Zero? A cup of water doesn't typically contain tritium.

You can fuse anything lighter than iron and it will release energy. The process with tritium is just easier. Stars can extract the nuclear potential energy from hydrogen and oxygen just fine.

I'd love to see someone fusing water at sea level.

The oxygen would be the hard part. You'd want a temperature of about 2 billion degrees Kelvin and have it compressed to a density of about 4 million kg/ml.

That's a lot of activation energy, but it takes money to make money. Err, takes energy to make energy.

> activation energy

Which was the very point of my example.