Shield Emitters are a module in the ships that emit a shield around the ship they are placed on. They can deflect and absorb as much power as their rating allows them to. What this means that if a shot is the same frequency and same amount of energy as the Shield emitter emits it will go right through the shield causing damage. This is why Ion based weaponry is so dangerous as they can change frequency to attack their enemies.
200 gW Shield Emitters can last in combat for up to 1 hour of sustained power. These are found on smaller vessels.
250 gW Shield Emitters are interestingly more powerful and they when paired with a higher rating of shield emitters will cause them to power up however they will drain power from the stronger shields.
300 gW Shield Emitters are standard on Heavy cruisers but there is enough space on them for a 500 gW shield Emitter.
500 gW Shield Emitter are the most powerful shielding available They can sustain the shields at optimal power for up to 2 days of sustained power.
Note: When it says hours or days it requires the ship to not be in offensive so all weaponry cannot be used or the shield will not last. As a design mistake found in all ships the weaponry and shielding are on the same power plant.