Power off while updating pc
That's a best case scenario, worst case is your existing install is irreparably damaged.
As dalchina pointed out, the best insurance to something unforeseen like this is to have a backup/image of your present installation.
Many recommend Macrium Reflect, there are other 3rd party backup utilities, that's something you need to decide what's best for you.
There's also the built in windows backup and restore function.
You go to shut off your computer, and rather than just shut down, it decides it’s time to install a bunch of updates.To shut down without installing updates, select “Shut down” from the drop-down list. You can also shut down your PC immediately from the login screen. Then, in the lower-right corner of the login screen, click the power button and select “Shut down” from the popup menu. Lastly, if you need do do this from a script, you run the following shutdown command in a Command Prompt window.Type the following command at the prompt and press Enter. In a few rare cases, a Windows Update has been known to cause a problem, but that’s very uncommon.So you should get those updates when they come out, in order to keep your computer as safe and secure as possible.
Bottom line: I had my system back and fully operational in 7 1/2 minutes. Even if it did download "completely" and then you lost power I wouldn't trust it to be complete.