Calculating total power requirements for data centres

Part of data centre planning and design is to align the power and cooling requirements of the IT equipment with the capacity of infrastructure equipment. Schneider Electric presents methods for calculating power and cooling requirements and provides guidelines for determining the total electrical power capacity.

Introduction

With the adoption of scalable 'pay as you grow' uninterruptible power supply (UPS) architectures, such as Schneider Electric’s APC Symmetra range, the data centre manager can simply add modules as needs grow.  However, it is easy to lose sight of the future electrical needs of the data centre or data room within a larger facility.

Sizing the electrical service requires an understanding of the amount of electricity required by the cooling system, the UPS system, and the critical IT loads.  The power requirements of these elements may vary substantially from each other, but can be accurately estimated using simple rules once the power requirements of the planned IT load are determined.  In addition to estimating the size of the electrical service, these elements can be used to estimate the power output capacity of a standby generator system, if one is required. 

Needs assessment

The needs assessment essentially establishes the availability needs of the business applications being processed by the IT equipment.  A business process that is not time sensitive may require no internal redundancies to increase availability.

More time sensitive sites may require a degree of redundancy in key component systems and have configurations of an “N+1” topology. The most critical applications requiring total availability (7x24) would have a 2N topology where critical systems would be completely redundant. 

No matter what the actual UPS system design configuration, the core issue of providing sufficient power to the critical load and keeping it cool must be carefully addressed.  Underestimating the required capacity may result in future power disruptions when forced to increase capacity, and over estimating leads to excessive initial installation costs and higher ongoing maintenance expenses.

Determining electrical power capacity

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

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

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

1.1.Add up 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 the device to get the VA, which approximates the amount of watts the device will consume. 

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

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

Future loads

Data centre loads are dynamic, and the IT equipment will be under an almost constant state of change during the lifetime of the facility.  A realistic assessment of the scope and timing of future changes and upgrades should be developed by the IT organization to allow proper planning for the initial determination of power requirements.  The ‘downstream’ elements of the electrical power and distribution system can be scaled, or adjusted to known loads and future loading, but the electrical power service supplying the data centre physical infrastructure (DCPI) has to be sufficiently sized to carry the known load to be made for installing additional capacity without incurring excessive downtime. 

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

UPS loads

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

 Battery charging is a significant but intermittent power consumer.  Under normal operation with a charged battery the charging load is negligible.  However, when a battery has been partially or completely discharged the battery charging power can be on the order of 20% of the rated UPS load.  Although this load only rarely occurs, the generator and service entrance must be sized accordingly. 

Lighting loads

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

Cooling

Cooling systems vary widely in efficiency but can be broken down into chilled water systems and direct expansion systems.  Chilled water systems are generally more efficient and a rule of thumb for 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 steady state values.

Sizing the electrical power system

Two important numbers have been determined that will assist in estimating the size of the electrical system are the total critical load and the total cooling load. The electrical supply must be large enough to support the sum of these two numbers, plus the lighting load. 

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

Final electrical capacity computation

Once the total electrical capacity is estimated in kW, two critical determinations can be made:  the first is an estimate of the electrical service needed to supply the data center, and the second is the size of any standby power generator capacity that may be needed to achieve the desired availability. 

Sizing of the electrical service 

The electrical service can be calculated as follows:

1.1.Take the total electrical capacity required in kilowatts and multiply by 125% to meet code or regulatory requirements.

2.2.Determine the three phase AC voltage of the service entrance to be supplied by the utility. 

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

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

Conclusion

Assessing the electrical power required to support and cool critical loads is essential when planning the development of a facility to meet availability expectations.  By employing the above process, a reasonable estimate of the power requirements can be made.  This will help specify the size of the data centre physical infrastructure components and once the sizing determination is made, conceptual and detailed planning can go forward with the assistance of a competent systems supplier or a consulting engineer. Cost estimation can then be made based on the size and reliability configuration determined from the power needs assessment. For more details, please download Schneider Electric white paper 3 Calculating Total Power Requirements for Data Centres at www.apc.com/whitepapers