BR1500MS2 Output Seems Inconsistent

[Newton-W]

I bought this UPS to keep my ATT WiFi router and digital phones up during outages. To test it, I plugged a small lamp with a 40 watt bulb into one of the backup outlets. When I unplug the unit from the wall outlet, the light bulb in the lamp flickers every few seconds. Is that normal? I ran "tests" on it using PowerChute and all passed. At this point, I don't feel confident enough to use it on my equipment.

Thanks in advance,

Newton