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)

To 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: Using meters and faceplates to find your equipment’s power requirements

Most modern power distribution equipment has a built-in meter that displays the usage of power. In the PDU’s LCD readout below, you see can see both the primary and redundant PDUs are pulling 9 amps:

PDU LCD readout
PDU LCD readout

Manufacturers are also required to display acceptable voltage ranges and amps drawn per load on the faceplate of the equipment:

Equipment faceplate showing acceptable voltage range and amps drawn per load
Equipment faceplate showing acceptable voltage range and amps drawn per load

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

9 amps * 208 volts = 1,872 watts

The reason that we use only one of the 9-amp 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 one fails.

We find that the cabinet’s equipment is pulling 1,872 watts (almost 1.9 kilowatts).

Make sure to leave wiggle room for “power creep,” as all IT equipment consumes more power over time.

Method #2: Using hardware lists to determine your equipment’s power requirements

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:

  • 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, one of our customers might list the following pieces of equipment:

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

Let’s find our maximum power usage for all six pieces of equipment. First, we look up the manufacturer power specifications online to find:

  • Dell PowerEdge R420 has a 550-watt rated power supply.
  • Juniper EX4200-48T has a 320-watt rated power supply.
  • FortiGate Fortinet 310B can draw a max of 5-3 amps across 100-240 volt systems. We know we’re looking for the maximum power usage in watts. (And we know that to calculate watts, we need to multiply amps by volts.) The 310B datasheet says our max ranges from 5 to 3 amps. Because a device actually draws fewer amps the higher the voltage, our max is actually the smaller number: 3 amps. For volts, the datasheet gives a range: 100-240 volts. We can assume it’s a 120-volt circuit because that’s standard for data centers in the United States.

So to determine the maximum power usage at any one time, we first have to convert everything into watts:

  • 4 Dell servers: 4 servers * 550 watts each = 2200 watts
  • 1 Juniper switch: 320 watts (leave as is)
  • 1 FortiGate firewall: 3 amps * 120 volts = 360 watts

Then, add them together:

2200 watts + 320 watts + 360 watts = 2880 watts

The maximum amount power usage for these six pieces of equipment is 2880 watts.

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. It’s important to note, however, that IT equipment rarely reaches its maximum power limit.

At SCTG, we guarantee 100% uptime on power (and bandwidth!). Part of our unblemished success at doing so is the depth of discovery and analysis that our sales engineers undertake. The other part is the level of redundancies built into our data centers (like this one).

All it takes is a basic formula to right-size your power requirements. And if you need someone to double-check your work, you can always contact us.

SCTG

111 W. Jackson Blvd #1600
Chicago, IL 60604

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