An uninterruptible power supply (UPS) device is used to ensure electronic devices have a few minutes of power when the main supply of power goes out. This ensures all the necessary critical files are saved. Some UPS devices also feature a surge protector to prevent sudden power spikes from affecting devices connected to it.
Some users connect a UPS to a power strip since the location of the electronic devices is far from the power outlet. This is not advisable since it may cause the UPS to remain on battery power while it is being used. This will happen since the equipment will draw more power from the UPS, compelling it to remain online all the time. Due to this, the battery will be worn out faster since it will be used most of the time while the UPS is connected to a power strip.
A power strip may also increase the current flowing into the UPS, which can potentially increase the chances of a power spike. The risk of a power spike will increase if the UPS is daisy-chained using a number of power strips.
Connecting the UPS to a power strip will also void the Equipment Protection Policy (EPP). The EPP is a guarantee that the manufacture will pay for the repair of any equipment connected to the UPS if they are damaged due to power line issues. The manufacturer will also replace the UPS if it is damaged by these issues.
In the end, it is important to plug a UPS directly to power source to prevent any issues that will put all electronic devices connected to it at risk.
References:
http://www.apc.com/us/en/faqs/FA158852/
http://www.geek.com/xyzcomputing/daisy-chain-your-ups-571188/
http://www.apc.com/ca/en/support/product-support/registration-warranty-services.jsp
http://www.apcmedia.com/salestools/ASTE-6Z7UYR/ASTE-6Z7UYR_R0_EN.pdf?sdirect=true