I plan to get a 400watt ups in case of power loss. Will the power supply in the ups power my computer all the time or will the power go straight through the wall to my computer? Im pretty sure 400watt is not enough and it may just go boom after extensive use, but it should be enough for when the power goes out in this rotten house.
This will be my first ups so pardon the dumb question :-)
It depends on what UPS you purchase, most of the cheaper UPS's are line-interactive meaning that it uses a Autotransformer that switches voltage as it see's fit depending on what kinda power is coming out of the wall outlet. With this one it runs on A/C power via the wall when it detects overvoltage/undervoltage it makes up for the voltage with a transformer instead of using the battery at any time it detects a power fluctuation unless it's a really big spike or complete AC power loss then the battery will intervene.
Here's a visual to better understand
There are double conversion UPS's that filter all output power through the batteries first. This is the best protection but the most expensive.
So to answer your question your computer will draw AC power from the wall and switch to battery power when needed(unless you have a double conversion UPS)