Early Game Reactors

Like many people, I use my Commander to build more energy. However, unlike any other early game building it could build, the reactors put me negative in energy while constructing them. It takes the construction of some 10 reactors before I can get back above.

The commander itself provides more power then he needs to build. He and the first factory share his 3k input 1.5k/1.5k
You are building too many Fabricators that cost power before you have the generators to fuel them.

Commander Production: 3000
Generator production: 600

Commadner Consumption: 1500
Basic factory consumption: 675
T1 Fabricator consumption: 1000 (+10% for Air fabricator)

I think its your fabbers puling you under.

but it is only happening when building reactors, but when the commander was working on a factory a second earlier and a second after, the energy is fine.

How many engineers and factory's did you have on at the same time while building the power plants?

AFAIK one standard build for start is:
Commander -> 2 bots factories
The first bot factory build 2 fabbers then infinite dox, the second bot factory build infinite dox
With those 2 fabbers: 1 build metal extractors, the other energy plants (infinite)

Then the commander build 2 energy plants, then he build only factories. Then you can start to pull off more fabbers while keeping your eco in a good shape. Try to stay à 100%.

Use the key [CTRL] + your order, to get just 1 fabber in any producing factory.

It's coincidence I would think. Factories stop draining resources when they are in their roll-off phase, so that's probably the reason it seemed to be going up and down.

All fabbers (inc. commander and factories) have their own drain rates for metal and energy, and those rates stay the same whatever they are building.

The comanders power income covers his own consuption + 2 Factories consumption (power wise).
In turn 1 T1 Fabber needs 1.67 T1 Reactors to be powered.

Compared to the free commadner income T1 reactors have a miniscule input (1 Commander = 5 T1 Reactors)
In GW efficieny and power cost reduction techs can furter change up the math.

Note that you do not get less power consumption if you lack the metal to do full building. Power consumption is fixed based on the unit that is doing the building and in no way connected to metal cost.