Perhaps you have read our earlier report and are fully convinced about how an uninterruptible power supply (UPS) can protect against untimely power outages and eliminate the dangers of a power surge on your expensive new smart home appliances.
But buying your first UPS can be an intimidating experience. How do you make sense of specifications such as “watts”, “KVA” or “VA”? And what exactly does a tolerance rating of “500 joules” or “800 joules” mean?
Making Sense of Power Terminology
As noted in a blog post by Sarah Jane Hannon of Schneider Electric, a common confusion by consumers is the distinction between the watt (W) and volt-amp (VA) ratings and how they relate to the load that a given UPS can handle. The watt rating is used to identify the amount of utility power required to run a piece of equipment, while the VA rating is used for proper sizing of wiring and circuit breakers, she explains.
While equipment like incandescent light bulbs tends to have the same VA and watt ratings, the challenge arises for hardware where the VA and watt ratings differ significantly – such as with consumer-grade personal computers. This is typically due to the use of built-in power supplies with a high “power factor”, which is the ratio of the watt to VA.
UPS come with both maximum watt and VA ratings, though it is a de-facto industry standard for a watt rating that is 60% of the VA rating for small UPS systems. For instance, the popular APC Back-UPS Pro 1000 is a 1 kVA (The “k” in kVA is a short form for kilo, which denotes 1,000) device rated at 600 watts.
How does that help you find the right UPS though? For a start, understand that neither the watt nor the VA rating of the UPS can be exceeded by the appliance you are attempting to protect. And if the reliability of the equipment you are protecting with your UPS is absolutely vital, then a Schneider Electric white paper recommends (pdf) choosing a UPS with a watt rating greater than or equal to the VA rating of the protected appliance for a guaranteed safety margin.
Of Joules and eP Joule
Finally, what of “joules”? If you recall, many UPS incorporate surge protection functionality. In this case, the number measured in “joules” is the amount of energy that the UPS can safely dissipate in the event of a power surge. A unit with up to 1,000 joules of surge protection is typically considered adequate for most office equipment and small appliances, though you may want to consider a rating of 2000 joules or more for expensive hardware or PCs storing irreplaceable data.
You may see some UPS with “eP joule” ratings, which is used by many APC home UPS products. According to Hannon, “eP joule” is used because these UPS products were designed not to blindly absorb a power surge. Upon reaching the maximum voltage that the attached equipment can be subjected to, excessive voltage is shunted safely to the ground for safe “pass through” protection.
Of course, you don’t have to be a power expert to shortlist a suitable UPS for your needs. Additional information on how to make UPS selection easy and stress-free can be found here.
