As the achievement states, you will have to amass a TOTAL output of 999,999 kW. Which would include a lot of playtime/grinding. You don’t have to have the amount held at once.
From playing through the comrades story and by the time you’ve powered all nodes you’ll have approximately 400k+ kW so far.
You can choose to carry on playing and grinding away for the remaining power or it can be bought from a fisherman in old Lestallum. He can be found across the far side of the map from where you arrive, then going down the stairs to the left. He’s fishing on the rocks nearby.
He sells various amount of meteorites, most importantly though;
500,000 kW for 220,000 Gil
100,000 kW for 50,000 Gil
He does sell smaller amounts but buying one of each of these amount just after you’ve powered all nodes will net you the correct amount and the achievement.
If you don’t have the Gil by the time you’ve finished, try selling leftover weapons, parts etc to make up the deficit.