Backup Battery Runtime Calculator

Worked example

A clinic continuity exercise can estimate runtime from tested usable battery energy, steady demand and power limits. Account for reserve, conversion loss and auxiliary draw.

A fictional battery starts with 12 kWh of stored DC energy and keeps 2 kWh in reserve. At 80% conversion efficiency it supplies 8 kWh AC. A constant 3 kW load first falls short after 2 hours 40 minutes.

Try it yourself

The calculator opens with the fictional example already filled in. Change one input, choose Calculate locally, and compare the result to the original values below. Use Reset fictional fixture to start over.

An unknown result means the available inputs do not support a value. Check the calculator’s Warnings tab for what is missing or uncertain.

AC energy delivered 8 kWh AC
AC demand unmet 4 kWh AC
Elapsed time to first shortfall 2.666667 hours
Energy used by auxiliary equipment 0 kWh DC
Remaining stored energy 2 kWh DC

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Calculator

Approach (label only)Changing the label does not improve capacity, speed, error rates or confidence. Enter measured differences explicitly.

Comparison conditions and human review

Equal-resource comparison conditionsRecord budget, staffing, resources, information release dates, scenario conditions and evaluation criteria. Material differences can make options not comparable.

Evaluation period

Geography / exercise boundary

Human reviewers and roles

This evaluation depends on a real transaction

Supplied human authority gateThis declaration is not legal certification. Blocked, pending or assumed gates stop transaction-dependent evaluation.

Keep units consistent within each table. Enter zero when the amount is known to be zero. Leave optional amounts blank when they are unknown. If a required value is missing, the calculator will ask for it.

Advanced: complete input envelope and optional fields

Edit optional fields using the method dictionary. Apply validates and recalculates the entire input. Unknown values remain null.

Versioned input JSON

Import local JSON or CSV

Files stay in this browser. Maximum 1 MiB. JSON imports are recalculated. CSV supports flat observation tables, with column types taken from the displayed fixture. Empty cells become null.

Workbench JSON / input envelope

CSV target table

CSV file, matching the target table headers

Compare the arrangements

Keep physical resources and evaluation conditions equal. With identical inputs, all three approaches produce identical arithmetic. The comparison refuses material differences in declared conditions; it does not verify those declarations.

Show only differing metric rows

Privacy: comparison inputs may contain sensitive notes or operational details. Storage is off by default. Do not enter personal records or sensitive infrastructure details. Reproduction exports include supplied inputs and notes, including raw small-cell inputs suppressed from CORE-009 result tables. Suppression of displayed results does not make a reproduction export safe to publish.

Save comparison inputs on this device until I clear them

Executive brief and reproducible export

Public exercise notes

Review for personal, confidential or sensitive material before saving or sharing. No automatic submission occurs.

Read the generated brief

Method, inputs and limits

Account for battery energy, reserve, inverter power, auxiliary draw and load over a declared outage schedule. Method version 0.2.0. Size or certify an electrical installation, model charging, or account for startup surges, degradation or temperature.

CORE-020 input dictionary
Input	Unit / type	Required interpretation
initialKwh — Initial usable battery energy	kWh DC / number	Declared stored energy at the start of the exercise.
reserveKwh — Energy reserved	kWh DC / number	A floor at or below the initial stored energy.
inverterKw — Inverter output limit	kW AC / number	Constant real power available at the AC output.
efficiency — Discharge conversion efficiency	fraction / number	Between 0.01 and 1. Enter 0.9 for 90%.
auxiliaryKw — Continuous battery-side auxiliary draw	kW DC / number	Optional controls, cooling or inverter standby draw measured at the battery. Enter zero if already included in the efficiency.
periods — Ordered outage intervals	hours and kW AC / number	Each row has hours and constant loadKw. No charging during the exercise.

Visible mathematics / decision rules

Demand = load kW × interval hours.

AC power delivered while active = min(load kW, inverter output limit).

Battery draw = delivered AC power ÷ efficiency + auxiliary DC draw. Active time is capped by the energy above reserve and interval length.

Closing DC energy = opening DC energy − battery draw × active time.

What this result cannot establish

Load, auxiliary draw and conversion efficiency are constant within each interval. Auxiliary draw stops at the reserve floor.
A load above the inverter ceiling is short from the start of the interval. Otherwise a shortfall starts when the battery reaches reserve.

What would falsify the assumptions?

Compare simulated interval energy and loss of load with an authorized load test at the declared reserve and efficiency.
Assumptions fail if the supplied definitions, units, period, scope or source records are wrong or incomplete.
An independent source audit or replication may contradict an input or the recorded dependency.
This method creates no resources. Physical conservation is checked in the inventory and transfer ledgers; descriptive graphs and checklists are not resource-flow models.
The same equations and review checklist can be reproduced with a spreadsheet or structured human-only process.
This is deterministic arithmetic, a descriptive comparison or a structured governance check, not a fitted causal effect or forecast. Fictional inputs remain an exercise.
Equal or worse measured errors, net review effort, disclosure burden or fallback under MAR would show no added benefit or added risk; no improvement is assumed.

Reproducible fictional fixture and independently calculated expectations

This fixture is synthetic. It does not describe a country, government, company or event.

{
  "observations": {
    "unit": "kWh",
    "origin": "synthetic",
    "sources": [
      {
        "id": "SYN-020",
        "kind": "synthetic",
        "label": "Fictional continuity exercise",
        "url": null
      }
    ],
    "initialKwh": 12,
    "reserveKwh": 2,
    "inverterKw": 4,
    "efficiency": 0.8,
    "auxiliaryKw": 0,
    "periods": [
      {
        "hours": 2,
        "loadKw": 3
      },
      {
        "hours": 2,
        "loadKw": 3
      }
    ]
  },
  "parameters": {}
}

Expected checks

{
  "delivered": 8,
  "unmet": 4,
  "firstShortfallHour": 2.666667,
  "closingKwh": 2
}

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Authoritative background references

NIST: Cybersecurity Framework 2.0 — Outcome taxonomy, not a calculated national risk score. Reviewed 2026-11-09. Reference, not endorsement or validation.
U.S. Energy Information Administration: Measuring electricity — Distinguishes power in kilowatts from energy in kilowatt-hours. Reviewed 2026-11-11. Reference, not endorsement or validation.

These sources inform definitions or review practices. They do not validate this implementation or endorse MAR.

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