The Hidden Cost of Undersizing Your PoE Budget

When specifying a PoE switch, it's tempting to calculate exactly what you need and buy accordingly. A 10-camera deployment at 15W each needs 150W, so a 180W switch should work, right? In theory, yes. In practice, you're setting yourself up for headaches.

PoE Classes Don't Tell the Whole Story

Devices negotiate PoE class at startup, but actual power draw varies during operation. A PTZ camera might idle at 12W but spike to 25W during movement. An access point draws more power under heavy load. If your budget is tight, these spikes cause problems.

When a switch runs out of PoE budget, one of two things happens:

• Priority-based shutdown: Lower-priority ports lose power entirely
• Denied power requests: New devices won't power on at all

Neither is acceptable in a production environment.

PoE Budget Calculator Calculate your total power requirements with headroom

The Growth Problem

Networks grow. That 10-camera system becomes 12, then 15. You add access points, VoIP phones, or IoT sensors. If your switch is already at 90% PoE capacity, every new device becomes a crisis.

The cost of adding another switch—plus the cabling, rack space, and configuration—far exceeds the premium you'd have paid for a higher-capacity switch initially.

Temperature Derating

Switch manufacturers rate PoE budgets at specific temperatures, typically 25°C (77°F). In a warm IDF closet running at 35°C, that 370W budget might derate to 300W or less. Check the datasheet—derating curves are often buried in the fine print.

Ambient Temp	Typical Derating	370W Switch Actual
25°C (77°F)	100%	370W
35°C (95°F)	80-90%	296-333W
45°C (113°F)	60-75%	222-278W

Cable Length Matters

Power loss over copper is real. A 100-meter run loses more power to resistance than a 10-meter run. If you're calculating based on device requirements alone, you're ignoring the power that never reaches the device.

For long runs to high-power devices, budget an extra 10-15% for cable losses.

The 30% Rule

A practical guideline: size your PoE budget for 70% utilization maximum. This gives you:

• Headroom for power spikes
• Room for growth
• Buffer for temperature derating
• Margin for cable losses

Yes, you'll pay more upfront. But you'll avoid the 2 AM call when cameras start dropping because the switch can't keep up with a cold-weather power spike.

Do the Math Before You Buy

Add up your device requirements. Add 10-15% for cable losses. Add 20-30% for growth and headroom. Then buy a switch that meets that number. It's not overengineering—it's realistic planning.