UPS selector & runtime calculator

There is no honest general answer, which is why this page will not print one. Runtime is set by the energy in the battery pack, the load drawn from it, and the efficiency of the inverter converting one into the other. Change any of those and the answer moves — so the only trustworthy figure is the manufacturer's runtime chart for that exact model, at your load. Every result above links to its product page for precisely that reason.

What can be said generally is the shape of the curve, and it surprises people: runtime does not scale linearly with load. Halving the load more than doubles the runtime, because a battery discharged hard delivers less of its stored energy than the same battery discharged gently. This is the practical argument for headroom — a UPS running at 40% of its rating is not just safer, it stays up considerably longer than the arithmetic suggests.

For most organizations the target is not hours, it is a clean landing: enough minutes to ride out the short outages that make up the large majority of utility events, and enough to trigger a graceful shutdown of anything that would otherwise lose data. That is typically a handful of minutes, and it is what a standard-battery UPS is designed around. Shutdown software or a network management card is what converts those minutes into an actual orderly shutdown, and it is worth ordering at the same time.

When you genuinely need fifteen, thirty or sixty minutes, the answer is a model that accepts external battery modules plus the right number of cabinets — or a generator, with the UPS bridging the seconds until it picks up the load. Both are configurations rather than filters, so send us the load and the hold-up time and we will scope it against real runtime charts.

One last variable that is easy to forget: batteries age. Sealed lead-acid strings are generally considered to be at end of service life after roughly three to five years, and they lose capacity steadily before that, faster in a warm room. A UPS that comfortably covered your shutdown when it was installed may not cover it in year four, so treat battery replacement as a scheduled item rather than a fault to be discovered during an outage.

Standby (offline) units pass utility power straight through and switch to battery when it fails. They are the cheapest option and are appropriate for a single desktop or a small appliance, but they do nothing about the sags and surges that never quite become an outage.

Line-interactive units add automatic voltage regulation (AVR), which corrects under- and over-voltage without touching the battery. That matters more than it sounds: in most buildings the common event is a brown-out, not a black-out, and a standby unit answers every one of them by discharging the battery, which wears it out. Line-interactive is the sensible default for network closets, small server rooms and edge racks.

Double-conversion (online) units rectify incoming AC to DC and invert it back continuously, so the load is permanently isolated from the utility and there is no transfer time at all. That is what you buy for a data-centre rack, for equipment that is sensitive to frequency, or where a generator's imperfect output has to be cleaned up. It costs more and runs less efficiently, and for a wiring-closet switch it is usually more than the job needs.

Waveform is the other spec worth checking on smaller units. Servers and desktops with active power-factor-correction supplies can misbehave on a simulated (stepped) sine wave and want a pure sine-wave output. If the product page does not say pure sine wave and the load is a modern server, ask before you order.

Outlets and plug types: count the receptacles you actually need and confirm the types. A rack UPS may present IEC C13 and C19 outlets where your gear expects NEMA 5-15P, and its own input plug must match the circuit it is going on — a 208 V unit will not serve a 120 V feed.

Rack space and depth: rack UPS units are heavy and often deep, and a battery module doubles the commitment. Confirm the U height, the depth against your rack, and that the rails supplied match your posts. Plan the weight distribution low in the cabinet.

Management: a network management card or USB/serial connection plus shutdown software is what makes the UPS useful during an event rather than just after it. Confirm whether the card is included or an accessory, and that the software supports your hypervisor or NAS.

Service life: budget for a battery replacement roughly every three to five years, and check that the model's replacement cartridges are user-serviceable if your site has no on-site engineer. For rack fleets, a hot-swappable battery matters more than the sticker price.