Think about it like this:
5 plugs, each can handle up to 15 amps. but the box can't handle more than 20 total... however you want to split those 20 amps up, you can and keep in mind that you can have more than 20 amps of stuff plugged in, just no more than 20 amps running at the same time.. examples to follow (think day routine and night routine, you have to run those power hungry MH's off some time, that frees up a LOT of capacity!)
the unit can handle a total of 20 amps spread across all the plugs. you have to remember that the common household socket that this unit will be plugged into is only rated for a total of 15 amps. Most homes have 20 amp breakers spanning across multiple sockets. so this unit is sized with that in mind. (20 amp sockets have one of the pins turned on end. the ones that look like a "face" - a standard plug you see in every house across the country, are rated at 15 amps. So if you have all the stuff plugged into a single outlet, I hope you've beefed up your household electric!!! Also remember, there are some things that won't need to get plugged into the unit. For example, I'm not sure I'd plug my canister filter into it. There's no need. It's always on, always running, unless I'm cleaning it. and the Koralia jets are always on, so really, they don't need to be plugged in either.
You could even go as far as to "overload" the box with more than 20 amps worth of equipment, as long as no more than 20 amps worth is running at the same time. (say, run the MH lights and your pumps and Actinics during the day, then at night, run a protein skimmer, LED moonlights, and chiller/heater - something like that)
as long as you're not running more than 15A worth of equipment at once across one socket and no more than 20A across the whole box, you're fine.
Here are some examples of how you could plug things in to the unit, I hope these help clear things up a bit:
plug 1 - 15 amp device (say, 3 400W Metal halide lights) - run during the day
plug 2 - 5 amp device (say a heater, or chiller) - run during the day
plug 3 - NOT USED
plug 4 - 5 amps (Moon lights and two extra power heads) - During the Night
plug 5 - 10 amps (Industrial Protein Skimmer) - During the Night
[hr]
20 amps during the day
15 amps at night
- OR -
Plug 1 - 4 amp device (like a whole bank of pumps)
Plug 2 - 4 amp device
Plug 3 - 4 amp device
Plug 4 - 4 amp device
Plug 5 - 4 amp device
[hr]
20 amps
- OR -
plug 1 - 10 amp device (a bank of CF lights or two MH lights and a bank of Actinics) - during the day
plug 2 - 5 amp device (1 big heater) - run at any time
plug 3 - 5 amp device (one big chiller) - run at any time
plug 4 - NOT USED
plug 5 - 10 amps - (Moon Lights and a tidal power head) - run at night
[hr]
Max of 20 amps during the day
Max of 20 amps at night.
- OR -
plug 1 - 2 amp device (Actinics)
plug 2 - 4 amp device (one 400W MH light)
plug 3 - 7 amp device (a heater/chiller or second MH light)
plug 4 - 1 amp device (LED moonlights and a protein skimmer)
plug 5 - 1 amp device (a coupe of power heads)
[hr]
15 amps
5 amps to spare
One final note: if you're running 240V equipment or 3 phase equipment (big industrial stuff), this unit will not work with it.