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FAQs - Energy Efficiency

Commone questions on energy efficiency and and how to reduce operational costs in server rooms and data centre environments.

When we look at a server room or datacentre in terms of energy efficiency, we are looking at loads on the demand-side of the electricity distribution infrastructure. Related terms include Demand Side Management (DSM) and Demand Side Response (DSR). Our objective is to find ways to improve the overall energy efficiency of the facility through behavioural changes, more efficient hardware use and where necessary hardware upgrade to more energy efficient infrastructure systems (uninterruptible power supplies and cooling systems). We can make recommendations for IT hardware upgrade but focus purely on the infrastructure and related systems.

We often find quick solutions to energy efficiency lie in critical cooling and air conditioning systems and the air flow path within the server room or datacentre. Typical issues are poorly sealed floor tiles or rack cabinets with missing or poorly fitted blanking plates. Other solutions to poor energy efficiency figures (leading to high PUEs) include not upgrading to the latest energy efficient solutions including file servers, cooling systems and uninterruptible power supplies.

We do not charge for an energy efficiency audit, if we can easily book the review into the diaries of our project managers. If a client requests an outside-working-hours audit or one that will involve us travelling over 250miles, we may have to make a special booking and will quote a reasonable price for this.

We provide a complete energy efficiency audit service. This can be booked through our projects team who can arrange a suitable time and date. During the visit, we will do a site walk through to gain an overall view of the project and to identify easy-fixes and the hardware systems that will require review. After the visit, we provide a detailed energy efficiency report with recommendations for improvement and where necessary training and system upgrades.

PUE standards for Power Usage Effectiveness and measures the energy efficiency of a server room or datacentre.

PUE = Total Facility Energy / IT Equipment Energy

The reciprocal measure of PUE is Data Center infrastructure Efficiency (DCiE)

DCIE = IT Equipment Power / Total Facility Power x 100%

PUE and DCiE were developed by the Green Grid ( and are widely accepted energy efficiency measures that can also be used for bench marking.

It is important to note that PUE does not factor in sustainability and how the electricity used to power the facility is generated e.g. nuclear or renewable power sources. In addition, PUE does not consider energy reuse. Some larger datacentres may reuse heat created during the cooling process which in turns reduces the total energy consumption of the facility.

The energy efficiency of a computer or server room air conditioning unit is measured using the Energy Efficiency Ratio (EER).

EER is a ratio of the cooling capacity measured in British Thermal Units (BTU or Btu) compared to the power input in Watts.

The higher the EER, the more energy efficient the air conditioner.

Kilowatts (kW) measures the amount of power required by a device to the work it is employed for. If you have a rack of servers in data centre with a total power requirement of 5kW then this is the amount of power that must be supplied from your electrical supply to support the load.

Kilowatt-hours (kWh) mesaures the amount of energy used by a device within 1hour. Energy storage is typically measured in kWh. In our example, if you have a 10kWh hour energy storage system this would provide 2hours of power backup for our 5kW load i.e. 10kWh / 5kW = 2hours. If the load of 5kW is reduced to 2.5kW through for example, load-shedding, then the result becomes 4 hours i.e. 10kWh / 2.5kW = 4hours.

Kilowatts can be converted to Kilowatt-hours using the formula:
Load Power requirements (kW) x the time the load is to run for (hours) = answer in kW.h

The formula to convert load power requirements in kilowatts (kW) to the energy used in an hour (kw) is:

Load power (kw) x time run for (hours) = answer (kWh)

If you have a generator rated at 100kW with a day-tank of fuel for 8hours, the total load in kWh is:

100 (kw) x 8 (hours) = 800kWh

If the load on the generator is only 50kW then the same coniguration would provide a total kilowatt-hours of:

(100 / 50 ) = 2 × 800kWh = 1600kWh

Kilowatt-hours (kwh) measures the amount of energy provided or consumed within 1hour.

If you have a 50kW standby diesel generator then it can produce 50kWh of energy per hour. The amount of time a diesel genertor can run for is dependent upon its fuel tank. A standard day tank may be sized to provide suficient fuel to run a generator for 8hours at full load.

In this example a 50kW generator with an 8hour runtime fuel tank will provide: 50kW x 8hours = 400kWh of energy. If the power load in kilowatts (kw) is reduced by 50% through load-shedding then the runtime available from the fuel tank doubles but the total energy storage capacity of the generator remains the same.

(50 / 2) = 25kw x 16hours = max 400kWh of energy.

Renewable Energy Factor (REF) is a metric to measure the percentage of renewable energy (RE) versus the total power usage in a data centre. The metric helps to assess carbon emission mitigation. The higher the REF the more sustainable the data centre and diverse its energy sources.

The REF formula = Total Data Centre Energy Consumption (annual in kWh) / the Renewable Energy owned and controlled by the Data Centre (kWh).

The nearer the REF is to 1, the greater the percentage of RE used by the data centre. The metric does not take into account, extra capacity of renewable power generated by the data centre site but not used.

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