Shield generators are among the most dangerous units in the game. They're fast, they can move over land and water. They're relatively cheap.
They don't have weapons. But that doesn't matter. Their use as a force multiplier exceeds any other land unit.
I will be discussing these units primarily in a UEF context and especially the T2 mobile shield generator (MSG). but the same logic applies to any static or mobile shield generator.
The most obvious feature of the shield generator is the fact that it instantly adds a huge amount of health to units under its umbrella. In the case of the UEF mobile unit, the hit point increase is 4000. Compare that to the health of the Pillar which is 1200. The shield generator also comes out substantially faster and cheaper than the Pillar-- let's call it 25% each.
Consider an engagement composed of two Pillars on team Red vs one Pillar and one MSG on team Blue. The Pillar's damage per second (DPS) rate is 53. It will kill an enemy Pillar in 22 seconds. Two, focused, will kill an enemy pillar in 11 seconds, with both alive, one at half health.
With a shield generator involved, the picture is substantially different. It will take 36 seconds for the two Red Pillars to chew through the 4000 HP health outlay of the MSG, not taking regeneration into account (see below.) But they will never get there. Instead, one Red pillar will be dead at 22 seconds, and the other Red pillar will not be able to destroy either the shield generator or the Blue pillar before it is killed, leaving both the Pillar and MSG unscathed on team Blue. This alone would make the MSG an incredible unit, easily worth its cost.
But the situation is even more extreme. The MSG has a standing regeneration rate of +55 per second. That's roughly the DPS of one Pillar. After one Red Pillar is dead, the second will be doing no damage to the shield and the shield will still be regenerating, albeit very slowly.
But the situation is even MORE EXTREME than that!
After its initial health of 4000 is exceeded, the MSG has a downtime, then enters a super regen phase of +167 per second! It will be back to its full health in 24 seconds. Though its shield does not function during that regeneration period, this means that multiple MSGs have a very interesting property.
They cover for each other during that crazy tank regen period.
Think about that for a second. Let's say you have four of them on team Blue, and four Red Pillars. The Pillars have a combined DPS of 212, versus an initial shield health of 16000. This means that it would again take 75 seconds for Red to wear through the Blue shields.
But now, once they get through that first shield, that first MSG will be protected under the shield umbrellas of three other MSGs for a total of 9000 health, which would take the three Pillars 42 seconds to wear down.
Unfortunately, they will never get there. After 24 seconds, the first MSG will be recharged to full shield health. And this example does not even take the +55/second normal regen rate which if spread across all 4 shields would nullify all 4 Pillars.
But none of this takes into account the REALLY extreme advantage of the MSG.
Let me introduce the Law of Diminishing Firepower or Lanchester's Law.
When two multiple-unit groups are involved in a battle, the DPS on either side is not constant but instead decreases over time as units are eliminated. This is why advantage for a larger force snowballs: not only does the total health of the smaller force decrease but the DPS of the force decreases.
This is the reason why it's usually better to focus fire, destroying one unit at a time, rather than spreading out the firepower. The quicker a unit dies, the faster the aggregate DPS of that side decreases.
Units with high health are not subject to diminishing firepower in the same way as units of lower health. As an example, take a Percival. It has roughly the same DPS and health of eight Pillars. It also has roughly the same build time and cost.
But it will win a fight with those eight pillars every single time. Why? Every four seconds it will kill a Pillar. It would take all eight Pillars 21 seconds to kill the Percival. In that time it has destroyed five Pillars, cutting the DPS of the Pillars by more than 50%. Because they are subject to diminishing firepower, the DPS of the Pillars is decreasing while the Percival's DPS stays the same, meaning that the Percival can easily handle its weight in Pillars, at an 8x numerical superiority: facing one pillar at a time there's just no contest.
Here's the crux: MSG units make normal units immune to the effects of diminishing firepower. Six Blue Pillars and two MSGs has somewhat less firepower and far more health than a Percival. The eight Red Pillars will never get through both shields.
Now let's bring micro into the picture. A good micro player will be dancing his protected units in and out of the coverage of the shields. He will also be cycling depleted shields to the back of his formation and cycling forward new ones. He will keep a logistics line of MSGs running to the front line-- even if you only use them once and they are never allowed to regen, they are something like potions, and represent battlefield supply logistics in a way that is not commonly appreciated.
Because of the regen rate, in some situations it's better to let your MSGs deplete rather than cycling them out prior to depletion. That means that you are getting more usage out of the crazy depleted regen rate. But only do that if you have multiple MSGs so that they can cover for each other during the refractory period before the shield regenerates, otherwise it's better to retreat them when the shield gets low. On the other hand, if you have enough MSGs and you are able to spread incoming damage between them, shield damage to any given unit will be trivial and easily taken care of by the normal shield regen rate.
MSGs are also very useful in siege defense in combination with static shield structures. They will typically only stop direct fire weapons from reaching the static shield, but they will stop high-powered, low-rate-of-fire shots without a problem. They will also easily allow the shield structure to regenerate. T2 shields have far higher regen rates when depleted, meaning that when under fire an MSG right next to the shield structure can save the day, meaning that attacking units have to overcome a +600/second tank regen!
MSGs are indispensable for artillery operations. In combination with T3 artillery, they vastly extend the lifespan of the artillery vehicles, covering them while deploying and undeploying and essentially making counterbattery fire useless-- one MSG can take SIX mobile T3 artillery shots, and again the protected battery will always win. By raw metal costs you could have SIX MSGs per T3 mobile artillery, which would make your battery next to invulnerable to counterbattery fire. If you add fire-and-maneuver, your battery will be unstoppable by a more expensive battery with no MSGs!
MSGs also synergize with Titans in a predictable way, giving the Titans somewhere to retire while still staying in the fight. Firepower is the primary advantage of Titans and they should not be allowed to be destroyed. Much better to cycle them back under MSG shields where they can continue to contribute DPS to the fight. Then when their shields have recharged, run them out again to take some of the incoming DPS load off of your MSGs.
MSGs are very good for raiding groups. They will keep even a small T1 or T2 force alive much longer than it has any right to be, meaning that response teams going after these raiding groups will need to be disproportionately large, taking more units away from his main battlefield presence. Since most people don't use MSGs, response groups may end up being too small to win engagements against even small raiding groups.
Lastly, MSGs can be used to support battlefield engineer operations like reclaim, repair and building static defense lines after a battle or even under enemy fire, something that would be much harder without MSGs. Cover your engineers with MSGs and let them build firebases directly behind the front lines! Or have them build dragontooth revetments in front of the MSGs in the middle of a fight! When your engineer group is supported by MSGs and effectively invulnerable, you will be much more likely to use them for battlefield reclaim, so the MSGs will have a compound economic effect in addition to simple force preservation, because reclaim that you get is reclaim that your opponent won't get.
There are a couple of negligible weaknesses to MSG units.
First, it's possible to run under the shield and shoot them up because they have low health. This can be avoided by micro, by using them in combination with other units (like you should be!) and by using dragonteeth around the MSG.
Second, they take power. At -100, power requirement for MSGs seems nontrivial. It takes one T2 PGen to power five of them. But when you compare that to what you get in return, the cost is negligible. Would you rather build protected PGens or constantly have to replenish your dead units, in addition to the cost of sending them all the way to the front?
Third, your generators become a valuable center of gravity for the shielded units. Make your PGens unappealing targets by shielding them and giving them flak and tacmissile defenses.
If you are using shielded units correctly, your gains in battles will gigantically offset the above disadvantages. He will have to constantly use his factories to replenish force health that you get at negligible cost out of your power pool, he will not have access to the crazy battlefield regen, AND his groups will be subject to Lanchester's law and yours will not!
Mobile shield generators are just shockingly overlooked. In my opinion they are the single most overpowered unit I have ever seen in ANY RTS GAME, and I have been playing them since the very beginning. I think a lot of people don't actually understand how they work. If they did, they would use them much more than they do.
- MSGs offer unbelievable health for the price. Nothing else comes remotely close. For the same metal cost as a Percival you can have 40,000 HP, and that's RAW HP, to say nothing of tank regen HP. This means that you should do everything within reason to put a shield between armor and incoming damage.
- At range, shield generators are essentially invulnerable to damage from units at the same cost. A combat group containing shield units has a gigantic advantage over units of the same cost without shield generators, to such an extent that the group with shields will nearly always win.
- To save on power, turn the shields off on MSGs that are not at the front line or are in a reserve force.
- Except in specific situations, the MSG should generally be the first unit out of a T2 factory. Even if under attack, it can shield engineers while they put up T1 point defenses. If under rush from a small number of T1 land units the Pillar should be the first unit out.
- If you have a T1 army already, the MSG should UNQUESTIONABLY be the first unit out. Two of them will turn a reasonably sized T1 army into a Percival with a high rate of fire and therefore much lower overkill than the Percival will have on an opposing T1 army.
- Always, ALWAYS fight under shield generators if you can help it. And do your best to make sure! After all of the above, it's plain to see that to avoid them is simply foolish.
- Always build sufficient power. As illustrated above, the metal cost of the power plants is effectively negligible compared to the cost of rebuilding destroyed units.
- Come on, after all this you are arguing? A few pillars and a couple of MSGs are a pocket Percival at T2!!!