The hitpoint-system in stellar impact revolves around four basic statistics: structure, armor, hull and shields.
As you can see, the healthbar is divided into three parts: a blue bar, which represents your shield, a grey bar, which represents your armor and seperate smaller chunks of the green bar. Armor only takes 1/3 of the damage.
Every time you take so much damage that your health goes below one of the borders you gain a critical system damage. The effect of these damages is as this:
You can reduce the number of critical system damage you are taking by the use of the makeshift repairs skill.
Armor only takes 1/3 of the damage. So a shot that would usually deal 60 damage only cost you 20 structure points. That means that the number of structure points is no clear indication of your total toughness. A cruiser has 1750 structure points. Lets assume for a second that this cruiser would have 0% armor and 0 shields. This way he could take about 9 shots from a small turret. However, the very same cruiser with 100% armor could take 9*3=27 shots.
To get around this problem the community has developed a concept called effective hitpoints. Basically, if a structure point is armored, it counts as three effective hitpoints. This way, damage is damage and weapons that ignore armor simply deal three times the damage. To calculate the number of effective hitpoints use the following formula:
a: armor multiplier 35% = 0.35
An example, for a not-upgraded cruiser:
$ ehp=1750-0.35 \cdot 1750 + 3 \cdot (0.35 \cdot 1750) = 2975 $
a: armor %
u: structure upgrade %
v: armor upgrade %
This brings us to the following effective hitpoints for various upgrade-paths:
However, keep in mind that each point in armor also regenerates an additional 0.1 structure points per second, even under fire.
Shields start recharging after 7 seconds. This can be reduced, however, by the use of the shield derivation device. Shields charge 100 points per second.
There is an external effective hitpoint calculator here: here.