Introduction
A watt is a unit of power that tells you how much energy a device uses each second. When you build a PC or work with electrical circuits, you need to know the wattage so you can pick the right power supply or stay safe. This wattage calculator helps you do both.
Use the PC Build Estimator to add up the power draw of your CPU, GPU, storage, RAM, and fans. The tool then tells you what size power supply unit (PSU) you need, shows a full wattage breakdown, and estimates your monthly electricity cost. Use the Electrical Wattage tab to calculate watts from voltage and current, or find amps from watts and volts. It supports DC, single-phase AC, and three-phase AC circuits. Every calculation includes a step-by-step solution so you can see exactly how the answer was found.
How to Use Our Wattage Calculator
This wattage calculator has two modes. The PC Build Estimator takes your computer parts and tells you how many watts your system needs and what size power supply to buy. The Electrical Wattage mode solves for watts or amps using voltage, current, and resistance values you provide.
PC Build Estimator
Number of CPUs: Pick how many processors your build has. Most desktops use 1. Workstations and servers may use 2.
CPU Manufacturer, Socket, and Model: Choose the brand first, then the socket type, then the exact chip. The wattage fills in based on the model you pick.
Number of GPUs: Pick how many graphics cards you plan to install, from 1 to 4.
GPU Manufacturer and Model: Choose the brand first, then the exact card. The wattage fills in based on the model you pick.
SSDs: Enter how many solid-state drives are in your build. Each one adds about 5 watts.
HDDs: Enter how many mechanical hard drives you have. Each one adds about 8 watts.
Case Fans: Enter how many cooling fans are in your case. Each one adds about 3 watts.
RAM Sticks: Enter how many memory modules you have. Each one adds about 5 watts.
Optical Drives: Enter how many DVD or Blu-ray drives you have. Each one adds about 20 watts.
USB / PCIe Peripherals: Enter how many extra USB-powered or add-in card devices you use. Each one adds about 10 watts.
PSU Form Factor: Pick ATX for standard desktop cases or SFX for small form factor builds. SFX power supplies have lower maximum wattage options.
Electricity Rate: Type the price you pay per kilowatt-hour. You can find this on your electric bill. The default is $0.13/kWh.
Daily Usage: Enter how many hours per day you use your PC. This is used to estimate your monthly energy cost.
Press Calculate to see your total system wattage, recommended PSU size, safety status, and estimated monthly electricity cost.
Electrical Wattage: Calculate Watts
Current Type: Choose DC, AC Single Phase, or AC Three Phase. This controls which power formula the calculator uses.
Power Factor: This field appears for AC circuits only. Enter a value between 0.01 and 1.00. Use 1.00 for simple resistive loads like heaters.
Voltage Type: This field appears for three-phase AC only. Choose line-to-line or line-to-neutral based on how your voltage is measured.
Voltage, Current, and Resistance: Fill in any two of these three fields. The calculator detects the right formula and solves for power in watts. Use the dropdown next to each field to change the unit. This follows the same relationships described by Ohm's law, which ties voltage, current, and resistance together.
Output Unit: Choose the unit for your result, such as μW, mW, W, kW, or MW.
Press Calculate to see the power in watts along with a full step-by-step solution.
Electrical Wattage: Calculate Amps
Current Type: Choose DC, AC Single Phase, or AC Three Phase. This sets the correct formula for solving current.
Power Factor: This appears for AC circuits only. Enter a value between 0.01 and 1.00.
Voltage Type: This appears for three-phase AC only. Choose line-to-line or line-to-neutral.
Power: Enter the power value and pick its unit from the dropdown.
Voltage: Enter the voltage value and pick its unit from the dropdown.
Output Unit: Choose the unit for your current result, such as μA, mA, A, kA, or MA.
Press Calculate to see the current in amps along with a full step-by-step solution.
What Is a Wattage Calculator?
A wattage calculator helps you figure out how much electric power a device or system uses. Power is measured in watts (W). One watt is the amount of energy used per second. The more watts something needs, the more electricity it draws.
How Watts Are Calculated
The basic formula for power is P = V × I, which means watts equal volts times amps. Volts (V) measure how hard electricity is pushed through a wire. Amps (A) measure how much electricity flows. When you multiply the two, you get the total power in watts. If you know the resistance in ohms (Ω) instead, you can also use P = V² ÷ R or P = I² × R. All three formulas give you the same answer, so you just pick the one that matches the values you have. These relationships all come from Ohm's law.
AC vs. DC Power
DC (direct current) flows in one direction, like in a battery. AC (alternating current) switches direction many times per second and is what comes out of your wall outlets. AC calculations include an extra number called the power factor (PF), which shows how efficiently the current is used. A power factor of 1 means no energy is wasted. Most real devices have a power factor between 0.8 and 1. Three-phase AC, common in large buildings and industrial equipment, adds a √3 (about 1.732) multiplier for line-to-line voltage.
PC Power Supply Sizing
Every part inside a computer (the CPU, GPU, RAM, drives, and fans) uses a certain number of watts. To find the right power supply unit (PSU), you add up the wattage of every component and then add 20% extra headroom. This buffer keeps the PSU from running at full load, which helps it last longer, stay cooler, and run quieter. You then round up to the nearest standard PSU size, such as 550W, 650W, or 750W.
What Is 80 Plus Efficiency?
An 80 Plus rating tells you how efficient a power supply is at turning AC wall power into DC power for your PC. An 80 Plus Bronze PSU wastes less energy as heat than a basic unit, while Platinum and Titanium waste even less. A more efficient PSU saves money on your electricity bill over time.
Why Wattage Matters
Knowing your wattage helps you in two big ways. First, it lets you pick the right-sized power supply or circuit so nothing overloads. An overloaded PSU or circuit can overheat or shut down. When wiring circuits, make sure to check voltage drop over long cable runs and use the right wire size for the current being carried. Second, it helps you estimate your electricity cost. Your electric company charges by the kilowatt-hour (kWh), which is 1,000 watts used for one hour. If you know your total watts, how many hours you run your devices each day, and your electricity rate, you can calculate your monthly bill.