electrical

Normally a switch has a maximum voltage rating (and may have one for ac and another for dc) that shows the ability of the switch to interrupt the arc upon opening.

It will also have a maximum current rating that shows the ability of the contacts to carry that current without overheating and possibly fusing.

In such a case you should not multiply the two together to get a maximum power rating since that power rating would be based on the maximum voltage only.

Normally the ac or dc voltage rating of the switch which should be printed on the frame of the switch and secondly the maximum current carrying capacity. If you now want to know the max Watts you can use that info to calculate it.

It is , however, not good practice to try and use a switch under maximum current conditions. Rather use one with at least a higher current rating than what you are going to need. The voltage is ok. as good switches can normally tolerate a voltage at least twice the rating.

Assume your (actual) voltage is 120 volts ac and the maximum ampere rating for that voltage is 15 amps. To answer your question multiply 120 VAC by 15 amps equals 1800 watts maximum. Use caution though, because equipment condition and extented operation or loose wire connections can drive your load beyond maximum watts rating.

leave a 15-20% allowance below rating for unforeseen conditions.

In the U.S., the ratings of most switches are molded into the case. These ratings may be repeated on the packaging and package inserts. Most wall switches are rated for purely resistive loads on single phase AC at 250 volts or less. Switches rated for DC service, or for loads with a large reactive component (like motors) typically cost more, but probably won't explode or start a fire from arcing.

Printed catalogs are a useful source of this kind of information, and a good search engine on the internet will give you access to a huge additional amount. Have you taken advantage of the search function on CR4 to see if anyone else has addressed this issue? It's a good beginning.

To obtain Watts, multiply Amps by Volts.

In simple terms, in the UK, domestic lighting is normally attached to a circuit protected by either 5A fuses or a 6A circuit breakers. All switches have their operating current embossed upon them, so selecting a switch with a continuous rating of 5 to 6A or above will be satisfactory for these circuits.