The Supco SPP6E is rated at 208 to 277 volts, not the 120 volts that you would need.
The SPP4E is rated at 100 - 130 volts, just right!
http://www.supco.com/web/supco_live/product.php?department=1&category=1&subcategory=1  You can look up the specs on each here.  
You might be looking at a PTC booster.  They do not have the instant recycling ability, in other words they use a resistor that gets really hot, then the resistance increases, taking the capacitor out of the wiring circuit.  Then it MUST wait about 4 minutes to cool enough that the capacitor can work effectively again.  If still hot, it will not do any good.  So if the compressor does not start in 5 seconds, it will still be to hot when the compressor overload tries to start it again, but the capacitor will not be able to assist.
The SPP4E will recycle instantly.  So if the compressor does not start, the compressor internal overload trips, and resets in about 45 seconds, then the compressor needs 'extra' help to restart, the SPP4E will be ready to start the compressor (even after 4 seconds of wait time). 
The SPP6E rated at 200 - 250 volts will also 'work' but will wait until it has over 190 volts across the terminals to 'drop out' the capacitor.  While the E-series does have a electronic drop out after a few seconds.  However it is much better to drop out at 120 volts (once the compressor is up to speed, and less than 2 seconds most of the time) to lessen stress on the compressor start winding.  
Good luck
Fred.