SENTINEL PRO ER models have more powerful battery chargers and no internal batteries yet when purchased with matching battery extension packs offer longer runtimes. Based on an on-line double-conversion (Voltage and Frequency Independent) UPS technology they offer the highest levels of reliability and protection for critical servers, voice and data systems In terms of communications, each Sentinel Pro has a built-in mimic panel with an LCD to show input and output voltages, and battery charge status each SENTINEL PRO also has USB communication ports as standard, with a built-in expansion slot for additional adapters, protocol converters and related contact cards. Model numbers SEP1000ER (1000VA), SEP2200ER (2200VA), SEP3000ER (3000VA)