It doesn’t work that way because of the phenomenon of “inrush current.” Although LED bulbs run on very low wattage overall, when they start up there is a huge spike in current. For this reason you will find that every switch model will list its specifications in the user guide in two categories:
Resistant load, meaning incandescent and Halogens and
CFLs and LEDs.
Also note that the total that the switch can handle will be reduced if you have broken off the side tabs, which act as heat sinks. This is true for both incandescents and LEDs.
Seriously, the exact specifications will be printed in the user guide for each individual model. You need to read the guide, find the specification, and follow it. Also note that the total that the switch can handle will be reduced if you have broken off the side tabs, which act as heat sinks.
So if you give us the model numbers for the switches that you are looking at, we can help you find the max loads for those, with the understanding that the max load for LEDs will be different—and much lower than the max load for incandescents. (Also, always round down, not up, when calculating max load for any electrical devices. In fact a lot of electricians will tell you to aim for 10% below the max load, but that’s a matter of personal preference.)
Let’s look at the newest GE in wall dimmer. Model 14294
The max load specs are 600w incandescent, 150w LEDs. But it drops if you have it in a double gang or triple gang set up where you had to break off the side tabs.
One more thing: wattage can vary quite a bit from One LED brand/model to another, so always check the specs on the bulb as well. A 60w incandescent Equivalent LED might be anywhere from 7W to 12 or 13 depending on the specific model.
In your case, for your 21 W LEDs with the Z wave 14294, you could safely support 7 of those specific bulbs with that specific switch.
To support 9, you would need the Leviton 1000 W equivalent switch, which can support up to 450 W of LEDs.