I’ve noticed that questions about researching Protoss shield upgrades get asked every once in a while. But there are no definitive explanations on why it isn’t done much. Some say to never get them, or only get them after all the other upgrades have been exhausted. In fact, outside of “when there’s nothing left to upgrade”, I haven’t heard a rational to research them. Which of course makes me wonder – are there any good times/situations to get Protoss shield upgrades?
First, though, I should look at the reasons why they are not gotten often. As I see it, there are three common reasons.
Reason #1 – prohibitive cost
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Denoting price as minerals/gas:
The shield upgrade levels cost 200/200, 300/300, 400/400. The total for all shield levels is 900/900.
The ground weapons, ground armor, and air weapons levels all cost 100/100, 175/175, 250/250. The total for any single tech path is 525/525.
The air armor levels cost 150/150, 225/225, 300/300. The total for all air armor levels is 675/675.
It is clear the shield upgrades are much more expensive than any other upgrade. Purchase all 3 shield upgrades and only Ground Weapons Level 1 (1000/1000 total), or get all 3 ground weapon and all 3 ground armor upgrades for an extra 50/50 (1050/1050 total).
Reason #2 – usefulness
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Most Protoss units already have some amount of armor, so gaining armor upgrades increases that edge. Going from 0 to 1 is less “good” than going from 1 to 2 (and from 2 to 3 and so on). The reason is that the linear change in damage reduction causes a nonlinear change in the percentage of damage reduction. This has been highlighted by others, but I will outline it here.
Take the marine, for example. It does 6 damage. Against Zealot shields, it does the full 6 damage. Against Zealot health points, it does 5 damage (Zealots have 1 armor). That is a 17% damage reduction (as compared to the full damage to shields). Now you have the choice of buying either Ground Armor Level 1 or Shield Level 1. If you buy the shields, you reduce damage to their shields by 17% (from 6 to 5). If you buy the armor, you reduce damage to their health points by 20% (from 5 to 4). Therefore, the armor upgrade is better proportionally.
However, some units break this concept. For the same percentage-based reasoning, any unit which receives more than +1 damage per upgrade (Roach/Thor/Immortal) or does splash damage (Siege Tank/Collossus/Ultralisk) reduces the benefits of armor and shield upgrades in general. Particularly units like the Thor, which are more likely to do overkill than have to shoot an extra shot due to armor concerns. Against these units, damage upgrades are typically better, except where there are specific breaks in the number of attacks needed to kill/be killed.
Important to remember against Terran, the shield upgrade is very fragile and it is the only forge upgrade that can be removed with a spell (Ghost’s EMP Round). In fact, the Ghost can remove all the shields from the following units with 1 EMP: Probe, Zealot, Sentry, Stalker, High Templar, Dark Templar, Observer, Warp Prism, Immortal, Phoenix, Void Ray. If that feels like a lot of units, turns out, it is. The only units that have any shields left are Colossus, Carriers, Archons, and Motherships. So unless you are going heavy on these 4 units (which is very cost prohibitive) or micro High Templar to neutralize Ghosts, shield upgrades can be very risky against a good Terran player who checks what upgrades you are getting. And getting High Templar or Archons can trigger even a poor player into getting Ghosts as an obvious “counter”.
Reason #3 – timing
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Upgrading shields spends time the forge could be using to upgrade something else. In many high level games, which involve timing pushes, those pushes are often based on attacking just as a particular upgrade is finished. Weapons and armor upgrades cost less, so you can start them sooner and have a slightly larger force than you would have with a shield upgrade push.
All that being said, I still think that there are situations when getting the shield upgrade is useful. If you tend to play a balanced Gateway/Stargate style, the Shield upgrade affects both units. In this sense, it is cost effective to get one upgrade that covers both sets of units (not to mention Phoenix and Void Ray have no base armor, so shield upgrades are just as strong as armor upgrades). Even more so if you get units that are good at harassing and/or have strong shields relative to health points. Stalkers, Dark Templar, and Void Rays all fit this description. They aren’t used much, but Carriers can also benefit from shields more than armor. Or specifically, their Interceptors do. Interceptors do not heal when they return to the Carrier. And they have 0 base armor. So upgrading their shields provides the same percentage damage reduction as armor upgrades, while increasing their survivability better as they can regenerate shields while stowed away. And on that same tech path, Motherships, with their huge 350 shields, obviously benefit from additional shield levels. As a bonus, shield upgrades also work on buildings, so using cannons as defense becomes a little safer as well.
Viability in matchups:
PvZ – The Archon, dealing bonus damage to biological units, becomes stronger and can handle Zerglings and Mutalisks better. Additionally, with the strengthening of Protoss air builds, the appeal of getting one 200/200 upgrade that works for ground and air units, rather than separate 100/100 (ground armor) and 150/150 (air armor) defensive upgrades may catch on. Phoenix can survive longer against Mutalisk due to reduced bounce damage, and the ability to kite and recharge shields becomes even better.
PvP – Without focusing on any critical breakpoints, a shield-upgraded Protoss player could do harass attacks, designed to “wound” the other player. By damaging the health of several units then running away, the shield-upgraded player will be better off, even if taking health damage as well. Repeated wounding attacks like this, over time as additional shield upgrades are researched, can be a viable way to pressure with fast units like Stalkers while creating an advantage over armor upgrades.
PvT – Clearly the most dangerous matchup to get shield upgrades. But there are situations where it still makes sense to get them. For example, look at a typical marine-tank Terran. Immortals work well against the tanks, but are fragile against marines. And once their shields are down, they become fragile against the tanks as well. But by strengthening shields, they can last much longer.