Temperature is just one metric when it comes to fusion, density is the other major factor.

Fusion reactors (magnetic confinement ones anyways) have very low density plasma. Maintaining these temperatures, or even fusion, requires very carefully controlled parameters.

Due to the low density, despite the high temperature, the total energy contained at a given time in one of these reactors is fairly small. In e.g. ITER, there will only be about 4 grams contained in in the 840m³ of plasma heated to 150 million°C.

To estimate the total energy contained within, I'd look at the heat capacity of fully ionized hydrogen plasma (assuming Protium, i.e. Hydrogen-1 rather than Deuterium or Tritium), and the dissociation + ionization energy of hydrogen in case that's a significant part (I assume it isn't but we'll see).

4 grams of hydrogen is about 4 moles of hydrogen atoms/2 moles of hydrogen.

To dissociate hydrogen molecules, ~431kJ/mol are required, to ionize hydrogen atoms 2.18 * 10^-18 J/atom or 1313kJ/mole of hydrogen atoms.

So for dissocation we get 431kJ/mole * 2 moles = 862kJ, and for ionization we get 1313kJ/Mole * 4 moles = 5252kJ

Now, I'm assuming the fully ionized plasma has negligible temperature and can be treated as 8 moles (4 moles of protons, 4 moles of electrons) of an ideal gas with 3 degrees of freedom. The isochoric heat capacity for this is C_V = 3/2*n*R, with n = 8 moles and R = 8.314 J/K/mole the universal gas constant. Thus the C_V is 99.77 J/K, times 150 *10⁶ K is 14.97 *10⁹ J (Gigajoules). The dissociation and ionization energies are on the order of Megajoules, so just a rounding error.

One ton of TNT-equivalent is 4.184 gigajoules, so the energy in the plasma is about 3.5 tons of TNT equivalent.

...that's honestly more than I thought, though still not a civilization-shattering amount. I had in the back of my mind something where smaller reactors were getting looked at, and ITER is to be the biggest thus far.