I went by the spec's sheet for the c7000 chassis. 6x power supplies, peak at 2400W each (worst possible case).
You should never see a load near that unless you have an A/C failure, or you just have a huge load on the blades, or you go crazy with storage blades. Depending on which blade config's you use, you will probably not utilize all 6 psu's. Ideally you would configure them in a redundant mode.
But, even assuming half peak load (50% of 14.4kW), you will still be at 7.2kW, which will more than put you over your threshold for a 20kW UPS. Remember, you do not want to run your UPS 100% - ideally around 80% maximum. Besides having a couple of minutes run time, you risk popping breakers if you have a power spike from several server spinning up at once.
According to APC, all of the Symmetra PX UPS are scalable. This is of course, assuming that your model matches their current spec sheets. It may also not make sense to upgrade the UPS vs. adding an additional unit.
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=sy20k40hYou may want to talk to your building maintenance or electrician, your breaker panel may not be maxed out.
