Power and UPS Sizing for Edge AI Deployments

TL;DR

UPS protection at the edge prevents data corruption, pipeline crashes, and hardware damage from unexpected power loss. Sizing a UPS correctly requires knowing total node load (compute, cameras, switch), desired runtime on battery, and the UPS efficiency curve. For most edge AI nodes, a compact online or line-interactive UPS at 300–600 VA provides 5–15 minutes of runtime — enough for a clean shutdown or to ride through brief utility interruptions.

Why UPS at the Edge

Data center servers operate on redundant power feeds with automatic transfer switches and generator backup. Edge nodes in a retail location, a warehouse, or an outdoor cabinet are typically connected to a single utility circuit with no redundancy. Power events — brief sags, brownouts, and hard cuts — are far more common at the building level than at a colocation facility.

The consequences of unprotected power loss at an edge AI node include:

• NVMe SSD filesystem corruption or write-in-progress data loss (particularly severe without power loss protection in the drive)
• Docker container state corruption requiring manual recovery
• Inference pipeline crash requiring on-site restart if watchdog recovery fails
• Hardware damage from voltage sag on sensitive compute platforms
• Loss of video footage covering the event window

A properly sized UPS eliminates all of these failure modes during outages short enough for graceful shutdown, and suppresses voltage anomalies that can damage hardware even without a full power cut.

Total Load Calculation

Measure or estimate the sustained power draw of every device on the UPS circuit:

A typical 8-camera Jetson Orin NX node total: ~170–200W sustained. Always use measured values from a calibrated power meter rather than rated TDP when available — actual draw is often 20–30% below rated maximum.

For component-level power breakdown, see the 8-camera reference architecture and networking for edge AI for PoE switch power specifics.

Runtime Math

UPS runtime on battery is determined by battery capacity (Wh), load (W), and inverter efficiency (η):

Runtime (hours) = Battery capacity (Wh) × η ÷ Load (W)

Typical UPS inverter efficiency: 85–92%. Example:

• Battery capacity: 216 Wh (common in a 600 VA / 360W UPS)
• Load: 180W
• Efficiency: 88%
• Runtime = 216 × 0.88 ÷ 180 = ~1.06 hours

For a 5-minute graceful shutdown target at 180W load, you need:

Required Wh = 180W × (5 ÷ 60) ÷ 0.88 = ~17 Wh

Most commercially available compact UPS units exceed this comfortably. The practical constraint is usually the VA (volt-ampere) rating, which must exceed the load's apparent power (which may differ from real power for switching power supplies due to power factor). Use a UPS VA rating at least 1.2–1.6× the measured watt load to account for power factor.

Note: battery capacity degrades over time. A UPS battery at 80% state of health (typical after 2–3 years) provides 80% of rated runtime. Plan for battery replacement on a 3-year cycle for 24/7 deployed UPS units.

Failure Scenarios UPS Addresses

• Hard power cut: UPS transfers to battery instantly (online UPS) or within 4–8ms (line-interactive). Either is fast enough to prevent compute platform reset.
• Voltage sag (brownout): Line-interactive and online UPS regulate output voltage during sags, protecting equipment from under-voltage damage.
• Voltage surge: UPS AVR (automatic voltage regulation) clamps surges before they reach connected equipment. Surge-only protection strips do not provide runtime on outage.
• Brief interruption (sub-second): Most utility flicker events last under 200ms. A UPS on battery through a brief interruption prevents any equipment disruption.
• Extended outage: UPS provides runtime for graceful shutdown sequence. After shutdown, the site has no power regardless — the UPS goal is clean shutdown, not indefinite operation.

UPS Form Factors for Edge

Compact tower (desktop form factor): 300–1500 VA. Fits on a shelf or in a small cabinet alongside networking gear. Most common choice for single-node edge deployments. Brands offer USB or serial communication ports for graceful shutdown automation.

1U or 2U rack-mount: For deployments in equipment racks (wiring closets, server rooms at the edge site). Higher VA ratings available. Better for multi-node installations sharing a single UPS.

Compact DIN-rail DC UPS: For industrial cabinet deployments. Takes 24V or 48V DC input and output, with an internal battery pack. Eliminates AC-DC conversion loss when the edge compute runs on DC. Relevant when the full node is already DC-powered.

Mini tower (sub-200 VA): Adequate for single Jetson Nano or Pi-class nodes without cameras. Not sufficient for nodes with PoE switches and multiple cameras.

UPS Type Comparison

Graceful Shutdown Design

A UPS without graceful shutdown automation is only half a solution. When the UPS transfers to battery, the compute node must be notified so it can execute a clean shutdown before the battery is exhausted.

Most UPS units include a USB communication port. The open-source NUT (Network UPS Tools) daemon reads UPS status via USB and can trigger system shutdown when battery reaches a configured threshold (e.g., 30% remaining or 3 minutes estimated runtime left).

Shutdown sequence for an edge AI node:

1. UPS transfers to battery → NUT detects event → NUT notifies system
2. Inference pipeline receives SIGTERM → flushes in-progress video segment → saves checkpoint state
3. Docker containers stop gracefully
4. NVMe sync and unmount
5. System shutdown complete within 60–90 seconds

Design and test this sequence before deployment. A graceful shutdown that takes 3 minutes but only has 2 minutes of battery is not a working solution.

Common Pitfalls

• Sizing UPS by VA rating alone: VA is apparent power. Watts is real power. A 600 VA UPS may only support 360W real load. Verify both the VA and watt ratings against your measured node load.
• Not automating graceful shutdown: A UPS without NUT or equivalent automation just delays the crash until the battery dies. Automate clean shutdown as part of initial deployment, not as a future improvement.
• Forgetting camera PoE power in load calculation: The PoE switch draws its rated power budget from the AC outlet, not just the switch's own consumption. If cameras draw 96W total through PoE, that 96W comes from the UPS via the switch.
• Not replacing batteries on schedule: Lead-acid batteries in UPS units degrade significantly after 2–3 years of 24/7 float charging. A UPS reporting "battery OK" may provide 50% of rated runtime if the battery is old. Test periodically by measuring actual runtime under load.
• Undersizing for cold weather: Lead-acid and some lithium battery chemistries lose significant capacity at low temperatures. An outdoor cabinet UPS in a cold climate may deliver 70–80% of rated capacity in winter. Size accordingly.
• Using consumer surge strips instead of UPS: Surge strips protect against voltage spikes but provide no runtime on power loss. They do not prevent filesystem corruption or pipeline crashes from hard power cuts.

FAQ

What is NUT and how does it work with a UPS?

NUT (Network UPS Tools) is an open-source daemon that communicates with UPS hardware via USB, serial, or network. It monitors battery status and can trigger system shutdown scripts when configurable thresholds are reached. Available in standard Ubuntu and Debian package repositories.

Can I use a lithium battery UPS for outdoor installations?

Lithium-based UPS units offer higher energy density, faster recharge, and better cycle life than lead-acid. They are well-suited to outdoor installations within their temperature range. Verify the battery's operating temperature range against your deployment environment's extremes.

How long should my UPS runtime target be?

5–10 minutes is sufficient for graceful shutdown in most deployments. If the site has a generator that starts automatically within 30–60 seconds, 2–3 minutes of runtime is adequate. Extended runtime (hours) requires external battery packs and significantly larger UPS units.

Should the UPS protect just the compute node or the whole installation?

Protect everything connected to the pipeline — compute node, PoE switch, and cameras — on the same UPS circuit. If the PoE switch loses power, cameras stop streaming and the inference pipeline crashes anyway, regardless of compute node power status.

What happens to in-flight video when power cuts?

Without a UPS, the current video segment file is likely corrupted. With a UPS and graceful shutdown, the pipeline can flush the in-progress segment to disk cleanly before shutdown. Some systems use memory-mapped ring buffers that survive brief power events if the UPS maintains power long enough.

Do I need a UPS for a node on a PoE injector?

Yes. PoE injectors are passive — they pass AC power to the switch and have no battery backup. A UPS upstream of the PoE injector protects the entire chain. Size the UPS to include the PoE injector's full output load, not just its idle consumption.

Recommended Reading

• 8-Camera Edge AI Reference Architecture: Full Power Budget Breakdown
• PoE Switch Power Draw in the Networking Layer
• Fanless Enclosures: Thermal Design and Power Constraints
• Storage Layout: Write Endurance Under Sustained Power
• Jetson Deployment Checklist: UPS and Graceful Shutdown Setup