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There are two ways to go about this: Find the output current rating on the power supply (I assume the Lorex NVR powers the cameras?) and multiply that by its output voltage. That will give you its approximate max wattage. UPS' are rated in VA (volt-amps), but it'll be close enough. The other way is to actually measure the current draw on the AC side using a true RMS ammeter. (That's the best way.) Then multiply the measured current draw times the line voltage. Next you have to determine whether the point is to protect your surveillance system and allow it to survive short-term outages, or to have it stay up for an extended time when there's an outage. (I note you list a "12V 10" PSU. If that's 12V at 10A, which would probably be about right for an 8-camera system [assuming about 15W/camera], then your want a minimum 120VA UPS. Depending upon the design, that may give you anywhere from 5-10 minutes of uptime [SWAG]). To a degree you can extend uptime by going with a higher-than-recommend-VA UPS, but there are diminishing returns with each bump in VA capacity. Reason is reduced efficiencies as UPS VA capacity goes up. For truly extended runtime you need an "extended runtime" UPS. They don't have greater VA capacity, but more battery. (They also take longer to recover because there's more battery capacity to charge back up. There ain't no free lunches.) E.g.: With a 120W load, a 120VA UPS may give you about 5-10 minutes of uptime, but a 360VA UPS won't necessarily give you 15-30 minutes. In fact: My old power-hog Dell 1600SC server had a 700VA APC SmartUPS on it. The computer + peripherals drew about 200W. The UPS had a runtime of only fifteen minutes with brand new batteries. Speaking of batteries: Make sure to buy a UPS with user-replaceable batteries. They have about a three-year lifespan. Also: Get in the habit of doing run-time tests about every quarter or so, to extend battery life. The UPS manufacturer should have a chart listing estimated run times for each of their UPS' vs. expected loads.