Sizing Uninterruptible Power Supplies for Energy Efficiency

By: Alison Campbell

Rising electricity costs and the prospect of energy rationing mean it's never been more important for uninterruptible power supplies to be correctly sized. Increasingly, power protection customers are being urged to be energy efficient and play their part in tackling climate change. If an uninterruptible power supply is significantly over-sized it will run inefficiently. 'Undersizing' will introduce the risk of system overloads.

This is particularly true for a power-hungry end-user such as a datacenter. The latest high-end servers require more power to operate - and greater cooling resources. According to some reports, every megawatt needed to power datacenter hardware draws another 1.5 MW to cool it!

Is just simple math?

According to business continuity experts, the proportion of business continuity plans invoked by power failures has jumped recently. Why? Either energy supply is less reliable or existing uninterruptible power supply installations are unsatisfactory - or non-existent!

Historically, power protection has been oversized so that, at full capacity, the system itself isn't overloaded. Oversizing also leads to higher installation and ongoing maintenance costs. Undersizing, however, especially in a busy datacenter where equipment is being continuously added and switched in and out, will soon cause problems.

Whilst an on-line uninterruptible power supply has a built-in automatic bypass for emergencies, running close to design limits with regular overloads is bad practice. Some oversizing is always best.

The secret of power protection sizing

Firstly, the equipment being protected (the 'load') must be categorized into critical, essential and non-essential loads.

Critical Loads are the IT and electrical components that make up the business architecture and without which business continuity would be lost: servers, computers, storage devices, telecommunications, security and building management systems. Their power protection will probably require some extended runtime to keep equipment running continuously; probably also redundancy so that if a power outage occurs, and one UPS is unserviceable, another can take over.

Essential Loads are vital to the business (datacenter, machine room or some such) but in their absence some functionality can exist. Some essential loads may need redundancy to be built into power protection, but in many instances back up needn't be as robust as for critical loads.

Non-essential loads are those that the business can survive without until power is reinstated: printers and canteen facilities for example.

The Power Factor in UPS sizing

An understanding of the importance of 'real power' is crucial for power protection sizing. Kilowatts (kW) are a measure of the real power drawn by the load whereas kilovolt-amps (kVA) are a measure of apparent power. The difference between the two is the power factor (pf) and its size can present problems when specifying uninterruptible power supplies.

The greatest efficiency comes from operating at a power factor of 1.0 or 'unity'. Though computer equipment has been approaching this in recent years, many UPS systems have not kept pace. The solution is to specify an uninterruptible power supply with as high an output power factor as possible - with a power factor of 0.9 set to become the standard. UPS manufacturers like Riello UPS have set the bar at this level with new energy efficient products being released onto the market.

As well as being correctly sized, power protection systems should consist of units offering an input power factor of not less than 99%. This should enable users to cut energy wastage by up to 40%. Harmonics generation and heat output should also be low and today's UPS should offer a small footprint so as (in the case of a datacenter) not to take up too much valuable revenue earning space.

Scalability

Finally, and looking to the future, power protection should be 'scalable'. This means additional uninterruptible power supply modules can be added without having to rip out and replace the unit each time power protection requirement increase.

As always, time invested in assessing and categorizing loads, determining their size and power requirements and calculating a correctly sized power protection system will pay dividends. Naturally, UPS sizing consultancy is available from manufacturers such as Riello.

The way to enjoy peace of mind and cost savings from a correctly-sized power protection system, is to think about slightly oversizing UPS, categorizing loads, calculating power factor and being mindful of scalability.

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