By 5-20 I assume you mean an assortment of say 5,10,15, and 20 Amp.
Peeling the next layer of the onion, Watts = Volts x Amps. If you look at the 18V output, 100 Watt Z-1000 you'll see a 6 Amps breaker. 18 Volts x 6 Amps = 108 Watts.
I'm not familiar with how the ZW can "allocate" its 275 Watts to the various outputs. But assuming you'd set the ZW output to 18V, then 10 Amps would be 18V x 10 Amp = 180 Watts. I'd start there.
Note that 5 Amps would only be 90 Watts so you'd be no better off (so to speak) than the Z-1000.
