Puerto Rico uses the same 110-volt AC (60-cycle), two-prong-outlet electrical system as in North America. Plugs have two flat pins set parallel to each other. European visitors should bring adapters and converters, or call ahead to see whether their hotel has them on hand.
Consider making a small investment in a universal adapter, which has several types of plugs in one lightweight, compact unit. Most laptops and mobile phone chargers are dual voltage (i.e., they operate equally well on 110 and 220 volts), so require only an adapter. These days the same is true of small appliances such as hair dryers. Always check labels and manufacturer instructions to be sure. Don't use 110-volt outlets marked for shavers only for high-wattage appliances such as hair dryers.