How to calculate THD using only the first harmonic?
You can't. It's not possible.
The T in THD is for &quot;total&quot;, and the whole point of giving a THD instead of just the power of the first harmonic relative to the fundamental is exactly that most nonlinear systems have behaviour that's complicated enough so that you can't know how much power is in the other harmonics (and to how much total power that sums up) from just the first harmonic.