edit: fixed
The Burner Generator burns fuel at a constant rate regardless of how much or how little power it is generating. A series of generators will seem to distribute the power needs of the network among the generators evenly, causing all of them to be on even if much less power is needed. But under a certain threshold, they generate free power and won't burn any fuel at all.
For instance: I have twenty burner generators which are all on, and they burn coal at the same rate whether they are hooked up to one radar or three radars. But if I have them running four active inserters, they generate the power for free and never burn any coal. But then if I disconnect ten of the generators while the inserters are running, the other ten fire up and start burning coal at max rate even though the power demand is no more than about 5% of the maximum these can supply.
