So how long does an Uninterruptible Power Supply last when the power goes down? The answer varies according to the size of the system and the amount of power required. If you’re running an office with several dozen computers in it, along with a server room, then you’ll need an accordingly sized UPS. If you’re just running one computer, then you might get away with a smaller one.

A UPS’s power capacity is measured in Voltage-Amps (rather than Watts, which measures the power consumed rather than supplied). As a general rule, a UPS can support a load of around 9kW for every 10kVA.

If you’d like to learn more, please get in touch with one of our specialists!

The kVA-rating determines the total amount of power that the device can supply at any one time, but this needn’t imply anything about how long the supply will last for.

kW vs kVA

As crude as the following is, it is best to describe the relationship between kW & kVA in terms of a pint of “carbonated amber liquid”: -

The glass is 100% full but only the kW (in this case the liquid) is useful whereas the kVA (the foam) is wasted. Typically, this relationship is 80% (a poor bartender indeed!) so the “Power Factor” is said to be 0.8.

This means that whilst a UPS system can produce 100kVA of power. The equipment being powered by the generator can only use 80% of it (80kW at 0.8 power factor).

 How can I calculate the amount of time I have?

It’s very difficult to say with any precision how much time you’ll have, because of another complicating factor: the non-linear nature of a UPS battery’s discharge. The power-capacity and the rate of discharge don’t correlate evenly; small changes in the former can produce big changes in the latter.

1000VA UPS might be able to power a small office computer for around seven minutes. Make the upgrade to a 10kVA UPS, and you might see this figure jump up to around twenty-sevenminutes. Go too small, on the other hand, and the available time shrinks to just a few minutes or less.

Is it better to go bigger?

Many manufacturers, such as Riello UPSAPC UPSVertiv UPS and Eaton UPS, will provide an estimate of how long the UPS can run at full capacity. However, as conditions will always vary, it is worth obtaining a larger UPS than you believe necessary in order to ensure full coverage. If the battery in a UPS is drained too frequently due to it supplying a heavier load than expected, then its total capacity will begin to suffer, and you’ll need to replace the battery sooner.

Can I ride out a power cut with a UPS (Uninterruptible Power Supplies)?

A UPS is designed to safeguard sensitive equipment and afford you the time required to shut everything down properly. What it isn’t designed to do is ‘ride-out’ a power-cut; there is only a finite amount of power stored within a UPS and a power-cut could be a long-term problem. The solution to a mid to long-term disruption would be a generator. If you’d like to immunise your business against protracted outages, then a generator system and a UPS make excellent partners: together, they’ll keep the lights on for extended periods.

What about the automatic shutdown?

If you want to be especially safe, you can automate the shutdown process via a USB or RS-232 connection. In a setup like this, the UPS will instruct your servers to shut down the moment the power goes out. This saves you the trouble of racing into the office to save your data and eliminates the opportunity for human error.

What UPS should I choose?

Businesses come in a range of shapes and sizes, and so too do the UPS devices that support them. A good UPS does more than a shield from power outages. It’ll also correct minor problems in the power supply, like voltage spikes and brownouts, and thereby ensure that your equipment enjoys the longest possible lifespan.

If you’re in the market for a UPS, you might be intimidated by the breadth of options. That’s where we can help: we can recommend a UPS that’s appropriate for your needs, perform the installation for you, and give you advice on how to get the optimum performance out of it. We can also help to configure your servers so that they automatically power-down in the event of a blackout through the use of a UPS.


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UPS Systems have been supplying backup power solutions for over 25 years and can propose a solution regardless of the size of your requirement. If you have any questions or queries, please contact us. We can supply small UPS (Uninterruptible Power Supplies) to support a home office, to a larger UPS to keep the central server running or a diesel generator to manufacturing equipment.  

UPS Systems plc supply a wide range of uninterruptible power supplies including those from Riello UPS and Eaton UPS as well as the UPS battery packs designed to go with them. UPS Systems plc also offers various diesel generators and industrial generators including 60kva generator80kva generator and 100kva generator from a wide range of manufacturers including AKSA generatorSDMO generator and Pramac generator.