Having played Sins in online multiplayer for over a year-and-a-half, I don't think I've EVER seen anyone build that around a planet.  I'm not sure anyone even thinks about it or considers it at all.  That tech might as well not even be on the research tree.

Even with a starbase, it will still reduce the planet population and hence the taxes.  Also, the threat of the Nova forces the enemy to waste money on starbases.

Prior to Entrenchment it was considered pretty useless.  Now it's a little less useless than it was before.

Isn't that a problem that needs to be addressed?

So you believe that the TEC superweapon in fact hasn't been nerfed by starbases, and nothing needs to be done?

(Note that I am NOT implying that TEC is a weak race.  I'm just noticing aspects of the game that seem a little screwed)

The problem with the Shield Generator would be its position in the tech tree. The structure itself is at Tier 5 Civilian and provides 30% bombing mitigation (last I checked, a Novalith does 3500 dmg, so it gets reduced to 2450 dmg). At Tier 7 is a tech that upgrades the mitigation to 50% and then 80% (so Pirate sieges, which normally do 25 damage, are reduced to 5).

Although it can make planets 5 times tougher (it would take 9 Novalith warheads to bring down a 6k HP planet protected by a SG with no chance to regenerate), its so high up in the Civ tech tree its not worth pursuing in the overwhelming majority of cases.

InfiniteVoid -- are you certain that the Shield Generator will protect against a Novalith strike, or are you just speculating that it would reduce the damage?

So, anyway, how much lower on the tech tree would they need to put it before people started building them and how would it impact the game?

The shield generator is a bit of a problem for me, and I don't think lowering its tier level alone would bring it into the realm of usefulness. 

The shield generator (and the auxiliary government starbase upgrade) both are useful in one and only one situation: where you do not have the ability to stop the enemy from bombarding, and the enemy does not have the ability to destroy the tactical structures in the gravity well.  The case of the novalith notwithstanding, this implies that you have a relatively thick static defense network somewhere in this gravity well, and the enemy fleet is clearly superior to your own.  There's one very important subtext here: this planet is very important for some reason, because you've invested a lot of money in static defense.

The starbase offers a very permanent solution: the enemy can't claim the planet until the starbase is downed, and this means it's the attacker who must come and get the defender, not the other way around.  A shield generator offers a very temporary solution: no matter how upgraded it is, the attacker will eventually break the planet.  In other words, the defender must eventually come after the attacker.  It does nothing but buy a little extra time which may or may not be enough to do something about the situation.  In practice, if five minutes isn't enough time, ten minutes probably isn't either.  Unless you're at a very special stage of the game (for instance, an ally recently knocked out an enemy and a bunch of dominos are about to fall) entrenched battles rarely turn so spontaneously.  Chances are you need a permanent solution. 

That's a big reason to pick a starbase over a shield generator, but there are two other reasons.  The first is that if the enemy cannot take this planet in a reasonable amount of time, a common solution is just to bypass it and move on to the next planet.  A starbase will greatly punish an enemy for doing this, perhaps giving you enough of an edge to beat them in a straight fight.  A shield generator not so much.  The second reason is that the enemy might just decide to rush your defenses and take out the shield generator.  No one in their right mind would attempt to rush a starbase without a massive advantage, so a starbase is also much more difficult to counter.

A shield generator may be cheaper, but it's very hard to compare with a starbase for effect, and for this reason I think that simply tweaking its cost and position in the tech tree is insufficient.  I believe the shield generator needs to be completely retooled so it does something that a starbase cannot offer.  This is very similar to the issue of PJI's and starbases.  Both have a role in punishing people who attempt to bypass, but in different ways.  With a PJI, you get some extra time and an attack of opportunity before the enemy leaves.  With a starbase, the enemy takes significant damage upon arrival in the next well.  While their role is the same, the exact effects are different and this gives the PJI a very real role even when a starbase is already in play.  A shield generator, on the other hand, is just a weak version of the starbase's effect.

Forget the shield generator, its a sily idea anyway

As for the Novalith being useless with the starbase planet-lockdown thingy, i reckon that the big bomb impact should retard the planet in more ways than population growth. The description says massive damage to infrastructure, so lets make it reduce population caps and pretty much any of the planet upgrades. THAT makes it worthwhile even if the SB prevents loss of control

My idea for additional effects of the shield generator:

Allows permits to be issued for city expansion and growth.  Attracts more colonists from the system to live there because of the protection and safety of the shield generator.

ADDED BENEFIT: POPULATION INCREASE.

A few suggestions on the matter of improving the shield:

1. Reduce enemy culture influnce, but provide no bonus to culture.

2. Give it an AOE ability that gives an area of space the mitigation bonus, meaning everything in that space is harder to kill.

3. Give it an offensive ability, firing a beam that pushes ships in a wave.

4. Allow it to target Starbases instead.

5. Finally, let it enhance all the buildings in orbit, giving them better survival ratings.

Either as AdmiralSol says: give it an actual shield equal to the real hp of the planet, or make the planet immune to bombardment until the generator is destroyed. The generator's hp would probably need to be buffed in that case and maybe given a shield of its own.