* World electricity production: 25000TWh/year https://en.wikipedia.org/wiki/Electricity_generation
* Energy per Helium atom: 17.6MeV https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi...
So per year (with a 100% efficiency [1]) you would produce as a side effect 25000TWh/17.6MeV ~= 3E31 Helium atoms.
The Avogadro constant is 6,022E23 and that produces 22.4 liters of gas, so you get 1.2E9 liters, that is 1.2E6 cubic meters. Let's round that to 1 million cubic meters per year.
The worldwide production of Helium is 140 million cubic meter per year https://www.statista.com/statistics/925214/helium-production...
[1] A 100% efficiency is too optimistic. The second law and the nasty details of the real world reduce the efficiency a lot. Let me guess 50% from https://en.wikipedia.org/wiki/Energy_conversion_efficiency#E... But this is good! If you waste half of the produced energy, you need the double of fusion plants, and that means the double of Helium balloons.