To get your device's power consumption, you can contact the manufacturer directly or find it yourself by looking at the power adaptor's (charger) Tech Specs. If the Tech Specs don't give you the wattage but only voltage and amperage, just simply multiply them to get the Wattage:
Voltage x Amperage = Wattage
Take Lenovo Thinkpad X1 Carbon for example, on its power adaptor it says 'OUTPUT: 20V 2.25A'. We can then use the equation above to calculate Lenovo Thinkpad X1 Carbon's power consumption:
20V x 2.25A = 45W, which is workable with both Omni20 and Omni13.
Note: Your laptop charger's output spec shows the maximum wattage it will output when your computer is under a high load. Most of the time your charger will not be outputting this much power.