An outdoor camera device with local AI and a siren lives on a strict energy diet: one Cat6 cable carrying PoE+ when mains are up, and a small battery when they are not. Sizing that battery starts with knowing where the watts go.
The four consumers, ranked
Cameras and night IR dominate continuous draw — two 4–5 MP sensors with illuminators are the single biggest steady load. Edge AI inference comes next and is bursty: classification runs on frames, not continuously, so duty cycling matters more than peak numbers. The siren is short and enormous — tens of times the idle draw — but it is measured in minutes per year. Standby and heartbeat is the small, relentless baseline.
The battery sizes to the siren, not the camera
Mains power the cameras; the battery’s job is siren bursts plus a short standby window. Targeting “two hours of standby and ten cumulative minutes of siren” produces a very different (and much smaller) cell than naïvely backing up continuous recording. Writing the standby and siren assumptions down is the single most valuable line in an energy budget.
PoE is a design input, not a detail
PoE+ Type 4 defines how many watts arrive, and cable runs eat margin. Devices that negotiate power classes carelessly either pay for headroom they never use or brown out on long runs. We treat the power negotiation table as part of the specification, reviewed alongside the thermal plan.
Heat and battery chemistry
An enclosure that reaches 60°C in summer sun ages lithium cells fast. Battery placement away from heat-soaked surfaces, and charging limits that respect temperature, extend service life more than any “high capacity” cell choice does.