Note that the RS is the version (vs the XS) that is expandable with the BR24BP battery pack. I've got two of the RS1500s, and recently we had a 90 minute long power outage. The standard 1500s can't keep my PCs (three) and network equipment (routers/cable modem, etc.) up for more than 20 minutes. With the add-on pack, I expect to get close to the 90 minute duration (longer than that, go ahead and shut the large loads down!).(I use the other 1500 for my Home Theater setup. PCs can't take fickering power, and neither can DVRs (which are really just PCs) or rear projection TVs with high-wattage bulbs; good to have something to smooth out the power and also let the fan cool down the bulb!)Highly recommended, with or without the external batter pack!(also, I can't even find the XS on the APC website... old model?)