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Determining Power Requirements

One of the hardest concepts when considering data center colocation is determining how much power equipment needs. There are many ways to find out, but no matter what method you use, all computations involve three electrical concepts:

  • Current (amps)
  • Voltage (volts)
  • Electrical power (watts)

How do they help you calculate power draw?

They’re applied to a simple formula:

amps * volts = watts

This formula determines how much energy a piece of equipment uses at a given moment.

Method #1: Meters and Faceplates

faceplate Faceplate

In much of today’s modern power distribution equipment, a built-in meter displays the usage of power. Manufacturers are also required to display acceptable voltage ranges and amps drawn per load on the faceplate of the equipment:

power req PDUs

In the PDU's LCD readout, you see can see both the primary and redundant PDUs are pulling 9 amps.

Modern IT equipment like this usually accepts voltage ranges from 100-240 V and is compatible with both 120 V and 208 V power. These particular PDUs are APC AP7941, which are rated for up to 30 A on 208 V circuits (80% of 30 A according to the National Electrical Code for safety reasons). Because we know that the equipment that’s plugged into the PDUs are pulling 9 amps, we can plug the values into the formula:

9 amps * 208 volts = 1,872 watts

NOTE: The reason that we use only one of the 9 A values is due to how primary and redundant power are configured. Primary and redundant power means two or more power supplies from different sources of power. Because the PDUs have the same gear connected to each, they should draw the same amount of power. When planning for power redundancy, each circuit (primary and redundant) must be sized to handle the total load of both in case of failure.

We find that the cabinet’s equipment is pulling 1,872 watts (almost 1.9 kilowatts). Make sure to leave room for “power creep,” as all IT equipment consumes more power over time.

Method #2: Hardware Lists

If you don't have a PDU with amp readouts, you can determine power requirements using a complete hardware list. You'll have to research the manufacturer power specifications for each piece of equipment to determine:

  • What is the CPU/RAM/HDD/SSD configuration of the equipment?
  • The purpose of the equipment (DNS, database, application server, web server)
  • The age of the equipment (newer equipment will have more efficient power supplies)
  • Special requirements like “Power-over-Ethernet” (common with network switches)

For example, assume a customer lists the following pieces of equipment:

  • 1 Juniper EX4200-48T switch
  • 1 FortiGate Fortinet 310B firewall
  • 4 Dell PowerEdge R420 servers

After looking up the manufacturer specifications, we find:

  • Juniper EX4200-48T has a 320 watt rated power supply
  • FortiGate Fortinet 310B has an average power consumption of 136 watts and can draw a max of 5-3 amps across 100-240 volt systems
  • Dell PowerEdge R420 has a 550 watt rated power supply

To determine the maximum power usage at any one time, we calculate:

320 watts + (3 amps * 120 volts) + (4 * 550 watts) = 2880 watts

The maximum amount of power that these six pieces of equipment can consume at any one time is 2880 watts, however IT equipment rarely reaches its maximum power limit. Even though the exercise above calculated “maximum power usage at a given time," it did not take other variables into consideration.

Knowing the maximum power required provides a base to factor in how the equipment is used and how much real-world power needs to be provisioned.

At ServerCentral, we're committed to providing 100% uptime on power (and bandwidth!), and part of our unblemished success at doing so is the depth of discovery and analysis that our sales engineers undertake.

All it takes is a basic formula to right-size power requirements.

Topics: Power Products and Services