Energy production is one of those things that adapts to your circumstance. At the beginning, just sticking a solar or two next to each Mex is sufficient to make sure you're on top of things (As an added bonus - having the solars there by the mex lets them contribute to overdrive right away without setting up connections yet. On the downside, it's metal spent by a frontline constructor that might be better spent on a turret or something... It's a balancing act.
The early game ends when the mexes in each base are claimed and defended, and the battle lines are clear. The mid-game is when players begin to ramp up energy, and connect their grids. Only when your grid is connected does building a larger power plant pay off - the return on investment gets better based on how much metal the power plant cost, but you also don't get any energy until it's finished, and it doesn't count toward overdrive unless your power plant and mexes are connected to the same grid. The best next step for more power is to build a geothermal reactor. If there's no geothermal rift on the map in a defendable location (tap F4 to see where geos are), most players will build a fusion in their base to get that early energy spike, allowing them to repair stuff and feed off of reclaim. Don't go immediately for an advanced geothermal (i.e. click the upgrade button on the geothermal), or a singularity reactor - a stealth raider group or airstrike can probably get to it and destroy it early if you're not careful, causing significant collateral damage and wasting all the resources you put into it (4000 isn't cheap!).
A mid-game alternative, if you're struggling to get energy up but don't have the means to invest in a fusion plant right away, is to go to a high point on the map that provides 1.5+ energy minimum for your wind turbine (click the wind turbine and hover it over the area to see how much it'll return), and build a whole bunch of them packed together. You'll have to defend it well, but it'll give you the energy boost you need to keep going.
You can tell you've hit the late game when players are skirmishing with more than one of their factory's demi-strider units (Grizzly, Crab, Lance, etc.), or when players are beginning to build significant clusters of units (think thug/felon balls, recluse swarms, dominatrix balls, etc.). At this point, you're going to want to build a significant jump in energy to keep up through overdrive. A single singularity reactor provides a significant jump in overdrive, but subsequent ones have significantly reduced returns on investment, so unless you've really got the breathing room, that second batch of 4000 metal might be better used elsewhere.
Now, specifically regarding the turbines question: Turbines work best on maps that are at high altitude. However, they're cheaper, and are much easier to place on steep surfaces, so many players use them as tethers to connect an outlying mex to their main energy production center.
