Choosing an online double-conversion UPS well is a site-survey exercise, not a catalogue pick. Build a load inventory, rank each load by what an outage actually costs, convert real watts to kVA at a 0.8 power factor with a growth margin, then fix runtime, input window and monitoring before you request a quote. Sites that skip the inventory and buy against a headline kVA number are the ones that later discover a hidden load with no protection, or a unit running at 95 percent capacity with no headroom for a new server.
List every device that must stay powered, with its real measured wattage rather than the fuse rating on the plate. Workstations, controllers, switches, scanners, small instrumentation and the network gear that ties them together all count. Ignore the marketing kVA on the box; what the battery has to cover is the sum of true watts. A clipboard survey of one afternoon beats a guess for the life of the installation, because the unit is sized once and then expected to protect everything behind it for five to ten years.
Not every load deserves the same protection, and ranking them stops you overspending on a socket that only feeds a desk lamp. Group them into three tiers: life-safety and process-critical at the top, business-important in the middle, and convenience at the bottom. The top tier sets the runtime and the topology; the middle tier shares that protection; the bottom tier is deliberately left out so the battery budget goes where it matters. A clear tier list also makes the later vendor questions unambiguous, because you can state which loads are in scope and which are not.
The core conversion is straightforward. Divide total true watts by the output power factor, then add headroom for load growth and inrush. As a working equation:
kVA needed = (sum of watts) / 0.8 / 0.75
The 0.8 is the typical output power factor of a modern UPS, so a 6 kW load is 7.5 kVA before margin. The 0.75 divisor leaves 25 percent headroom, so the same load calls for 10 kVA. That headroom absorbs a new cabinet, motor inrush and the fact that real sites always grow. Size to the 75 percent rule and the unit runs cooler and lasts longer than one pinned at its nameplate edge.
Runtime is a business decision, not a technical default. If a generator starts reliably within five minutes, the battery only has to bridge the start and stabilise the bus, so five to ten minutes is enough. If there is no generator, runtime becomes the question of how long the process must keep running unaided, and that can be twenty minutes or more. State the runtime target up front, because it drives battery count and footprint more than anything else in the specification. A unit sized for the wrong runtime either costs too much or fails the first real event.
An online UPS accepts a wide input voltage and frequency window, which is exactly why it is the right call wherever a generator is in the path. Generators wander in frequency during load steps, and a topology that simply follows the input will either reject the source or pass that wander straight to the load. Confirm the acceptable input range in writing, and confirm the unit holds output frequency steady through a generator transition. This single check removes more field failures than any efficiency argument, because a UPS that drops the load during a transfer has done the one thing it was bought to prevent.
Protection you cannot see fails quietly. Require SNMP or a network card that reports battery state of health, not just remaining runtime, and that raises a remote alarm before a fault becomes an outage. Tie that alarm into the same supervisory bus as the rest of the site so a weak battery appears on a dashboard rather than during the next power cut. The common specification mistakes and the monitoring questions worth asking are collected in our field notes at https://www.upsboss.com/faq/, which most buyers review before writing the final bill of materials.
Suppose the cell draws a controller at 200 W, a switch at 50 W, two panels at 120 W each and a small instrument at 160 W. True watts total 650. Divide by 0.8 gives 812 VA, divide by 0.75 gives 1,083 VA, so a 1.5 kVA online unit is the sensible pick with margin. Runtime target is eight minutes to cover a generator start, so the standard battery string fits. The whole exercise took one survey and one formula, and the result protects exactly the loads that matter without paying for capacity the cell will never draw. The full range of online units sized this way is listed at https://www.upsboss.com/products/.
If you send us your load inventory and runtime target, our engineers will return a sized configuration with a single-line diagram and a written efficiency curve at your operating point. Start from the product range at https://www.upsboss.com/products/ or open a requirement through the contact page.
Key takeaway: select an online UPS by surveying true watts, tiering loads by outage cost, converting at 0.8 power factor with a 25 percent margin, and fixing runtime and monitoring before you quote, because a unit sized to the site protects the right loads for its whole service life instead of failing the first event it meets.
Why divide by 0.75 instead of just adding a flat margin?
The 0.75 divisor builds in 25 percent headroom in one step, which covers load growth, motor inrush and the fact that units run cooler and live longer below nameplate. A flat additive guess leaves you arguing later about whether there is room for the next cabinet.
Do I need online topology if the grid here is stable?
Only if a generator or a sensitive load is in the path. For a tolerant office load on a clean supply, line-interactive is cheaper, but the moment a process controller or a generator enters the picture, online double-conversion is the safer specification.
How do I know the runtime target is right?
Ask how long the process must survive without mains. With a reliable generator, five to ten minutes bridges the start; without one, runtime is whatever the business decides it can tolerate, and that answer should come from operations, not from the UPS datasheet.
Contact: Frank Zhang
Phone: +86-135 5688 8641
Email: frank@upsboss.com
Add: Jufeng Road, Guangming Street, Guangming District, Shenzhen City, Guangdong Province, China
