If you have a PSU from one of these manufacturers, then not only you can know about the wattage but can possibly know how reliable your PSU is. put up a big label on their PSUs that will list out the voltage, current, and power ratings.

PSU manufacturers like Cooler Master, Seasonic, Corsair, Thermaltake, Gigabyte, EVGA, etc. However, the specs will always be mentioned on one side least which may or may not be visible to you based on the direction your power supply is installed. It’s easy to know about the power supply of your PC if you have an assembled computer but still, there are some power supplies that don’t mention their power capacity in big fonts as we see on a lot of premium quality power supplies nowadays which makes it harder to guess how big it is. In both cases, one method of checking the power supply is universal and that is manually opening the chassis side panel. There are a couple of ways you can know about your PSU in either case. Generally, it is easy to figure out the power supply in the first scenario but it’s also not impossible to find your PSU model if you have a pre-built computer.

Speccy psu voltage