Sizing a UPS Properly: VA, Watts, Power Factor and Real Battery Runtime

The question we actually get

"We bought a 3 kVA UPS. The power cut for five minutes and the server still went down. Why?" Nearly every time, the answer is one of three things: the UPS was sized on VA instead of watts, the batteries were old and cooked, or nobody set up automatic shutdown. This guide walks through sizing the way we do it on site, with the numbers.

Why this matters more in Laos

Power cuts, flicker and voltage swings are normal. Worse in the rainy season, worse again in the provinces. In many areas we measure a big voltage sag between 17:00 and 19:00 when every house switches on air conditioning and rice cookers. A UPS isn't just for blackouts. It protects power supplies and hard drives from the over- and under-voltage that quietly wears equipment out every day. Lightning season adds surges on long cable runs, and many sites rely on a generator that doesn't produce clean power.

VA versus watts

• VA (volt-amperes) is apparent power
• W (watts) is the real power equipment uses
• Power factor (PF) = W ÷ VA

W = VA × PF

Most current small online UPS models are rated at PF 0.9 or 1.0, so a 3,000 VA unit delivers 2,700 W or 3,000 W. Budget line-interactive units are often rated around PF 0.6, which means the same 3,000 VA only delivers 1,800 W. Read both numbers on the datasheet and keep the load under both. Modern servers and switches have power-factor-corrected supplies, so for sizing you can treat their watts and VA as almost equal. The UPS rating is the limit that matters.

Three types of UPS

The international standard IEC 62040-3 sorts UPS designs by how much they protect the load:

Where power is unstable or you run a generator, an online UPS is the safe choice. Generator voltage and frequency often wander enough that other types keep flipping to battery and draining it for no reason.

Which type for which job

Sizing step by step

1. List every device that will plug into the UPS. Monitors, printers and air conditioners don't belong on it.
2. Find the watts actually drawn. Measure with a clamp meter or a metered PDU, or use the datasheet's typical figure. The maximum printed on the power supply label is usually far above real draw. A server with two 750 W supplies may draw 250 W.
3. Add 20–30% for growth and so the UPS isn't running flat out.
4. Check against the UPS watt rating, then the VA rating. Both must be higher than your load.
5. Decide the runtime you need: enough to ride out short cuts, or enough to shut down cleanly, or enough to wait for the generator.
6. Read the manufacturer's runtime chart at your load and add external battery packs if needed.

Example 1: a small server room

• Add 25%: 1,260 × 1.25 = 1,575 W
• A 2 kVA online UPS rated at PF 0.9 gives 1,800 W. That covers 1,575 W with a little room. If you know more servers are coming, go to 3 kVA now; it costs less than replacing the UPS in two years.
• At 1,260 W, the internal batteries of a typical 2–3 kVA online UPS last somewhere around 5 to 15 minutes. That is enough for short cuts and a clean shutdown, not for waiting out a long outage.

Example 2: a branch network cabinet

With 25% margin that is about 190 W. A 1 kVA line-interactive unit rated 600 W runs this at roughly a third of capacity, which is where you want it. Runtime from two internal 12 V batteries at this load typically lands around half an hour, but check the chart for the exact model.

Runtime without fooling yourself

Battery energy needed (Wh) ≈ load (W) × time (hours) ÷ inverter efficiency

1,260 W for 30 minutes at 0.9 efficiency: 1,260 × 0.5 ÷ 0.9 ≈ 700 Wh actually delivered.

The catch: lead-acid (VRLA) batteries give up much less than their label when discharged fast. Over 30 minutes you may get only 50–60% of the 20-hour rating, so 700 Wh delivered needs roughly 1,200–1,400 Wh of nominal battery. Batteries also lose capacity as they age, and are normally replaced when they fall to about 80%. Size so that runtime is still enough on old batteries. So the manufacturer's runtime chart is the number to trust, not a calculation from the label. For long runtimes, pick a model that takes external battery packs (a long-backup model).

Our UPS runtime calculator, under Tools on this site, gives a first estimate before you look at datasheets.

Plan the shutdown, not just the runtime

Runtime only helps if something uses it. Install the UPS vendor's shutdown software or a network management card, and connect it to your hypervisor and NAS. Then time it: how long does it take to shut down all virtual machines, then the hosts, then the storage? If that takes 8 minutes, set shutdown to start while at least 12–15 minutes of battery remain. Test it once a year with a real pull of the input breaker, outside working hours.

UPS and generator

A UPS isn't built to carry hours of load at a sensible price. For continuous operation: a UPS for 10–30 minutes plus a generator with an automatic transfer switch (ATS). The UPS covers the 10–30 seconds the generator needs to start and take load.

Generator size depends on the UPS input design. Modern online UPS units with IGBT (active) rectifiers draw clean current, and a generator of about 1.25–1.5× the UPS kVA is usually enough. Older units with thyristor rectifiers draw distorted current and need about 2×. Remember the UPS also recharges its batteries after a cut, which adds load. Confirm the ratio with both manufacturers before ordering, and turn off ECO or high-efficiency mode on UPS units fed by a generator.

Batteries and heat

VRLA batteries are designed for around 25°C. For every 8–10°C above that, battery life roughly halves. A battery sold as "5-year design life" usually lasts 3–4 years in a cool room, and a UPS sitting in a 35°C electrical cupboard may need new batteries every two years. Lithium-ion handles heat better and often lasts 8–10 years, at a higher upfront price. Either way, schedule regular self-tests and keep the results.

Installation details that matter here

• Put the UPS on its own circuit breaker, sized to the UPS input current in the datasheet.
• Earth it properly. Many older buildings have weak or missing earth, and that defeats surge protection.
• For 3 kVA and up, fit an external maintenance bypass switch so the UPS can be serviced without shutting down the rack.
• Leave the space around the UPS that the manual asks for. Fans pull dust in; clean filters during the dry season.
• Keep a record of the battery install date on a label on the UPS.

What we keep finding

1. UPS bought on VA without looking at watts.
2. Load sitting above 80% all the time.
3. Shutdown software never installed, or installed and never tested.
4. A laser printer or kettle plugged into the UPS. The inrush current trips it into overload.
5. The UPS in a hot room with no AC, batteries swollen after two years.
6. Batteries never replaced, so a UPS that showed 20 minutes on day one now gives 2.

Our recommendation

Servers and core network gear get online double-conversion, no debate. Branches and network cabinets where power isn't too bad can use line-interactive with AVR. Design for a load of 50–75% of UPS capacity. Hook the UPS into monitoring so you know the moment power drops or a battery weakens. And order spare batteries early, because imports from Thailand can take weeks.

Send us a list of what will plug into the UPS, or photos of the rating labels on the back of each device. We'll work out the load and tell you the UPS size and the runtime you'll really get.