Understanding the Actual Power Budget of PoE Switch

One of the factors that affect power consumption on PoE switches is the power budget. Different PoE switch models have different power budgets. Some managed PoE switches for large enterprise networks can supply up to 400W, but for a home network, it’s better to use a basic unmanaged PoE switch with a lower power budget.

What Is PoE Budget?

When considering power usage for a hardware installation, you will need to make sure the PoE Switch or Power Sourcing Equipment (PSE) can meet the power requirements of Powered Devices (PDs) such as IP cameras, Voice over IP (VoIP), and Wireless Access Points (WAPs).

All PoE switches have a limited Budget——the total amount of power they can provide in watts to PDs at the same time.

The Actual Power Budget – The Power over Ethernet (PoE) switch sends both the data and power to the remote IP device, so you can remove the AC outlet that is used to power the device. As we all know, electric devices only take the power they need. For example, an access point only draws 5W even when a 100W power adapter is being used.

The following table lists the power consumption rates of some common network devices and how much power they need to operate.

How to Choose the PoE Switch to Meet the Power Demanding?

IEEE has a set of standards for PoE (Power over Ethernet). There are three types of pluggable connectors we can use when working with Ethernet cables: IEEE 802.3af, IEEE802.3at, and IEEE802.3bt. The output power on IEEE802.3af is 15.4W, on IEEE802.3at the output is 30W and on IEEE802.3bt the maximum output is 90W.

However, these are not the actual power budget! This is the maximum power that PoE switches inject into the network cable. The remaining budget for PoE switches is 12W for PoE, 25.5W for PoE+ switches and 71W for PoE++ switch (type 4 class 8). This difference in output is due to power loss over the course of the cables. Eventually, the heat created by this loss will be dissipated.

The new IEEE 802.3bt standard is the latest standard that employs all 4 twisted pairs to send the power. This new standard will make a difference for power hungry applications like access control, POS, interactive displays, cellular base stations, etc.

How to calculate the power in a PoE system?

We often need to calculate how many devices can be plugged into a PoE switch. To better illustrate this calculation, I would first like to introduce two important components to understand when we calculate power consumption on a PoE switch:

Power Sourcing Equipment (PSE)-this is equipment that delivers power over Ethernet. It is typically a PoE switch or PoE injector that comes with a predetermined Total PoE Budget.

Powered devices (PDs)- these are pieces of hardware that need power and connect to a PoE switch, such as IP cameras, WAPs, and VoIPs. They also come with a variety of demands, ranging from 6W to 65W even higher.

As a rule of thumb, the total sum of these demands cannot exceed the power budget from the PoE switch. So, how many devices can you link to a PoE switch? It all depends on two different factors: the maximum power output from your PoE switch, and the max power requirements from all your devices.

One way to measure a PoE switch’s power is the PoE Budget, which is the total amount of power that is available for the switch to distribute across PoE-compatible devices. For instance, a 16-port 802.3 PoE+ switch has a 200W PoE Budget. With this much power, the max power requirement for all your devices cannot exceed 200W.

One thing we always missing is the power loss inline. The actual power you receive for all those edge devices will be less than 200W. As we mentioned above, the power will be lost and converted to heat, we can’t ignore those power losses.

This conservative method is subtracting the power from the total power budget for each device. If the IEEE802af device is attached to the PoE, it takes 14.4W, and it will take 30W with PoE+ edge devices. Eventually, this method will reserve 20-30% additional power, but more is always better than less right? You have kept some power margin in the system.

How Can I Reduce the Power Consumption of a PoE Switch?

Using Energy Efficient Ethernet (EEE)-PoE switch with Energy Efficient Ethernet (EEE) is a technology defined as IEEE 802.3az to reduce switch power consumption during periods of low network traffic and aims to reduce power consumption by more than 50%. It also has a green function which not only saves the link load power and saves the power used for transmission, but also detects link status and adjusts the power according to cable length. Note that your device port and connecting device must support 802.3az for this PoE switch to imply the Energy Efficient Ethernet, otherwise, the switch will work as a regular PoE switch.

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