In February 2025 a grid failure knocked out power to nearly all of Chile, and in a single afternoon everyone found out who had actually sized their backup. Some racks rode it out like nothing happened. Other UPSs — the ones that "lasted an hour" according to the salesperson — died at minute 12, with the client staring at their watch. The difference wasn't luck. It was understanding three or four numbers the datasheet only mumbles. Let's say them out loud.
Watts vs volt-amperes: the pint and the foam
First trap in the UPS aisle: they're sold in VA, but your load lives in watts. They are not the same thing, and mixing them up is the fastest way to buy small.
Picture a pint of beer. The full glass is the volt-amperes (VA): apparent power, everything the circuit has to push around. The beer you actually drink is the watts (W): real power, the part doing useful work. The foam is reactive power — it takes up room in the glass and in your cables, but it quenches nobody's thirst. The power factor (PF) is how good your bartender is: PF = W / VA.
Modern IT gear, with power-factor-corrected supplies, runs at PF 0.9 or better, and most current UPSs are rated with an output PF of 0.9. The rule worth tattooing on your forearm:
1000 VA × 0.9 ≈ 900 usable watts. A "1 kVA" UPS does not feed 1000 W of servers; it feeds 900, and that's if the manufacturer was feeling generous. It works in reverse too: a 500 W load asks the circuit for 500 / 0.9 ≈ 556 VA. If you juggle kW, kVA and amps all day, the three-phase power calculator does the gymnastics for you, single- or three-phase.
Runtime is not linear: the Peukert effect
Second trap, and this one is sneakier: a battery at half load doesn't last twice as long — it lasts more than twice as long. And at full load it lasts less than simple arithmetic promises.
Blame the chemistry. When you pull heavy current from a lead-acid battery, part of the energy burns off in internal resistance and the reaction can't reach all the active material in time: you extract less total energy. It's like running. At a jog you can cover 40 km; at a full sprint you won't finish 10. You don't just tire "proportionally faster" — you blow up.
This is the Peukert effect. For a typical VRLA battery (exponent ≈ 1.2), cutting the load in half multiplies runtime by 21.2 ≈ 2.3, not by 2. In numbers: a UPS that gives 10 minutes at full load delivers about 23 minutes at 50% and roughly 53 minutes at 25%. That's why serious manufacturers don't publish "the runtime" — they publish runtime-vs-load curves, and it's why the UPS runtime calculator asks for your actual load, not just the model number.
The three layers of backup
Nobody sizes a single box "for the whole outage". Backup power is designed in layers, each with its own timescale and its own purchasing logic:
The UPS exists for the seconds and minutes: it absorbs the flicker, conditions the power, and buys time for everything else. The DC battery bank (the telecom-heritage 48 V) buys hours. The generator buys days — provided somebody remembered the diesel. As a table:
| Layer | Covers | Sized by | Calculator |
|---|---|---|---|
| UPS | seconds → minutes | VA (with PF) + runtime curve | ups-autonomia |
| 48 V DC bank | minutes → hours | Wh, DoD, efficiency → Ah | banco-baterias |
| Generator | hours → days | running kVA + motor starts + altitude | generador-kva |
Worked example: 500 W that can't even blink
A textbook field case: a comms rack with 500 W of measured load — switch, router, radios, a small NVR. The client wants 4 hours of runtime because that's how long outages last in their rural area. Layer by layer:
- Measure the real load: 500 W. With a clamp meter or the PDU readout — not by adding up nameplates. Nameplates declare worst case and typically inflate reality 2–3×.
- UPS: convert to VA. 500 W / 0.9 PF ≈ 556 VA. A 1 kVA UPS sits at ~56% load: the comfort zone. The manufacturer's curve at that load gives on the order of 10–15 minutes — enough for the next layer to take over, not enough to survive the afternoon.
- Battery bank: define the energy. 4 h target × 500 W = 2000 Wh.
- Convert Wh to Ah with the three factors. Ah = 2000 / (48 V × 0.5 DoD × 0.9 efficiency) = 2000 / 21.6 = 92.6 Ah. The 48 V is the bank voltage; the 0.5 is the depth of discharge that respects lead-acid lifespan; the 0.9 is the inverter's cut.
- Round up to a commercial battery: 100 Ah. To build 48 V out of 12 V units: 4 × 12 V / 100 Ah batteries in series (series adds voltage, keeps the Ah).
- Check it backwards. 48 V × 100 Ah = 4800 Wh gross; × 0.5 × 0.9 = 2160 Wh usable; / 500 W = 4.3 h. Target met, with a whisker of margin.
Generator: margin, motors and altitude
When the outage is measured in days, the genset comes in — and here the traps are mechanical as well as electrical.
The running margin
Base rule: running kVA × 1.25. A 40 kW load at PF 0.8 is 40 / 0.8 = 50 kVA running, so the recommended generator is 50 × 1.25 = 62.5 kVA (or the next commercial size up). That 25% isn't padding: it covers voltage regulation, the harmonics from switched-mode supplies, and the growth that "wasn't in scope" but always shows up.
Motors start like brutes
An induction motor with direct-on-line (DOL) starting pulls around 6× its rated current during startup. A 5 kW pump that runs at 6.25 kVA can demand ~37.5 kVA for a second — more than half of our 62.5 kVA generator, just to get spinning. If the genset feeds air conditioning, pumps or compressors, size for the worst start (or specify soft starters / VFDs). The generator sizing calculator accounts for exactly this.
Altitude charges a toll
A naturally aspirated diesel loses power where the air thins out: the rule of thumb is ~3.5% per 300 m above 1000 m. At a site 2500 m up, that's 17.5% less: your "62.5 kVA" generator delivers ~51.6. To get a real 62.5 up there, you need ~76 kVA on the nameplate. In Chile this is no footnote — it's Calama, it's the altiplano, it's half the mining industry.
Breakers: the 80% rule
All this power flows through a circuit breaker, and breakers have their own golden rule: for a continuous load (more than 3 hours straight — in other words, a rack), the breaker should not run above 80% of its rated capacity. The thermal element heats up, and a hot breaker trips when it feels like it, not when it should.
Example: a 3000 VA load at 230 V is 3000 / 230 ≈ 13 A. Minimum breaker: 13 / 0.8 ≈ 16.3 A. On the IEC ladder (6, 10, 13, 16, 20, 25, 32…) the 16 A falls short by three tenths — the right pick is the 20 A. Three tenths of an ampere separate an installation that passes inspection from one that trips every Friday at 6 pm. The load and breaker calculator gives you the standard size directly, single- or three-phase.
PUE: cooling costs money too (the bonus round)
One last number, on the house: every watt that enters the rack leaves as heat, and removing that heat also burns energy. PUE (Power Usage Effectiveness) is total facility power divided by IT power. A PUE of 1.6 — common in small rooms without aisle containment — means that for every watt of compute you pay 0.6 extra in cooling and losses: 37.5% of your electricity bill computes nothing at all.
In money: 10 kW of IT at PUE 1.6 is 16 kW total → 140,160 kWh per year → at $0.15/kWh, about $21,000 a year. Dropping PUE from 1.6 to 1.4 saves more than most maintenance contracts cost. The energy cost and PUE calculator converts your case into currency and CO₂.
Now, off to the field: measure your rack's real load this week (clamp meter or PDU, not nameplates), run it through ups-autonomia and banco-baterias, and write down your batteries' purchase date in the same spreadsheet where you track everything else. Batteries age like contracts do: silently, until the day of the outage.