UPS Energy Efficiency Calculations

The topic of UPS efficiency continues to rise up the corporate IT and datacenter manager’s agendas. Whether you are looking to install a new uninterruptible power supply or upgrade an existing UPS installation, improving operational efficiency can not only contribute towards capital cost recovery but will also lower total costs of ownership over the life time of the system.

For many people, paying for energy has become an almost de facto part of everyday life; whether its diesel for a car, gas for heating a home or the electricity used by a business. We sometimes complain about the cost of energy without really tracking its usage, look to identify wastage areas and analyzing the true cost we are paying per kiloWatt.

Electricity is measured and charged for in terms of kiloWatt Hours (kWH) or of you are a very large consumer, in terms of MegaWattHours (MWH) or GigaWattHours (GWH). A typical business consumer may be paying £10p per kWh (or less) for its electricity and this is regardless of the systems that are using the electricity on site.

So, consider an IT suite that may be drawing 20kW of power on average or 20kWh of electricity (power consumed per hour). This site has no uninterruptible power supplies installed and if the mains power fails, the IT servers crash. A UPS system is then installed to protect the critical power path to the servers and other key components within the IT suite. The system keeps running when the mains power fails. Job done but with an operational overhead and this is the energy efficiency of the UPS.

The power drawn by the IT suite rise if a UPS system is installed and the size of the rise is dependent in the first instance on the operational efficiency of the uninterruptible power supply. Depending on UPS energy efficiency this could add a further 2kW to the power demands or 2kWh to the energy bill. At £10P per kW this may not sound significant but in terms of annual electricity bills this will be £1,752 of additional electricity to pay for. This level of charge rise proportionally and so for larger UPS installations the costs are far higher.
Another variable to bring into the UPS energy saving equation is operational loading. Efficiency is a measure of useful work performed by a machine or process in ratio to the total energy expended. For uninterruptible power supplies, energy efficiency is a non-linear curve. This means it varies depending on the % load applied to the UPS.

UPS manufacturers have invested in their R&D to develop more energy efficient systems and with a flatter non-linear curve where the optimum efficiency can be maintained over a wider load profile to as low as 25%. The reasons for this are fairly clear. Most UPS systems are not loaded to 100% but run at around 60-80% or sometimes less.

This has implications for older UPS systems and especially those based on transformer technology. A traditional transformer-based UPS will have a poorer energy efficiency as the load is reduced on it. A typical system may offer 90% efficiency at 80% load, dropping to 70% or less in operational efficiency as the load reduces.

In comparison a modern transformerless UPS system may achieve 97.5% operational efficiency at 90% load and maintain this level of UPS energy efficiency down to 25%. This level of operational performance is also typical of modular UPS systems, which are based on transformerless designs. A further advantage of modular UPS is that in N+X configurations, modules can also be set to go into ‘sleep-mode’ automatically to further reduce energy losses.

UPS energy efficiency is one of those operational cost factors that should not be ignored when it comes to a new UPS installation or upgrade. The Eco Power Projects team provides free energy efficiency audits and using its extensive UPS knowledge bank can provide efficiency cost benefit analysis between manufacturers and upgrade options.