Distribution & Demand · UPS / Battery Autonomy

UPS Battery Sizing Calculator: The Energy Method, and Why 3000 Wh of Load Needs 4076 Wh of Battery

A 6 kW load for 30 minutes is only 3000 Wh of output — but after inverter efficiency and depth-of-discharge limits it needs 4076 Wh of battery. How the energy method works, where the losses come from, and why a big-looking bank can still fall below its runtime target.

Published 2026-08-01 · EI Portal

The most common UPS battery sizing mistake is dividing the load straight into the nameplate Ah. A battery's rated capacity is not the energy you get to spend: the inverter takes a slice on the way out, and the allowable depth of discharge takes a slice before you ever start. A battery autonomy calculator exists to make those two losses explicit — and to show how far a tidy-looking bank really carries the load.

The energy method

Autonomy sizing is an energy balance. Start at the output, work back to the battery:

load W            = entered real power, or entered VA × power factor
      output energy Wh  = load W × runtime hours
      base battery Wh   = output Wh / inverter efficiency / allowable depth of discharge
      recommended Ah    = base battery Wh × (1 + margin) / DC bus voltage
      

The two division terms carry the physics. Inverter efficiency is the DC energy that never reaches the AC output; depth of discharge (DOD) is the fraction of the bank you are willing to take out before you stop. Together they give the headline that catches people out:

usable energy = nominal × DOD × efficiency
      

Nameplate Wh is not usable Wh — which is why the required battery energy always sits above the output energy.

Worked: 6 kW for 30 minutes on a 192 V bus

The tool's default case is a small UPS critical load: 6 kW at power factor 0.9 (6.67 kVA), a 30 min runtime target, a 192 V DC bus, 92% inverter efficiency, 80% allowable DOD, a 20% design margin, and a 60 Ah bank selected. The chain runs:

• Output energy: 6 kW for half an hour = 3000 Wh
• Divide by the 92% inverter efficiency → 3261 Wh
• Divide by the 80% depth of discharge → 4076 Wh base battery energy
• Add the 20% design margin → 4891 Wh
• At 192 V that is 21.23 Ah base required, 25.48 Ah recommended

So 3000 Wh of output needs 4076 Wh of battery before margin is even applied. Nothing is wrong with the load — the gap is the loss budget, and it is the part of the sum most often skipped.

Runtime is the same balance, run backwards

Give the calculator a selected bank and it reads the equation the other way:

estimated runtime = selected Ah × DC bus voltage × DOD × efficiency / load W
      

The 60 Ah bank draws a DC load current of 33.97 A and delivers an estimated 84.79 min (about 1.41 h) against the 30-minute target. The verdict reads "Autonomy covered": the selection sits at 236% of the margin-adjusted need, with 34.5 Ah spare.

Where the verdict flips

The verdict is a three-state comparison of the selected bank against the requirement, and the boundaries are sharp. On the same 6 kW case:

• ≥ 25.48 Ah (the recommendation) → "Autonomy covered"
• Drop the selection to 25 Ah → "Tight on margin" — it covers the 21.23 Ah base requirement but not the margin
• Drop it to 21 Ah → "Below autonomy" — below the base requirement itself, the boundary sitting at 21.23 Ah

The middle state exists so a marginal pass does not read as a confident one.

The trap at scale — the extended-runtime review

The same arithmetic that quietly inflates a 30-minute case bites hard on a long one. The tool's extended-runtime preset reviews a 4 kW load against a 180 min target on a 120 V bus, with 88% inverter efficiency, 70% DOD and a 20% margin — and a 150 Ah bank selected, two and a half times the hero's 60 Ah.

It fails. The base requirement is 162 Ah, the recommendation 195 Ah, and the verdict reads "Below autonomy": an estimated 2.77 h against the 3.00 h target, −44.8 Ah spare, the selection covering only 77% of the margin-adjusted need. The full walk-through — the covered case and this failure, side by side — is in the worked case, the 3000 Wh trap.

Different systems, same method

The equation does not change with the application; the basis does:

• Control panel DC UPS — 1200 W at pf 0.95 for 60 min on a 48 V bus, 90% efficiency, 70% DOD, 25% margin: base 39.7 Ah, recommended 49.6 Ah, and a 60 Ah bank is covered at 121%.
• Server room ride-through — 18 kVA at pf 0.85 (15.30 kW) for 15 min on a 384 V bus, 94% efficiency, 80% DOD: base 13.2 Ah, recommended 15.9 Ah, and a 20 Ah bank is covered at 126% with an estimated 23 min of ride-through.

High-voltage strings need few Ah for short rides; low-voltage buses and long runtimes need many. The method makes you own the efficiency and DOD in every case rather than hiding them in a nameplate number.

Check it live

The UPS / Battery Autonomy Basic Sizing tool puts the whole chain on one surface: the output energy, the base and recommended Ah, the DC load current, the estimated runtime for the bank you select, and the verdict — "Autonomy covered", "Tight on margin" or "Below autonomy". Change the load, the runtime, the DOD or the efficiency and watch the decision move.

It is a preliminary energy-method check, and it says so. Discharge-rate (Peukert) effects, temperature, ageing, end-of-discharge voltage, real runtime curves and UPS overload capability are not modelled. Full battery sizing to IEEE 485 (vented lead-acid) and IEEE 1184 (UPS systems) additionally applies a discharge-rate / temperature correction (the Kt factor) that this energy method deliberately omits — a high-rate 15-minute discharge does not deliver the same usable Ah as a slow one. The Ah figure is the nominal bank capacity at the entered DC bus voltage; series/parallel arrangement is not modelled, and there is no PDF or report export. The final selection is confirmed against manufacturer runtime curves.

Frequently asked questions

How do I size a UPS battery for a required runtime?

Use the energy method: output energy (Wh) = load W × runtime hours; base battery energy = output Wh ÷ inverter efficiency ÷ allowable depth of discharge; then capacity in Ah = battery Wh ÷ DC bus voltage, with a design margin on top. For a 6 kW load and 30 minutes on a 192 V bus at 92% efficiency and 80% DOD, that gives 21.23 Ah base and 25.48 Ah with a 20% margin.

Why does 3000 Wh of load need about 4076 Wh of battery?

Because usable energy = nominal × DOD × efficiency. The inverter loses a slice of every Wh on the way out (÷0.92 → 3261 Wh) and the depth-of-discharge limit stops you spending the whole bank (÷0.80 → 4076 Wh). The battery must be bigger than the load's energy by exactly those two factors.

How do I calculate UPS runtime from a battery's Ah rating?

Run the same balance backwards: estimated runtime = selected Ah × DC bus voltage × DOD × efficiency ÷ load W. A 60 Ah bank at 192 V with 80% DOD and 92% efficiency carries a 6 kW load for an estimated 84.79 minutes — not the bare Ah-times-voltage figure, because the same losses that inflate the sizing also shorten the ride-through.

What is depth of discharge and why does it change the battery size?

Depth of discharge (DOD) is the fraction of the bank's capacity you allow yourself to take out before the load must stop. Designing to 80% DOD means only 80% of the nominal energy is available, so the required nominal capacity grows by the inverse. Loosen the DOD and the required bank shrinks; tighten it and the bank grows.

Is an energy-method calculator enough for final battery selection?

No — it is a preliminary first-pass check. It does not model discharge-rate (Peukert) effects, temperature, ageing, end-of-discharge voltage, real runtime curves or UPS overload capability. Full sizing to IEEE 485 / IEEE 1184 adds a discharge-rate and temperature correction (Kt) on top of the energy balance. Use the energy method to catch an undersized bank early, then confirm the final selection against the manufacturer's runtime curves.