Power at stepper torque per phase?

I have a stepper motor with specs as attached. its rated as a 10V, 0.5A/Phase NEMA 14

So power = 10V x 0.5A = 5 Watts

But my half stepping on a 24VDC current at 0.5A it is:

power = 24V x 0.5A = 12 Watts

What is the maximum watts here at any given time? Is it 10V when single stepping at 20V when half stepping but it can be ran at 24V?

Edit: It is also being ran by a 10-24V stepper driver at 0.49A RMS current

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I reckon the wattage of the motor is 10 * 0.5 = 5 watts. This is the voltage that will drive 0.5 amps through the coil when the motor is stationary.

Stepper motors need higher voltages to maintain torque at higher speeds. In effect the combination of the stepper driver and the motor act as a buck-converter. The amount of time at 0.5 amps will be shorter at higher speeds because the inductance of the coil means that it takes some time for the current to rise. With a higher voltage the current will rise more quickly towards the limit, but not instantly.

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I have a stepper motor with specs as attached. its rated as a 10V, 0.5A/Phase NEMA 14

"Rated Voltage" is not a stepper parameter. Its a 20 ohm, 0.5A motor. If you want it to go fast with torque you'll
need a lot of supply voltage, perhaps 48V or more, and a stepper driver capable of that voltage. Winding
inductance determines performance and this motor is 13.5mH which is rather high.

A stepper driver is a current source, so the winding dissipation is fixed whatever the supply voltage. However
the higher the voltage of the supply and the lower the winding inductance the faster the current can change and
the faster the motor can turn.

For lower voltage use a much lower winding resistance/inductance would be needed, a few ohms, not 20,
and a few mH. For instance something like 1A 5ohms would be far better for 24V use.

Typical high performance NEMA17 motors are about 1.7A and 1.7ohms, and NEMA34 are ~4A and 0.5ohms. A NEMA14 of similar specification should around 1A 4ohms, and NEMA11 about 0.5A 10 ohms.