Science Squabbling

Total energy of a system is conserved. The bits of atoms getting squashed together are in a lower energy than when apart. This reduction in 'binding energy' is released as heat etc... Energy is very much being conservered.

The tricky part is getting them to get together in the first place, a viable fusion reactor needs to use less energy getting them together than is released when together. All energy generation has an 'input' cost.

I got there in the end but thank you, yours was the best explanation.