I want to know how to buy a battery for this which will give me atleast 2.5hrs back up time.
I can't decide whether to buy a UPS or an inverter. I heard inverter is less expensive and offers more back up time compared to UPS ? Correct me if I'm wrong. But UPS offers protection to my PC from all kinds of voltage fluctuations and lightning ? Is there a way I can buy an inverter and still protect my PC?
dont forget your monitor. you will need to factor this into your power consumption. without a monitor during a blackout you will not be able to accomplish anything. unless of course you are going to rely on the autobackup generator for this. personally i would put both pc and monitor on the ups.
i use a television for a monitor and i dont have it hooked up to my ups but i probably should. i only have my i7, gtx470 tower hooked up on the backup loop. i get around 5-7minutes ONLY out of the $200 ups model i use. its the one that is a tower that sits on your desk with the lcd screen. its one step down from the top in the series.
for hooking up both medium-heavy draw pc and monitor for a 20min runtime you are looking at $200+ if you buy retail.
i'm no electrical engineer and i couldnt tell you if a sinewave model is required. as long as the power is as clean as it is coming out of your wall socket i dont see an issue. you might want to search around on this.
what you need to know is the power you are drawing from the wall on average and multiply that by the time you want to run the device.
now... having regular batteries like they do for home solar cell power generation and an inverter is most likely a much cheaper option if you compare potential outputs. you could probably buy some sort of device to stabilize the sine waves if you so wanted. remember that batteries are not discharged 100% so you will need more battery in this case then your calcuations indicate.
you might want to talk to a company dealing with batteries and inverters to get an idea of costs. they should be able to take your draw and time into consideration. now also remember that you will need to charge these batteries and they might take longer than 2.5 hours to charge.