How much of that would be wasted otherwise though? You want cheap electricity to mine Bitcoin, so any energy that would otherwise be wasted is ideal. Before Bitcoin, some energy was literally just thrown away. With Bitcoin, it can be put to use for a profit instead.
Electricity thrown away is a very very trivial amount compared to the amount Bitcoin uses. And there are still surpluses that are thrown away. Bitcoin doesn't magically tap into the grid and run only when there is a surplus; virtually all miners run it regardless of the current energy price on wholesale markets.
