If you use a J tube overflow (Can't speak to others, its what I use) ,when the power goes out your water level falls below the level in the skimmer box. Thats the box that is on the inside of the tank that water pours over into for transport to the other side by the J tube(s). When this happens, balance should be achieved in the overflow and water stops passing from the inside box through the J tubes to the outside box. I say should, mine always has, but your milage may vary. A little balancing act (Thank you physics!!) occurs. There is no flow into the skimmer box to push water (build up pressure actually) to the other side so an equilibrium is achieved. Water in the skimmer, water in the tubes and water on in the collection box. (That is, the box on the outside that the drain attaches to) When the power comes back on, your pump restarts and begins to push water back to the tank. The tank level rises and begins to fall into the skimmer box again and like magic (What goes up must come down..) the water starts to move from the skimmer box through the J tubes and into the collection box for draining into your sump.
Now what I described above is the ideal situation. Something to watch for is siphoning. This happens when the pump in your sump (Dr Seuss anyone??) shuts off. Water then falls back (theres that physics thing again..) down your return tubes that would normally feed the return water. When this happens, it creates a natural siphon and again like magic, your have a gravity fed siphon draining your tank through your return nozzle/hose(s). This will only happen until the water level in the tank drops below the return nozzle. Once that happens, air enters the nozzle and POOF breaks the siphon. In my case, I only run the sump about half full (Or half empty, depending on your state of mind..) and it can handle the amount of siphoned water with ease. What some folks do is drill a small hole in the return nozzle/hose just below the water line. This way when the siphons starts, it drains far less water until the hole begins to (heh) suck air. This in turn, again, breaks the siphon. In this scenario, you don't have to sweat the amount of water siphoning back into your sump.
Another gotcha..If your sump pump restarts and your overflow box siphon stalls, you get x gallons of water pumped into your tank. 2 ways to approach this that I am aware of. 1 - Run your sump like I do and expect the worst. My sump can handle the back siphon and the tank can handle the extra sump water at the level I run my sump. Its close and I have come home to find my drain in the overflow with a crab in it and the water just *itching* to flow over...but it didn't. 2 - place your sump pump on a 'riser'. I have not done this personally, but it stands to reason that if you raise the height of your sump pump, then it will only pump back into the tank x gallons of water before the level in the sump falls below the intake of the sump pump which is now on riser or some rocks or whatever. Some folks freak if their pump runs dry for a few hours, but most pumps (at the hobby level..RIO, Dolphin, etc) will probably be just fine. Higher end pumps (Iwaki, the 'pro' grade Dolphins and other external pumps) do not tolerate running dry (Though some can be purchased that will run dry, depends on how much $$ your willing to part with..) Besides, if your siphon stalls, your not only going to have to potentially replace a pump, you get the happy happy joy joy feeling of pulling up carpet, steam cleaning or (God forbid) coming home to the fire dept adding some fresh water to your tank (salt water + overflow tank + electrical wires = ..well you get it..)
Now I don't mean to scare you off from using overflows. However, most 'pros' would not recommend them. I disagree. Course, I don't have 12 PHD's, one in marine biology with $$ to burn either, so, overflow I go..er, you know what I mean.
Matching the flow from the tank with the flow of your pump is not hard. You have a 46 gallon tank plus )lets just say) a 20 gallon sump = 66 gallons x 10 (thats 10 times per hour, I wouldn't shoot for anything lower) = 660GPH @ head height. (Typically 4' , but I do not know how high it is to the top of your tank, though if I did, wouldn't that be weird??) Most good pumps will show the rating at various head heights. (Dont shoot me, its just as an example..)
http://www.marinedepot.com/md_viewItem.asp?idproduct=TA3160 If you look at the link, at 4' its rated at 690GPH. Not bad. Now you should find an over flow rated for around 800GPH or so. The only examples I could link to off the top of my head were not J tube over flows. Sorry, but I stand by my tubes..heh. When you install the pump, you should make sure you have a valve in line with it. Most folks use ball valves, though others may use something else. Anyway, with the ball valve in line with your return pump, you can limit the flow of the return. This will allow you to ease back on the return flow should you out pace your overflow. (Its also where you would adjust the water level in your display tank if your over flow is not adjustable) Its very simple. The over flow should at least be able to keep up with your pump. If you find the water level too high in the display tank, ease back on the return with the ball valve. This in turn slows the return flow to the tank and lowers the level (Because you are letting the overflow run shallow) If the water level is too low in the main tank, simply open the valve more and allow the water in the display tank to rise. (Because you are letting the overflow become over whelmed a tad) In my case, I have a dual outlet overflow only one side of which is plumbed. I use a RIO 3100 with the valve wide open. The level in my tank stays at about 2 inchs or so from the rim of the tank. I will say some folks complain of 'gurgling' or bubbleing sounds from the overflow. I don't have this issue personally, but there are a couple ways to ease that too if your not into the natural sound of Niagra Falls in your living room..(Or wherever)
The balance between overflow and pump is achieved naturally. Your tank will run at a certain level with pump wide open and at another level 1/4 closed. Just depends on how you want it. I will say (As mine is setup) that some add a ball valve to the overflow line. This should never be used to restrict water flow into the sump. Lord help you if you end up with something caught in the ball valve. However, it is very handy for cleaning the sump
imo.
HTH
GL!