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About this tool
Calculate charger wattage, cable limits, battery charging time, USB-C PD profile fit, and safe power recommendations.
Charging power is simply volts times amps, and the speed you actually get is the lowest of three ceilings: what the charger is rated for, what the cable can carry, and what the device will accept. This calculator multiplies your charger's V x A label, clips that figure to your cable's current rating and the device's own power limit, applies a conversion-loss factor (86% by default) and then estimates how long it will take to move the battery from its current percentage to your target. It also names the USB Power Delivery profile the voltage corresponds to and flags heat risk when you sit near the device ceiling.
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Under Device Battery enter 'Battery capacity' with its 'Capacity unit', 'Battery nominal voltage' and 'Device max accepted watts'.
Under Charger Label enter 'Voltage (V)', 'Current (A)', 'Cable current limit (A)' and 'Efficiency (%)', then drag the Start battery and Target battery sliders.
Read Rated Output, After Cable Limit, Usable Power and Charge Time with the detected USB-C PD profile, then use 'Copy Summary' or export CSV as charger-wattage-calculator.csv.
Rated, cable-limited, device-accepted and usable wattage are broken out so you can see which link is holding you back.
Time above 80% is penalised, because lithium cells slow down in constant-voltage mode instead of charging linearly.
Reads the voltage as 5V standard, 9V/12V fast charge, 15V, 20V PD or Extended Power Range so you can match it to your device's supported profiles.
Multiply the output voltage by the output current: a charger labelled 9V x 2A is 18W, and 20V x 3.25A is 65W. If the label lists several profiles, the charger's headline wattage is the highest V x A pair it advertises, but only one profile runs at a time.
Yes. A standard USB-C cable is rated for 3A, and only cables with an e-marker chip carry 5A, so a 100W (20V x 5A) charger paired with a 3A cable delivers at most 60W. That is why the calculator caps current at the cable rating before working out usable power.
Because two real-world effects are applied on top. Conversion loss means only about 86% of the wall-side wattage becomes stored energy, and charging past 80% enters a tapering phase where current is throttled, so the last stretch of a charge takes disproportionately longer than the first.
No, under USB Power Delivery the device negotiates the voltage and current it wants, so a 65W charger will supply only the 5W a pair of earbuds asks for. The practical concern is heat rather than over-supply: sustained charging near the device's own accepted limit, especially above roughly 32C ambient, is what ages a battery faster.
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