UPS Sizing and Battery Runtime Reference

The short answer

A UPS must satisfy two separate ratings at once: its real-power rating in watts (W) must exceed your load's watt draw, and its apparent-power rating in volt-amperes (VA) must exceed the load's VA. The two differ by power factor (PF = W / VA), which ranges from about 0.6 on legacy units to 1.0 (unity) on modern power-factor-corrected models. A 1500 VA / 900 W unit is PF 0.6; a 1500 VA / 1500 W unit is PF 1.0. Whichever rating your load hits first is the true ceiling, so always check both. Runtime is then governed by stored energy, not by VA: approximate runtime (hours) equals usable battery watt-hours divided by load watts, then reduced for inverter and battery losses. A pack of two 12 V 9 Ah blocks holds 24 V x 9 Ah = 216 Wh nominal; at a 200 W load the ideal figure is 216 / 200 = 1.08 h, but real delivery is typically 30-50% lower.

Runtime is strongly non-linear: doubling the load far more than halves the runtime. Two effects compound. Peukert's law means a lead-acid battery delivers less of its rated amp-hours as discharge current rises (Peukert exponent roughly 1.05-1.15 for VRLA AGM, 1.1-1.25 for gel, 1.2-1.6 for flooded), and the inverter's fixed overhead consumes a larger share at heavy loads. Manufacturer runtime curves, not the simple formula, are authoritative. For topology, match protection to load sensitivity: standby (offline / IEC VFD) for basic desktop gear, line-interactive (VI) with automatic voltage regulation for network racks, NVRs, and routers on typical utility power, and online double-conversion (VFI) for equipment needing a continuously regenerated, zero-transfer-time sine wave. When required runtime exceeds what the internal battery gives, add external battery modules (EBMs) to a UPS designed to accept them; runtime scales roughly with added watt-hours until the charger or inverter current limit is reached. Load the UPS to about 50-80% of rating for efficiency and future headroom.

How to Size a UPS in Six Decisions

Sizing a UPS is a sequence of independent decisions, and skipping any one of them is where most under-built systems fail. First, total the real power (watts) of everything the UPS will carry. Use measured draw or nameplate watts, and for gear labeled only in amps multiply by nominal line voltage (in North America, 120 V). Second, total the apparent power (VA). If a device lists only watts, divide by its power factor (many switch-mode supplies are 0.9-1.0; older gear can be 0.6-0.7). The UPS you choose must have a watt rating above your watt total and a VA rating above your VA total, because it can be limited by either.

Third, decide the runtime you actually need. Most network and camera systems need only enough to ride through brief sags and to trigger a graceful shutdown or a generator start, which is minutes, not hours. Fourth, choose topology by how sensitive and how critical the load is. Fifth, pick loading headroom: target 50-80% of the UPS rating so you keep efficiency, runtime margin, and room to grow. Sixth, plan battery replacement, because VRLA cells are consumables with a finite service life.

Treat the watt limit and the VA limit as a pair of gates. A 1500 VA / 900 W line-interactive UPS running a 950 W load is overloaded even though it is under 1500 VA. The same UPS at 800 W and 1400 VA is also at its limit, this time on the VA side. Whichever number your load reaches first is the real cap, and prudent design leaves margin below it.

• Watts first: sum real power; convert amp-only labels as W = V x A (120 V nominal in North America).
• VA second: sum apparent power; if only watts are known, VA = W / PF, with PF typically 0.6-1.0.
• The UPS must clear BOTH gates: rated watts > load watts AND rated VA > load VA.
• Target 50-80% loading for efficiency, runtime headroom, and future expansion.
• Runtime need is usually minutes for graceful shutdown or generator start, not hours.
• VRLA batteries are consumables; plan replacement on a 3-5 year cycle at 25 C / 77 F.

VA vs Watts and Power Factor

Volt-amperes (VA) measure apparent power, the product of RMS voltage and RMS current. Watts (W) measure real power, the portion that does useful work. They are related by power factor: W = VA x PF, so PF = W / VA. Power factor is always between 0 and 1. For linear resistive loads it is near 1.0; for the switch-mode power supplies in IT gear it is set by the supply's design and its own power-factor correction. Legacy or uncorrected supplies can sit near 0.6-0.7, while active-PFC supplies common in modern servers, NVRs, and network switches run 0.9 to unity.

This is why UPS nameplates carry two numbers. A 1000 VA / 600 W unit is built around PF 0.6; a 1000 VA / 900 W unit is PF 0.9; a 1500 VA / 1500 W unit is PF 1.0. If your load is 700 W of active-PFC equipment (about 730-780 VA), the first UPS is watt-limited and cannot carry it, even though 730 VA is well under 1000 VA. Modern unity-PF UPS units remove that mismatch, which is why they are preferred for PFC IT loads.

Two more electrical details matter. Crest factor describes the peaked, non-sinusoidal current that capacitor-input supplies draw; UPS units are specified to tolerate a crest factor around 3:1, and exceeding it forces derating. Output waveform also matters: a true sine-wave output suits active-PFC supplies and motor loads, while a simulated (stepped) sine-wave output can cause some PFC supplies to run hot or drop to battery erratically.

• W = VA x PF; PF = W / VA; power factor ranges 0 to 1, typically 0.6-1.0 in practice.
• Legacy/uncorrected supplies: PF ~0.6-0.7; active-PFC IT supplies: PF ~0.9-1.0.
• Nameplate examples: 1000 VA / 600 W = PF 0.6; 1500 VA / 1500 W = PF 1.0 (unity).
• A 700 W PFC load (~730-780 VA) overloads a 1000 VA / 600 W UPS on watts, not VA.
• UPS crest-factor rating is typically ~3:1 for the peaked current of capacitor-input supplies.
• Prefer true sine-wave output for active-PFC supplies; stepped-sine can cause erratic transfers.

Runtime Math and Why It Is Non-Linear

Start from stored energy. A battery's nominal energy is watt-hours = volts x amp-hours. A 12 V 9 Ah block holds about 108 Wh; two in series give 24 V 9 Ah = 216 Wh. The first-order runtime estimate is runtime (h) = usable Wh / load W. At 200 W the 216 Wh pack gives 216 / 200 = 1.08 h in the ideal case. Real runtime is lower because the inverter is 85-92% efficient converting DC to AC, and because you cannot use 100% of nominal capacity: discharge stops at an end-of-discharge voltage (roughly 1.67 V/cell at high rate, 1.75 V/cell at low rate) to protect the cells.

The bigger correction is Peukert's law. Lead-acid capacity is rated at a slow reference rate, usually the 20-hour rate (C/20). Draw current faster and you get fewer amp-hours out, following C = I^k x t with a Peukert exponent k of roughly 1.05-1.15 for VRLA AGM, 1.1-1.25 for gel, and 1.2-1.6 for flooded cells. At UPS-style discharge rates near 1C, usable capacity commonly drops to 60-70% of the C/20 figure. Combine inverter loss and Peukert derating and the 216 Wh pack realistically delivers on the order of 130-150 Wh, or roughly 25-40 minutes at 200 W.

Non-linearity follows directly: doubling the load raises current, which lowers Peukert-usable capacity AND raises the fixed inverter-overhead share, so runtime falls by much more than half. A unit that runs 30 minutes at 200 W may run only 8-10 minutes at 400 W. For this reason, published manufacturer runtime curves at your exact wattage are the authoritative source; the formula is for a sanity check and for comparing options.

• Battery energy: Wh = V x Ah; a 12 V 9 Ah block = ~108 Wh; two in series = ~216 Wh.
• Ideal runtime = usable Wh / load W; at 216 Wh and 200 W that is 1.08 h before losses.
• Inverter DC-AC efficiency is ~85-92%; discharge stops at ~1.67-1.75 V/cell end voltage.
• Peukert: capacity is rated at C/20; near 1C only ~60-70% of that is usable.
• Peukert exponent k: ~1.05-1.15 AGM, ~1.1-1.25 gel, ~1.2-1.6 flooded (C = I^k x t).
• Doubling the load more than halves runtime; always confirm on the maker's runtime curve.

Topologies: Standby, Line-Interactive, Online

IEC 62040-3 classifies UPS by how independent the output is from input disturbances, which maps cleanly onto the three common designs. Standby (offline) is VFD, voltage and frequency dependent: the load runs on filtered mains and the inverter starts only on failure, with a transfer time typically 2-10 ms. It is inexpensive and efficient (often 97-98%) but offers no voltage regulation, so it suits basic desktops and non-critical gear on relatively clean power.

Line-interactive is VI, voltage independent: it adds an automatic voltage regulator (AVR) that boosts or trims voltage using autotransformer taps without going to battery, so it rides through the common sags and swells that would otherwise drain a standby unit's cells. Transfer to battery on a true outage is still a few milliseconds. Efficiency is high (commonly 95-98%). This is the practical default for network racks, NVRs, routers, PoE switches, and access-control heads, where power is mostly acceptable but variable.

Online double-conversion is VFI, voltage and frequency independent: it continuously rectifies incoming AC to DC and reinverts it to a clean sine wave, so the load always runs from the inverter and transfer time is effectively zero. It also isolates the load from frequency drift and provides the tightest output regulation, at the cost of lower efficiency (older units ~90%, modern units ~94-96%, with eco-modes higher). Reserve it for the most sensitive or mission-critical loads: core switches, servers, medical or lab equipment, and sites on generator power where frequency wanders.

• Standby / offline = IEC VFD: transfer time ~2-10 ms, efficiency ~97-98%, no voltage regulation.
• Line-interactive = IEC VI: adds AVR for sags/swells without battery; efficiency ~95-98%.
• Online double-conversion = IEC VFI: zero transfer time, cleanest sine, efficiency ~90-96%.
• Standby suits basic desktops on clean power; line-interactive fits network racks/NVRs/routers.
• Online suits core switches, servers, sensitive/medical gear, and generator-fed sites.
• IEC 62040-3 also grades output waveform (sine vs non-sine) and a tolerance-curve number (1 = strictest).

Worked Example: 200 W Network Rack / NVR

Consider a small security and network rack: a 16-channel NVR with drives (~40-60 W), a 24-port PoE switch reserving budget for cameras (measure actual draw, since PoE output can dominate), a router and modem (~15-30 W combined), and a fiber ONT or media converter. Suppose measured real power totals 200 W. If the supplies are active-PFC, VA is close to watts, perhaps 210-230 VA; if some are uncorrected, apply their power factor. Choose a UPS whose watt rating clears 200 W with headroom and whose VA rating clears the VA total.

A 1500 VA / 900 W line-interactive UPS carries this load at about 22% of its watt rating, leaving comfortable margin. Its internal 216 Wh (two 12 V 9 Ah) pack yields roughly 25-40 minutes at 200 W after inverter and Peukert derating, enough to ride through most sags and to trigger a clean NVR shutdown or a generator start. If you instead want an hour or more, you need either a larger unit or external battery modules. Note the PoE caveat: cameras drawing power through the switch add to the UPS load, so budget their wattage explicitly rather than counting only the switch's idle draw.

Set expectations with the runtime curve, not the nameplate VA. Two 12 V 9 Ah strings is the same 216 Wh whether the label says 900 VA or 1500 VA; VA sizing buys the ability to carry the load, while watt-hours buy the minutes. If load may grow (more cameras, a second NVR), size VA and watts for the future total and keep loading under 80% so a later addition does not push you into overload or collapse runtime.

• Example load: 16-ch NVR ~40-60 W, router+modem ~15-30 W, plus switch and ONT = ~200 W measured.
• Active-PFC supplies put VA near watts (~210-230 VA); apply PF for any uncorrected gear.
• 1500 VA / 900 W line-interactive runs 200 W at ~22% of watt rating (comfortable headroom).
• Internal 216 Wh gives roughly 25-40 min at 200 W after inverter + Peukert derating.
• Budget PoE camera wattage separately; cameras draw through the switch and add to UPS load.
• VA carries the load; watt-hours provide the minutes. Confirm minutes on the runtime curve.

When to Add External Battery Modules

When the required runtime exceeds what the internal battery delivers, add external battery modules (EBMs) to a UPS explicitly designed to accept them. Only certain line-interactive and most larger online units support EBMs, and each family lists which modules and how many strings it allows. Runtime scales roughly with total watt-hours added, so doubling the battery watt-hours roughly doubles runtime at the same load, until you hit two hard limits: the inverter's discharge current ceiling and the charger's recharge capacity.

Recharge time is the constraint people miss. A standard UPS charger is sized for its internal battery, so adding strings extends recharge proportionally unless the charger is upsized. A typical VRLA charger returns roughly 90% of capacity in about 3-6 hours per string; four strings on the same charger can take far longer, leaving you exposed to back-to-back outages. Higher-capacity or multi-charger configurations solve this but add cost. Keeping batteries near 25 C / 77 F matters too, because VRLA service life roughly halves for every ~8-10 C sustained above that reference, and cold reduces available capacity.

A practical rule: if you need only a few minutes for graceful shutdown or generator transfer, the internal battery is usually enough. If you need 30-60+ minutes of ride-through, or you are bridging to a generator with a slow start, size EBMs to the target watt-hours, verify the charger can restore them in an acceptable window, and confirm the UPS can supply the peak current at that runtime. For long outages, a UPS plus a properly sized generator is the correct architecture; the UPS covers the seconds-to-minutes gap while the generator handles hours.

• EBMs only attach to UPS models designed for them; check supported module count and strings.
• Runtime scales roughly with added watt-hours until inverter current or charger limits are hit.
• Standard charger recharges ~90% in ~3-6 h per string; extra strings extend recharge unless upsized.
• Keep VRLA near 25 C / 77 F: service life roughly halves per ~8-10 C sustained above reference.
• Internal battery suits minutes-long shutdown; EBMs suit 30-60+ min ride-through targets.
• For multi-hour outages, pair the UPS with a generator: UPS covers the transfer gap, generator the hours.

Frequently asked questions