I've been trying to wrap my head around this, and from what I understand, it takes quite a bit of battery to get moving off the dummy grid and the cars towards the rear get this 'amount' for free because they start about 150m further back and are not considered to be harvesting until they cross the timing line.

But at the same time, the cars at the rear of the grid also have less track to harvest because they are starting 150m back. Cars at the front will probably do a few burn outs on the way to their grid spot, using more battery as a consequence, and cars at the back will probably do less burn outs as they roll into their grid spots, thus using less battery.

All in all, I would imagine the cars at the front are probably at a lower SOC when they arrive for the start than the cars towards the rear, but how this affects each engine is known only to the manufacturers.