. In a DC circuit, or in an AC circuit that behaves like a purely resistive load, real power is approximately:

Power (W) = Voltage (V) × Current (A)

That formula is intuitive: voltage is like electrical “push,” current is how much charge flows, and power is how much energy is moved per second. But real-world AC loads often include inductance and capacitance (motors, transformers, power supplies). In those cases, not all current contributes to useful work. That is where power factor comes in.

Why AC Loads Need Power Factor

In AC systems, the current can be out of phase with the voltage. Some of the current is associated with storing and releasing energy in electric and magnetic fields rather than converting energy into heat or work. The result is that the product of the RMS voltage and RMS current (called apparent power, measured in volt-amps or VA) can be larger than the real power measured in watts. The relationship is described by power factor (PF), a number between 0 and 1 for most common loads.

For single-phase AC, real power is:

Real Power (W) = V × I × PF

Apparent power is:

Apparent Power (VA) = V × I

If PF is 0.8, you can think of it as “only 80% of VA shows up as real watts.” That does not mean the other 20% is wasted as heat in the device, but it does mean the wiring and supply still have to carry the extra current, which can increase losses and heating in cables and transformers. In many consumer bills, you pay for kWh (real power over time). In commercial or industrial settings, power factor can also affect billing or demand charges, which is why PF correction is a topic in larger installations.

Three-Phase Power in Practical Terms

Three-phase systems are common in industrial environments and larger equipment because they can deliver power more smoothly and efficiently. The most common real power relationship (using line-to-line voltage) is:

Real Power (W) = √3 × V × I × PF

In this calculator, the three-phase option assumes your voltage input is the typical line-to-line value shown on a nameplate (for example, 400 V or 415 V). If you are not sure whether a system is single-phase or three-phase, check the supply type, the number of hot conductors, or the equipment label. Using the wrong phase model can lead to current estimates that are significantly off.

Real Power, Apparent Power, and Reactive Power

AC power is often described as three related quantities. Real power (W) is what does useful work or becomes heat. Apparent power (VA) is the raw voltage-current product. Reactive power (VAR) is associated with energy that oscillates back and forth in fields. These are connected by a “power triangle,” where:

• VA is the hypotenuse (overall electrical loading)
• W is the horizontal component (usable power)
• VAR is the vertical component (reactive component)

You do not always need reactive power for everyday planning, but it helps explain why a motor can draw a lot of current without showing the same wattage increase you would expect from a heater. This calculator reports an estimated reactive component when PF is provided, so you can see whether your situation looks strongly reactive or mostly resistive.

Efficiency: Input Power vs Output Power

Efficiency describes how much input power becomes useful output power. A resistive heater is often close to 100% efficient at converting electrical energy into heat, but motors and power supplies can be far from perfect. A motor might deliver 750 W of mechanical output while consuming 900 W of electrical input, meaning its efficiency is about 83%. A power supply might deliver 60 W to electronics while consuming 75 W from the wall, meaning some power becomes heat inside the supply.

When you are sizing a supply or estimating a bill, the important quantity is usually the input power. When you are rating “delivered power,” output matters. That is why the Amps and Volts tabs let you specify whether the watts you entered represent input real power or output power. If you choose output power, the calculator divides by efficiency to estimate the input watts before calculating current or voltage.

How to Use the Watts Tab

The Watts tab answers the classic question: “I have volts and amps, how many watts is that?” Start by choosing whether your system is DC, single-phase AC, or three-phase AC. Enter voltage and current, then set PF and efficiency if they apply. For DC or simple resistive loads, PF can be left near 1.00 and efficiency near 100% if you just want a straightforward conversion.

The output includes real power (watts), apparent power (VA), reactive estimate (VAR), and an estimated output power using your efficiency setting. You also get an energy-per-hour value in kWh, which is a quick bridge between “watts right now” and “cost over time.”

How to Use the Amps Tab

The Amps tab is for sizing current draw: “My device is X watts at Y volts, how many amps will it draw?” This is the most common sizing question for wiring and breaker planning. Choose the system type, enter watts and voltage, then enter PF and efficiency if needed. The calculator returns an estimated current, plus the implied apparent power and kWh per hour.

For AC loads with PF below 1, the current can be noticeably higher than a naïve DC calculation would suggest. That difference matters because current is what heats conductors and affects breaker sizing. A device with low PF can “feel bigger” to the electrical infrastructure than its wattage alone implies.

How to Use the Volts Tab

The Volts tab is useful when you know power and current and want to infer the voltage level that would produce it. This comes up in battery planning, inverter sizing, and checking whether a certain current limit makes sense for a given power target. Enter the watts, current, system type, and PF/efficiency when relevant. The output includes the computed voltage and a reality check using apparent power.

Estimating Electricity Cost from Wattage

Electricity bills are based on energy, not instantaneous power. The Energy & Cost tab starts with a power value, multiplies by hours per day and the number of days in your period, and converts the result into kWh. Then it multiplies by your tariff (price per kWh) to estimate cost. This is a planning estimate, not a billing statement, because real tariffs can include tiers, taxes, demand charges, fixed service fees, and time-of-use pricing.

The most important part of cost estimation is using a realistic average power and realistic hours. Many appliances cycle. A refrigerator does not run at its peak wattage continuously; it turns on and off. Air conditioners vary by temperature and thermostat settings. Chargers draw less power once a battery is near full. If you use a nameplate wattage, you may want to treat it as an upper bound unless you know the device runs continuously at that level.

Common Real-World Scenarios

Wattage calculations show up everywhere, not just in lab circuits:

• Home appliances: converting a wattage label into amps to understand whether an outlet or extension cord is appropriate.
• Solar and batteries: estimating how long a battery can run a load and how big an inverter must be.
• Motors and pumps: using PF and efficiency to estimate the real input draw rather than assuming watts equal volts times amps.
• Servers and electronics: translating power supply ratings into energy usage for monthly cost planning.
• Workshop tools: checking whether a tool’s startup and running load are compatible with a circuit.

In all cases, you can get more accurate estimates by using measured values when possible. If you have a plug-in power meter, it often reports real watts and sometimes PF directly. Those numbers will typically match your bill more closely than voltage-times-current estimates for complex loads.

Important Limits and Safety Notes

This calculator is for planning and estimation. Real systems have details that can change results: voltage can vary, current draw changes with load, motors have startup surges, PF is not constant for some devices, and efficiency can change with speed and temperature. If you are working near the limits of wiring, breakers, or power supplies, treat the result as a starting point and confirm with datasheets and measurements.

Also remember that current and heat are closely connected. Even if wattage looks reasonable, high current can create hot connections, voltage drop, or nuisance breaker trips. If your estimate suggests unusually high current, consider whether you selected the correct voltage level, PF, phase type, and whether the device power is input or output.

Tips for Better Estimates

• Use real watts when you can: real power is what maps to kWh billing.
• Include power factor for AC motors and large supplies: PF can significantly change current.
• Use efficiency when power is “output rated”: motors and inverters often list output power.
• Think in kWh for cost: multiply by realistic hours, not worst-case nameplate time.
• Allow margin: design and safety decisions usually benefit from headroom.