Energy should be a very defended building, as they do countless tasks, from overdriving to powering defenses. You should be building energy steadily as the game progresses, and linking the energy to as many mexes as safe as you can.
Greed is good
Always try to get more metal than you currently have (preferably by claiming mexes), and always try to spend your metal as fast as you can. This cannot be overstated: maximize your metal throughput. Make more units, claim more ground, reclaim and mex more, repeat.
Have some more buildpower
Use caretakers for your factory needs, so other mobile constructors can make other things, and as the caretaker has a high nanopower, it is very good for factories on repeat. This allows you to have spare constructors, so you should make them reclaim, expand,repair, make defenses,create more powerplants, or, simply help the constructor make units.
Repair your units
While a unit leaves 40% of its metal cost in wreckage for recycling, a unit that is still alive and can fight is infinitely better than a dead one. Repair units when they are damaged, and if the fight is too strong, retreat. There may be honor in sacrifice, but alive units do more damage in the end.
Scout the enemy
Unlike other RTS games, scouting here is very important, as it stops you wasting units and allows you to prepare for a fight, or kill a small enemy group before they are ready. Using advanced radar or simple radar is a way of scouting,but sight via units is more accurate as it doesn't have radar wobble. Those that cloak (Infiltrator/Flea) are better as they are invisible and can close up on an enemy base and not be spotted.
Energy is a good resource...