Stormy Weather: An observation of POE Budget
Late the other afternoon we had a thunderstorm come through. Tis the season hereabouts. The wind picked up, pushing in dark clouds. The sky darkened quickly. Then the Unifi app on my Pixel 8 pinged to alert me to a network error. The network switch in our attic had exceeded its power-over-Ethernet (P.O.E.) budget.
What a novel set of events I thought. It was the first time I’d ever seen such a notification. I wonder why it occurred? It didn’t take long to work it out.
The device reporting the power problem is a Unifi USW Flex 5-port switch that lives in the attic. It receives power from the USW-24-POE main switch in the house. The POE+ (aka 802.11at) ports on the main switch can deliver up to 30 Watts over a single Ethernet cable. That is, up to its overall power budget of 95 Watts.
USW Flex can deliver up to 46 Watts when powered from a POE++ (aka 802.11bt) capable switch. When powered from mere POE+ the USW Flex is able to provide 20 Watts onward to network loads.
In our case, the USW Flex powers three Grandstream GSC3615 IP surveillance cameras and one Unifi UAP AC Lite Wi-Fi access point.
According to Grandstream the cameras draw around 4 Watts each. Ubiquiti says the Unifi AC Lite AP needs 6.5 Watts. So, in theory the load on the USW Flex is (3 x 4 W)+ 6.5 W = 18.5 Watts.
Of course, the power consumption detailed in the specs are worst case. The Unifi Network app lets me see the pattern of power drawn by devices attached to the USW Flex. In fact, the application lets me see a near real-time view of the power bring drawn by each port. It’s hard to see in this picture, click to see a larger version.
During the daytime the aggregate draw is 8-11 Watts. At night, that increases to 12-15 Watts. This is because two of the cameras go into night mode, turning on the IR LEDs to illuminate their field of view. Since there’s a light fixture at the front door, the porch camera looking in that direction normally never goes into night mode.
The light at the front door is controlled by Home Assistant. It turns on/off in sync with sunset and sunrise.
This arrangement has been in place for several years. It works well. We are normally well within the POE budget of the USW Flex.
However, on this one day, with the rapid onset of dark caused by the approaching storm, well before the projected sunset, all three cameras switched into night mode at the very same time. In so doing, they briefly needed more power than usual.
The USW Flex reacted by protecting itself. That caused the attached devices to power down. Unifi Network app reported the UAP-AC-Lite going offline.
Logs in the Blue Iris surveillance software show the camera streams lost, and eventually recovering.
level time object message
1 7/7/2026 8:07:16.578 PM Porch Signal: network retry
1 7/7/2026 8:07:16.633 PM FrontN Signal: network retry
1 7/7/2026 8:07:16.641 PM FrontS Signal: network retry
<…continues until…>
4 7/7/2026 8:09:49.477 PM FrontS Signal: restored
4 7/7/2026 8:09:49.472 PM FrontN Signal: restored
4 7/7/2026 8:09:52.381 PM Porch Signal: restored
Once the unusual power draw was eliminated everything came back without any intervention on my part.
For a time, I pondered how I might change things so this doesn’t happen again. I could pull another Ethernet line into the attic so the AC Lite AP was on its own run. Or I could buy a POE++ injector to deliver more power to the USW Flex.
Then it occurred to me, I really don’t need to do anything. This situation has occurred just once in the three years since I installed the USW Flex. It’s hardly a real problem.
I really enjoy this sort of sleuthing. It’s part of why I’ve (mostly) enjoyed being in a tech support role professionally. I really do like running a managed network at home, even if it has cost more over time.