USB-C PD Wattage Calculator
Charger says 100W, cable says nothing, and your device still charges slowly. Enter what your device needs, what your charger can give, and what your cable can carry to find out exactly which part is holding you back. Nothing uploaded.
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-Understanding USB-C Power Delivery and charging bottlenecks
How USB-C Power Delivery (PD) negotiates wattage
USB-C Power Delivery is a smart charging protocol where the charger and device negotiate the highest safe voltage and current combination they both support. Wattage is the product of voltage times amperage: a 20V charger at 5A delivers 100W (20V * 5A). The USB-C PD specification defines voltage profiles: Standard Power Range (SPR) caps at 20V, while Extended Power Range (EPR) allows up to 48V for higher-power devices. A charger's maximum wattage rating represents its ceiling, but the actual wattage delivered depends on three factors: what the device requests, what the charger can supply, and what the cable can safely carry. For example, a 240W charger connected to a device needing 65W through a 60W cable will only deliver 60W.
Cable ratings and the bottleneck
USB-C cables are rated by amperage, not wattage. Standard non-marked cables support 3A (60W at 20V SPR), e-marked 5A cables support 100W (5A * 20V), and EPR cables support 240W (5A * 48V). The cable can become the bottleneck - using a cheap 3A cable with a 240W charger limits delivery to 60W, regardless of the charger's or device's capability. Higher amperage is also more important for longer cable runs (over 2 meters), where voltage drop becomes significant. Always check your cable's e-mark label or documentation to know its true amperage rating.
FAQ
Why does my 100W charger charge my phone slowly?
The bottleneck is likely your cable or your phone's supported charging rate. If your phone only supports 30W fast charging, a 100W charger will only deliver up to 30W to that phone. If your cable is a standard 3A type, it caps at 60W regardless of charger or device capability. Upgrade to an e-marked cable and check your phone's official fast-charging specification to improve charging speed.
Do I need an EPR cable for an EPR charger?
Not always, but you should for maximum performance. If your device supports EPR charging (common on high-end laptops), an EPR cable (5A, 240W) is necessary to unlock full 240W delivery. Using a 5A non-EPR cable will cap you at 100W. If your device only supports SPR (20V max), an e-marked 5A cable is sufficient, and you do not need EPR.
How do I identify my cable's amperage rating?
Look for an e-mark label printed on the cable connector or body. It will show "3A", "5A", "240W", or similar. If there is no label, the cable is likely a standard 3A type. Thicker cables are sometimes (but not always) higher amperage. When in doubt, buy a certified e-marked 5A cable from a reputable manufacturer.