I’ve been replacing light switches in my house to Lutron switches, because I think they look pretty. I’ve noticed that Lutron sometimes specifies a switch as having max load of 600 watts, and sometimes as 5A. Why this variance? Since our electricity in the US is 120V, aren’t these two the same? Thanks!
VA and Watt are sort-of the same, but not exactly. The difference is based on the power factor.
From a practical standpoint with residential lighting these days it makes little difference because with LEDs (and even with fluorescent), 600 Watts is a lot of light. On the one hand, with incandescent bulbs (a simple resistance load), the power factor is essentially 1, which is ideal, LEDs and fluorescent use so much less power that for typical residential usage this more than makes up for any power factor issues. It is easy to hit 600 Watts with incandescents - 10 x 60 or 6 x 100. But with LEDs, a 100 Watt equivalent might use 14 Watts, so it would take an awful lot of lighting on one switch to get even close to 600 VA or 600 Watts, no matter what the power factor.
In commercial settings the power factor becomes a real consideration as even LED lights can add up when you are lighting a warehouse or an auditorium or other large area. Large customers often have charges based partly on the power factor and may install equipment to improve the power factor. But that is not normally the case for residential customers.
There is really not a difference smart switches at 600w / 120v = 5 amps. This is the load the switch can handle. Simple snap switches are usually rated at 15 amps. The reason the smart switches have lower ratings than snap switches is because the electronics in the switch create heat the higher the load the more heat generated to maintain a safe box temp most manufacturers limit the wattage or they have larger heatsink flanges. With the latest smart switches that are LED compatible some of those top out at 3 amps or 360w. 360 is a huge amount of light when using LED's so larger wattage are not really needed. Why different types of smart switches for LED's ? Old smart switches used a little current through the lamps to power themselves because LED's are so efficient this small current caused them to glow and or flash so a new type of switch was needed. These switches require a neutral to power the electronics (thus the new code requiring a neutral in switch boxes) . I hope this information helps you understand what is going on. But there is no difference between 600w and 5 amp rated switches.