You want an Uninterruptible Power Supply (UPS)
This one click here
will do you for 20 mins at 240W.
So assuming your setup is probably 100W for a typical PC + monitor. You should get nearly an hour.
If you have any power hungry accessories/cards,
go for a bigger capacity model. Or go for the next one up anyway, just in case/future expansion.
Search amazon for "UPS" to see various.
Or Google "uninterruptible power supplies"