GL Multitimer units come in 4,6 and 8 modules

Most households have 10amp circuits - so the maximum you can put through one plug is 2400W or 240V x 10amps.
These timers come with 1 or 2 plugs, so you can draw from 2 plugs. As long as they can draw 10amps you will get 10amps per side which is 2x10amps or 4800W.

You may be able to plug both supply plugs into the same circuit (e.g. sheds where 20Amp or more has been supplied to each PowerPoint) or you might need to run the lead to another PowerPoint. (another side of the room or another room to find a separate circuit)

If unsure about your power availability, look in your fusebox. If it is a 10amp circuit breaker, then on that circuit, you can only use 10amps for everything on that circuit - fans, pumps, lights, and a circuit may be to 1 room or multiple rooms. You can turn a circuit off to find out which powerpoints apply, as they are no longer working. If you have 15amp circuits then it will be 15amps to that circuit, and 20,30Amps and so on... 

10amp x 240V =2400W
15Amp x 240V = 3600W
20Amp x 240V = 4800W

These timers are available in 4 models

4outlet - single plug, 2400W 10amp total - one plug controls the whole timer

6 outlet - 2 plugs - one plug controls one side of the timer - if on one 10amp circuit in the house 2400W 10amps - if on 2x10amp circuits, 2800W 10amps per side

8 Outlet - 2 plugs - one plug controls one side of the timer - if on one 10amp circuit in the house 2400W 10amps - if on 2x10amp circuits, 2800W 10amps per side