UPS (uninterruptible power supply) is now underpowered. Are there low cost solutions ?

My current UPS was sufficient for my computer before I began mining RVN coin.

Problem is that with the added power drain from mining, my UPS just can't provide enough power to keep my computer alive anymore. I know I can buy a bigger ups, but it costs money (that I can't spare at the moment) and it seems that there's also a shortage of ups in my region… go figure :-p

Obviously, I'd like to protect my computer from power spikes and outages. Anyone knows how to upgrade/enhance my setup ?

My current ups has a 550VA (350W) output. My computer has a 500W psu. I'm proficient with electronic circuits and got plenty of parts and tools lying around. Please, if you suggest anything that includes li-po batteries, it must be completely hazard-free. I don't want to burn the house….