Agree with ejs’s calculations.
I’ve never been a fan of USB power. My reasoning is that for a given appliance wattage, the lower the voltage, the higher the current. Imho (ejs, correct me?) This has a number of possibly undesirable consequences...
1. As in this case, the USB source may be unable to supply sufficient current.
2. Under fault conditions of a poor (resistive) connection, the heat generated is proportional to current, so a low voltage (USB) solution can sometimes be more of a hazard than a higher voltage, lower current, solution.
3. Higher current applications can lead to a significant voltage drop over the length of a thin USB cable, so you no longer see the full 5V at the far end. This can cause things to just not work.
In practice too, low voltage USB does not necessarily equate to safety. I once managed to trap a fine Apple keyboard USB cable under the stand of a monitor, where it became progressively more chaffed over time. I only noticed it had become chaffed after seeing a few actual sparks and smoke, when it eventually shorted.