The Dell Poweredge R520 can hold up to 2 power supplies if you have the hot swappable chassis. Otherwise it holds 1 cabled power supply. The spare power supplies come in Auto-ranging Titanium efficiency, hot-plug redundant 750W AC power supply; Auto-ranging Platinum efficiency, hot-plug redundant 750W or 495W AC power supply; Auto-ranging Silver efficiency, cabled 550W power supply; 1100W DC power supply options. Make sure you are replacing or adding an additional power supply that matches the wattage of the current power supply installed. Always replace with a genuine Dell power supply specifically engineered for the Dell Poweredge R520.
If you are receiving the error message Dell Poweredge R520 power required exceeds PSU wattage you should look at installing the largest wattage power supplies. If you are currently using the largest wattage Poweredge R520 power supply you will want to try replacing them. One of the power supplies could be causing the Poweredge R520 to have a wattage error.