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How to size generator for data center

Generators are a key to data center reliability. Supplementing a battery based uninterruptible power supply (UPS) with an emergency generator should beconsidered by all data center operators. The question has become increasingimportant as super storms such as Hurricane Sandy knocked out utility powerstations and caused many downed power lines, resulting in days and weeks ofutility power loss.

Beyonddisaster protection, the role of a backup generator to provide power isimportant when utility providers consider summer rolling blackouts andbrownouts and data center operators see reduced utility service reliability. Ina rolling blackout, power to industrial facilities is often shut down first.New data center managers should check the utilities contract to see if a datacenter is subject to such utility disconnects.

So how to size the generator for data center becomes so critical.

  • General

Withthe adoption of uninterruptible power supply (UPS) architectures, the data centermanager can simply add modules as needs grow. However, it is easy to lose  sight of the future electrical needs of thedata center or data room within a larger facility.

Sizingthe electrical service requires an understanding of the amount of electricityrequired by the cooling system, the UPS system, and the critical IT loads.  The  power requirements of these elements mayvary substantially from each other, but can be accurately estimated usingsimple rules once the power requirements of the planned IT load aredetermined.  In addition to estimatingthe size of the electrical service, these elements can be used to estimate thepower output capacity of a  standby generator system, if one is required.

  • Needs assessment

Theneeds assessment essentially establishes the availability needs of the businessapplications being processed by the IT equipment.  A business process that is not time sensitivemay require no internal redundancies to increase availability.

Moretime sensitive sites may require a degree of redundancy in key componentsystems and have configurations of an “N+1” topology. The most criticalapplications  requiring total availability (7x24) would have a 2N topology wherecritical systems would be completely redundant.

Nomatter what the actual UPS system design configuration, the core issue ofproviding sufficient power to the critical load and keeping it cool must becarefully addressed.  Underestimating therequired capacity may result in future power disruptions when forced toincrease capacity, and over estimating leads to excessive initial  installationcosts and higher ongoing maintenance expenses.

  • Determiningelectrical power capacity

Properplanning when developing a data centre of any size begins with determining thesize of the critical load that must be served and protected.  The critical load is all of the IT hardwarecomponents that make up the IT business architecture:  servers, routers, computers, storage devices,telecommunications equipment, etc., as  well as the security systems, fire andmonitoring systems that protect them. This process begins with a list of all such devices, with theirnameplate power rating, their voltage requirements, and whether they are singlephase or three phase devices.  

The nameplate power requirements are the worst case power consumption numbers andin almost all cases, are well above the expected operating power level,sometimes by a factor of 33%.  Bearing inmind that not all devices are run at full load 100% of the time.  Alternatively, an advanced sizing calculatormay be used which gathers power consumption data from a wide range ofmanufacturers.

Withthe list of anticipated components comprising the critical load, the base loadcan be established by using a sizing calculator.  For IT equipment unlisted in calculators aswell as the power requirement for fire, security and monitoring systems, thefollowing process should be used:

1.1.Addup the nameplate power of the anticipated loads.  If the wattage is not listed on the device,it can be determined by multiplying the current (amps) by the  voltage of thedevice to get the VA, which approximates the amount of watts the device willconsume.

2.2.Multiplythe anticipated VA number by 0.67 to estimate the actual power, in watts, thatthe critical load will represent.

3.3.Dividethe number by 1,000 to establish the kilowatt (kW) load level of theanticipated critical load.

  • Future loads

Datacentre loads are dynamic, and the IT equipment will be under an almost constantstate of change during the lifetime of the facility.  A realistic assessment of the  scope andtiming of future changes and upgrades should be developed by the ITorganization to allow proper planning for the initial determination of powerr equirements.  The ‘downstream’ elementsof the electrical power and distribution system can be scaled, or adjusted toknown loads and future loading, but the electrical power service supplying thedata centre physical infrastructure (DCPI) has to be sufficiently sized tocarry the known load to be made for installing additional  capacity withoutincurring excessive downtime.

Oncean estimate is made for the amount of future loading, it is added to the baseloading information to establish the electrical critical load number in kW.

  • UPS loads

Thetotal electrical load power must include a factor for the inefficiency of theUPS system as well as the additional power required for battery charging. UPS efficiency varies between product models and varies dramatically depending onthe loading. Also, they are rarely run at the operating points where their advertised  efficiency is provided.

Battery charging is a significant butintermittent power consumer.  Undernormal operation with a charged battery the charging load is negligible.  However, when a battery has been partially orcompletely discharged the battery charging power can be on the order of 20% ofthe rated UPS load.  Although this loadonly rarely occurs,  the generator and service entrance must be sizedaccordingly.

  • Lighting loads

Lightingloads account is a function of data centre floor area: A good rule of thumb forthis type of load is 2 watts per square foot or 21.5 watts per square meter.

  • Cooling system

Coolingsystems vary widely in efficiency but can be broken down into chilled watersystems and direct expansion systems. Chilled water systems are generally more efficient and a rule of thumbfor power consumption is 70% of the total peak load being supported.  Direct expansion systems require about 100%of the total peak  load being supported. Note that cooling loads have start-up peak loads that exceed the steadystate values.

  • Sizingthe electrical power system

Twoimportant numbers have been determined that will assist in estimating the sizeof the electrical system are the total critical load and the total coolingload. The electrical supply must be large enough to support the sum of thesetwo numbers, plus the lighting load.

The steadystate power consumption of theloads within a data centre establishes the power consumption for purposes ofdetermining electrical costs.  However,the  electrical service and the generator power sources that provide power tothe data centre cannot be sized to the steady state values.  These sources must be sized to the peak powerconsumption of the loads, plus any derating or oversizing margins required bycode or standard engineering practice. In practice, this causes the  electrical service and generator sizing tobe substantially larger than might be expected.

  • Final electrical capacity computation

Oncethe total electrical capacity is estimated in kW, two critical determinationscan be made:  the first is an estimate ofthe electrical service needed to supply the data center, and the second is thesize of any standby power generator capacity that may be needed to achieve thedesired availability.

Sizingof the electrical service

Theelectrical service can be calculated as follows:

1.1.Takethe total electrical capacity required in kilowatts and multiply by 125% tomeet code or regulatory requirements.

2.2.Determinethe three phase AC voltage of the service entrance to be supplied by theutility.

3.3.Usethe following formula to determine the electrical service size to supply thedata centre, in Amps:

Amps= (kW x1000) / (Volts x 1.73)


Assessingthe electrical power required to support and cool critical loads is essentialwhen planning the development of a facility to meet availabilityexpectations.  By  employing the aboveprocess, a reasonable estimate of the power requirements can be made.  This will help specify the size of the datacentre physical infrastructure components and once the sizing determination ismade, conceptual and detailed planning can go forward with the assistance of acompetent systems supplier or a  consulting engineer. Cost estimation can thenbe made based on the size and reliability configuration determined from thepower needs assessment.

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