Power compacts obviously need their own specific power compact ballast so you cant plug them into a T5 fixture. They have 2 different pin configs. One is 4 straight pin the other is 4 square pin.. The lights only plug in one side.. Here is a picture.. Notice the bottom bulb doesnt have a plug ? Thats because the plug is on the otherside and looks like the one on top.
(note the one on top i cut the 4 square ping plug off and wired the 4 wires into the 4 straight pin pink bulb.. Because they dont have pink plant bulbs in the square pin config so i had to mod the plug a little.)
I've actually converted a 55's 24 inch T8 hood into power compact hoods.. I had a spare duel 65w power compact ballast (seperate from the fixture in the pics above) I just took the t8 bulb out.. Unscrewd the white reflector and i snipped the wires removing the T8 ballast and T8 plugs where you twist the light into you know ? And i wired the 65W duel PC ballast into the Hoods button to turn the lights on and off.. And i used an old computer monitors 3 pin power cable and connected the ballast wires and screwd the white flector back on with the ballast inside.. Then I got zip ties to hold the PC light inside the hood, and i did this to both of my T8 hoods so now my 92G tank has 4X 65W power compact lights. And since each bulb is split in two that means I would have the equiv of 8 T5HO bulbs.
The reason why power compacts are more efficient is because they put out more light for the length of the bulb.. For example the 65W bulbs im using.. are only 21 inches long... To get a T5HO light to put out that much light.. You need one that is 48 inches long and still that is only 54w.. The 36 inch long power compacts.. ARE 96 watts.. So to get the equivalent lighting that i have on my 92g tank with my 4x 65W lights.. I would need 5x 48 INCH long T5 lights... And 4X 48 inch long T5's wouldnt fit on my 92G tank, they would be hanging off the side or the front and the back of the tank would be dark and so forth..
And if you want to light a 48 inch long 55/75/90g tank, You would use either the 96W PC thats 33 inches or the 130W Power compact thats 44 inches.. So 1 single Power compact light you would need 3x 48 inch T5HO's... So anyway they are more efficient because they put out more light for equivalent sized T5's.
So to light my tall 27g hexagon, i would need 4x 18 inch T5 HO lights, to equal 1 single 65W 21 inch Power compact. Make sense now ? I used a single 65W power compact over my 27g hexagon and my plants were exploding in color.. Something i couldnt acheive using the duel T5 fixture i had and the T5 wasnt nearly as bright to the eyes either
Here are pics of how i did that