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UPS / Battery Backup Sizing Guide

A battery's rated amp-hour capacity is never the amount of energy you can actually count on delivering to your load — depth of discharge, round-trip efficiency losses, and end-of-life aging margin all reduce the usable portion, sometimes by 40% or more combined. Sizing a battery backup system off the rated capacity alone, without accounting for these factors, is the most common way a "should have lasted 4 hours" system runs out in 2.5.

Last updated: August 29, 2026

A battery's rated amp-hour label is the starting point for sizing, not the answer — three separate factors (depth of discharge, conversion efficiency, and end-of-life aging margin) each take a bite out of what actually reaches the connected load, and skipping any one of them is how a calculated runtime ends up longer on paper than in practice.

This guide covers lead-acid vs lithium-ion comparison, the three factors that reduce usable capacity, and a full worked example.

Battery Chemistry Comparison

ChemistryTypical Depth of DischargeNotes
Flooded Lead-Acid~50%Lower upfront cost, mature technology, requires periodic watering maintenance
Sealed Lead-Acid / AGM / Gel (VRLA)~60%Maintenance-free compared to flooded, moderate cost, moderate cycle life
Lithium-Ion (LiFePO4)~85%Higher upfront cost, much longer cycle life, lighter and more compact, minimal maintenance

A deeper safe depth of discharge means more of the same rated Ah is actually usable — this is a major reason lithium-ion batteries can deliver meaningfully more usable capacity than a lead-acid battery with the same nominal rating.

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The Three Factors That Reduce Usable Capacity

FactorWhat It RepresentsTypical Planning Figure
Depth of dischargeChemistry-specific safe discharge limit50-85% depending on battery type
EfficiencyInverter conversion losses, DC to ACCommonly ~85% as a planning figure
Aging factorDesign margin for capacity decline over service lifeCommonly ~80% as a planning figure
Combined usable factorAll three multiplied togetherAs low as ~34% of rated Ah in a conservative combined case
Usable Energy = Rated Ah × Voltage × DoD × Efficiency × Aging Factor

Worked Example — 200Ah Lithium Battery, 500W Load

200Ah, 12V LiFePO4 Battery, 500W Continuous Load

Illustrative example

StepCalculationResult
Rated energy200 × 122,400 Wh
Usable factor (85% DoD × 85% efficiency × 80% aging)0.85 × 0.85 × 0.800.578
Usable energy2,400 × 0.578~1,387 Wh
Runtime at 500W load1,387 ÷ 500~2.8 hours

Note this is well under the naive "2,400Wh ÷ 500W = 4.8 hours" figure a rated-capacity-only calculation would give — the combined derating factors matter significantly.

Common Mistakes

Sizing Off Rated Ah Capacity With No Derating At All

The full rated amp-hour figure on a battery's label is never what actually reaches the connected load in practice — depth of discharge, efficiency, and aging together can reduce usable capacity by well over half in a conservative planning case.

Applying Lead-Acid's Shallower Depth of Discharge to a Lithium Battery, or Vice Versa

Using a generic 50% DoD assumption for a lithium battery significantly understates its real usable capacity, while using lithium's ~85% DoD for a lead-acid battery risks shortening its service life through over-discharge — always match the DoD assumption to the actual battery chemistry installed.

Forgetting Inverter Efficiency Losses on Top of Depth of Discharge

Depth of discharge alone doesn't capture the full picture — the DC-to-AC conversion step loses additional energy as heat, and skipping this factor overstates the runtime the connected AC load will actually see.

Sizing a New System With No Aging Margin

A system sized to just barely meet its target runtime when the battery is brand new will fall short of that target as the battery ages through normal use — the aging factor is a deliberate margin for this expected decline, not a pessimistic overcorrection.

Choosing a UPS When a Larger Battery Backup System Was Actually Needed

A compact UPS sized for a brief bridge-the-gap runtime on a few sensitive devices is a fundamentally different tool from a larger battery bank meant to ride out hours of outage for a broader set of loads — using the wrong category for the actual goal leads to a system that's either wildly over- or under-built for the need.

Relevant Standards and References

RegionRelevant Codes / Guidance
United StatesUL 1973 covers batteries for stationary/backup applications; NEC Article 480/706 cover storage battery and energy storage system installation
Europe / UKIEC 62619 and related standards cover lithium battery safety for stationary applications; BS 7671 covers installation wiring requirements
IndiaIS 16270 and related BIS standards cover stationary lithium and lead-acid battery safety
Australia / New ZealandAS/NZS 5139 covers safety requirements for battery energy storage system installation
General guidanceManufacturer-published depth of discharge, cycle life, and warranty terms for the specific battery being purchased are the authoritative figures for that model — this guide's percentages are general planning references, not a specific product's spec sheet.

Final Verdict

Correct battery backup sizing means applying depth of discharge, conversion efficiency, and an aging margin to the rated Ah figure — the actual usable energy delivered to a real load is meaningfully less than the label alone suggests.

  • Match the depth of discharge assumption to the actual battery chemistry — lead-acid and lithium-ion tolerate very different safe discharge depths.
  • Apply inverter conversion efficiency on top of DoD — the DC-to-AC step loses additional energy as heat.
  • Apply an aging margin even for a brand-new system, since usable capacity naturally declines over the battery's service life.
  • Lithium-ion typically delivers meaningfully more usable capacity per rated Ah than lead-acid, offsetting its higher upfront cost over the system's life.
  • Choose runtime-mode vs sizing-mode calculation based on whether you're starting from a known battery or a target runtime goal.
  • Use a compact UPS for bridging brief outages on sensitive electronics; use a larger battery bank for genuinely riding out extended outages.

Related calculators

Use these calculators when you need to turn this reference information into project quantities:

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FAQ

Depth of discharge is the percentage of a battery's rated capacity that can be safely used before recharging, without meaningfully shortening its service life or risking damage — batteries are generally not designed to be discharged to 0% regularly. Different chemistries tolerate different depths safely: flooded lead-acid batteries are typically limited to a shallower DoD (commonly around 50%) to preserve a reasonable cycle life, sealed lead-acid/AGM/gel (VRLA) batteries tolerate somewhat deeper discharge (commonly around 60%), and lithium-ion (specifically LiFePO4, the common chemistry for this use) tolerates a much deeper discharge (commonly around 85%) without the same degree of accelerated wear — this difference alone can mean a lithium battery delivers meaningfully more usable capacity than a similarly-rated lead-acid battery of the same nominal Ah rating.
Lead-acid (flooded or sealed/VRLA) has a lower upfront cost per rated Ah and is a mature, widely available, well-understood technology, but has a shallower safe depth of discharge, a shorter cycle life (fewer full charge-discharge cycles before capacity meaningfully degrades), is heavier for the same usable capacity, and requires more careful maintenance (especially flooded types, which need periodic watering). Lithium-ion (LiFePO4) costs more upfront per rated Ah, but tolerates a much deeper discharge, delivers meaningfully more cycles before degrading, is lighter and more compact for the same usable capacity, and generally requires less maintenance — the higher upfront cost is often offset over the system's service life by needing less replacement and delivering more usable capacity from the same rated size.