how to keep my PC running if electricity cut off

A properly sized UPS (Uninterruptible Power Supply) will provide enough power for your equipment to continue running through a short power interruption. Most have software which will automatically save and close any open files and applications and soft shutdown your computer.

are you wanting to use it as normal or just long enough to shut it down?

As above, a UPS (Uninterpreted Power Supply) will power the PC for a short time.
They use an internal battery and are designed to power the PC long enough to save your work and properly shut down the PC.
Most will supply power for 10 to 20 min. depending on the amount of power needed.

oh that is great !! is "UPS" all what I need to tell the vendor (lol) ?

sobeit, the more effective the better !!

If the vendor doesn't know what a UPS is, go to another store.

I went there and it seemed TOO LARGE. its like an external power supply or something, I need the space!!

There are different sized models.

ARGMAN said:

I went there and it seemed TOO LARGE. its like an external power supply or something, I need the space!!

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that is what it is, an external power supply. the larger it is, the longer you can run it on battery until you can get it to shut down. if you have large monitors, and a lot of peripjerals you have the faster its going to use up the battery.

aha ... still weird for me since laptops can achieve that with minimum capacity inside it?

A UPS is designed to operate a PC long enough to save any open work and to help prevent any damage from a sudden power loss.
Another option is to install a backup generator.

aha ... still weird for me since laptops can achieve that with minimum capacity inside it?

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That is because laptops are designed for low power consumption as a high priority because it means longer run time on battery power. Desktops have different priorities, such as performance, ease of expansion, etc. Desktops could be built with much lower power consumption but that would significantly increase cost with no noticeable benefits for the buyer. Any manufacturer trying that in todays market would soon be out of business.

ooh that is it .. thank you VM guys