(1.08 x 10⁷ joules) = (1/2) (inductance) (300 Amp)²
(2.16 x10⁷ joules) = (inductance) (300 Amp)²
Inductance = (2.16 x10⁷ joules) / (300 Amp)²
= 2.16 x10⁷ / 90,000 Henrys
I get 240 Henrys .
This is a big inductance. Possibly the size of your house. To get a big inductance, you want to wind the coil with a huge number of turns of very fine wire, in a small space. In this case, however, if you plan on running 300A through your coil, it'll have to be wound with a very thick conductor ... like maybe 1/4-inch solid copper wire, or even copper tubing, You have competing requirements. There are cheaper, easier, better ways to store 3 kWh of energy. In fact, a quick back-of-the-napkin calculation says that 3 or 4 car batteries will do the job nicely.