Bingley,

Take the model, and look at the battery- in this case, googled "CyberPower PR1500LCDRT2U battery"

the battery in it is a 12 volt, 7 Amp hour battery battery
So Watts= volts * amps, so 7*12= 84 watt/hours available in battery.

Your Pc uses 15.5 watts per hour.
You cell phone battery holds 5.6 Watts when full

So in the best of all possible worlds, yous PC could run up to 5 hours. ((((84 divided by 15.5- 5.38)))

OR, you could charge your cell 15 times (84 divided by 5.6)

Or some combo of the 2....