Methods used to assess UPS efficiency

The importance of reliable UPS in business is beyond doubt. Most businesses also understand the need for a robust regime of UPS maintenance. What is not always appreciated is that there can be considerable variations between different models in terms of their energy efficiency. As with any kind of electrical equipment, the more efficient your UPS system, the cheaper its running costs. Most manufacturers specify an efficiency rating for their products, but there are two reasons why this might not be as helpful as it could be. One is that some users will simply pay no attention to it: they’re just glad to have a UPS that works. The other reason is that the efficiency rating may have been calculated by tests conducted under laboratory or factory conditions and do not necessarily reflect operational realities. Making your calculation as part of your normal UPS maintenance is a sensible part of your energy and resources management.

What is UPS efficiency?

This is measured as the level of power remaining after the UPS has taken the power it needs to operate. To make this clearer, by example, a UPS with an efficiency rating of 95% requires 5% for its own running needs. The remaining 95% can be used to power the load and any connected systems. The reason to be slightly cautious about manufacturers’ claims is that they are judged on the basis that the unit will run with a 100% full load. However, this is seldom the case in practice. As the load requirement falls, so does the efficiency level. For example, if you have a large UPS that only has a little load of 40%, its efficiency may be reduced to 85%. Efficiency rates differ from unit to unit, and the efficiency of an individual unit can vary according to the load level.

Why does efficiency matter?

The lower the efficiency rating, the more energy is practically squandered in the UPS unit’s operation. In the example above, the waste is 5%. The higher the efficiency, the smaller the loss of energy in terms of heat. UPS units with lower efficiency generate more surplus heat and can therefore require more powerful — and more expensive — air conditioning to maintain a safe ambient temperature. An operating efficiency improvement of just a couple of percentage points can create significant savings over the life of a UPS unit, which is generally about ten years.

How to assess UPS efficiency?

With apologies to those for whom mathematical equations revive unpleasant classroom memories, it’s necessary to express the calculation as a simple formula. This will tell you how much energy is wasted at operating levels below 100% efficiency:

((UPS kW Rating x Load %) / UPS Efficiency) — (UPS kW Rating x Load %)) x (24×365) x kWh Price.

If we assume a price of 10p per kWh (the real figure is somewhat higher) then the calculation for the running costs of a 100kW UPS would look like this:

100kW UPS at 90% load is 92% efficient = £6,855 per annum
100kW UPS at 25% load is 70% efficient = £9,386 per annum

Modular UPS systems

When thinking about efficiency and reliability, it is worth considering modular UPS systems, which are becoming very popular. They are constructed from individual UPS modules and provide a flexible alternative to the monolithic model of a single UPS unit, in which one failure can cause the whole system to fail. In a modular system, one faulty module can easily be replaced without interrupting the operation of the rest.

In terms of efficiency, modular systems can significantly reduce running costs, because it is possible to turn off selected modules if the load is lower than expected. Energy consumption falls, and the modules which remain in operation do so at maximum efficiency.

By using the same formula outlined above, modular systems can attain efficiencies of 97.5 per cent at 100 per cent load, even if the load is as low as 25%. The costs are much lower at these two extremities of the scale.

Efficiency assessment as part of UPS maintenance

You may just ask your UPS maintenance engineer to assess you if the technicality of this is a bit intimidating. The key thing is to do it, so you can see how much money you’re spending, how much you’re wasting, and how much you could save.