USB Power Delivery Calculator
USB Power Delivery lets a charger and a device negotiate a voltage and a current limit over USB-C. The power delivered is simply the voltage multiplied by the current, the basic DC power law. Enter the negotiated voltage and current to see the wattage, and add a battery capacity in watt-hours with a conversion efficiency to estimate an idealised charge time. The wattage figure is exact; the charge time is a best-case minimum because real charging tapers near full and conversion is never perfectly efficient.
USB Power Delivery formula
Power (W) = voltage (V) x current (A)
Energy from wall (Wh) = battery (Wh) / (efficiency / 100)
Charge time (hours) = energy from wall (Wh) / power (W)
Charge time (minutes) = charge time (hours) x 60
The power equation P = V x I is the definition of electrical power for a DC supply and needs no empirical figure. Charge time divides the energy the wall must supply (battery energy adjusted for efficiency) by the charging power.
USB-PD context
- USB-PD fixed voltages are 5 V, 9 V, 15 V, 20 V, 28 V, 36 V and 48 V.
- The original ceiling was 100 W (20 V x 5 A); USB-PD 3.1 Extended Power Range reaches 240 W (48 V x 5 A).
- Drawing 5 A needs an electronically marked USB-C cable; unmarked cables cap at 3 A.
- The source advertises power profiles and the sink requests one within the cable's limits.
- Real charge time exceeds the ideal because current tapers near full charge.
USB Power Delivery: frequently asked questions
How is USB Power Delivery wattage calculated?
Power in watts equals voltage in volts times current in amperes (P = V x I). This is the fundamental DC power relationship. A USB-C Power Delivery sink and source negotiate a voltage and a current limit; the maximum power they can exchange is the product of the two. For example, 20 V at 5 A is 100 W.
What are the standard USB-PD voltages?
The USB Power Delivery specification defines fixed supply voltages of 5 V, 9 V, 15 V, 20 V, 28 V, 36 V, and 48 V. The original 100 W ceiling used up to 20 V at 5 A. The Extended Power Range introduced in USB-PD 3.1 raised this to 240 W using up to 48 V at 5 A. Enter the negotiated values to see the result.
How do I work out charge time from wattage?
Charge time in hours is the battery energy in watt-hours divided by the charging power in watts, then divided by an efficiency factor. This calculator divides battery energy by power and by your entered efficiency. Real charging slows near full, so the figure is an idealised minimum rather than an exact prediction.
What does the 5 A current limit require?
Carrying 5 A over a USB-C cable requires an electronically marked (e-marked) cable that declares its current rating to the source. Cables without an e-marker are limited to 3 A. The voltage and wattage you can negotiate therefore depend on both the charger, the device, and the cable.
Why is real charge time longer than the calculated value?
Chargers reduce current as a lithium battery approaches full charge (the constant-voltage phase), conversion is never 100 percent efficient, and some power runs the device while it charges. The efficiency input lets you account for conversion losses, but the tapering near full charge means actual time is always somewhat longer than the ideal.
Official sources
- USB Implementers Forum: USB Power Delivery Specification.
- U.S. National Institute of Standards and Technology: SI units of electric current and power.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.