{
  "slug": "battery-backup-runtime",
  "title": "Battery Backup Runtime Calculator",
  "heading": "Home Battery Backup Runtime Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/battery-backup-runtime",
  "summary": "How long a battery keeps your chosen loads running during an outage.",
  "description": "Home battery specs quote usable kWh, but runtime depends entirely on what you're actually running. This calculator sums the wattage of the loads you select for outage backup, applies inverter efficiency, and divides your battery's usable capacity by that draw to get hours of runtime. It also flags whether your battery's continuous power rating (kW, not kWh) can even start your loads — a common gap where a battery has plenty of energy but not enough instantaneous power to run a well pump or AC compressor simultaneously with other loads. Depth-of-discharge and efficiency losses are built in so the number reflects real-world performance, not the marketing spec sheet.",
  "formula": "Runtime (hrs) = (usable kWh × 1000 × round-trip efficiency) ÷ total load watts.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=battery-backup-runtime",
  "inputs": [
    {
      "id": "usableKwh",
      "label": "Usable battery capacity",
      "kind": "number",
      "hint": null,
      "default": 13.5,
      "unit": "kWh",
      "min": 1,
      "max": null
    },
    {
      "id": "efficiency",
      "label": "Round-trip efficiency",
      "kind": "number",
      "hint": null,
      "default": 90,
      "unit": "%",
      "min": 60,
      "max": 100
    },
    {
      "id": "continuousLoad",
      "label": "Total continuous load",
      "kind": "number",
      "hint": null,
      "default": 800,
      "unit": "W",
      "min": 50,
      "max": null
    },
    {
      "id": "surgeLoad",
      "label": "Peak surge load (startup)",
      "kind": "number",
      "hint": null,
      "default": 2500,
      "unit": "W",
      "min": 0,
      "max": null
    },
    {
      "id": "batteryPowerRating",
      "label": "Battery continuous power rating",
      "kind": "number",
      "hint": null,
      "default": 5,
      "unit": "kW",
      "min": 1,
      "max": null
    },
    {
      "id": "cyclesPerDay",
      "label": "Hours of backup needed per day",
      "kind": "number",
      "hint": null,
      "default": 24,
      "unit": "hrs",
      "min": 1,
      "max": 24
    }
  ],
  "outputs": [
    {
      "id": "runtimeHrs",
      "label": "Total runtime at this load",
      "format": "duration",
      "hint": null,
      "primary": true
    },
    {
      "id": "daysBackup",
      "label": "Days of backup (at chosen daily hours)",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "dailyEnergyNeeded",
      "label": "Energy needed per day of backup",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "canStart",
      "label": "Can battery start your surge load? (1=yes)",
      "format": "number",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Usable battery capacity: 13.5 kWh",
      "Round-trip efficiency: 90 %",
      "Total continuous load: 800 W",
      "Peak surge load (startup): 2500 W",
      "Battery continuous power rating: 5 kW",
      "Hours of backup needed per day: 24 hrs"
    ],
    "outputs": [
      "Total runtime at this load: 15.2",
      "Days of backup (at chosen daily hours): 0.6",
      "Energy needed per day of backup: 19.2",
      "Can battery start your surge load? (1=yes): 1"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter usable battery capacity (kWh).",
      "Enter round-trip efficiency (%).",
      "Enter total continuous load (W).",
      "Enter peak surge load (startup) (W).",
      "Enter battery continuous power rating (kW).",
      "Enter hours of backup needed per day (hrs).",
      "Read your total runtime at this load on the right — it updates as you type.",
      "Hit Share to keep the scenario or send it to someone."
    ]
  },
  "scenarios": [
    {
      "name": "Conservative",
      "description": "Lower-end numbers — what if things land soft?",
      "values": {
        "usableKwh": 8.1,
        "efficiency": 60,
        "continuousLoad": 500,
        "surgeLoad": 1500,
        "batteryPowerRating": 3,
        "cyclesPerDay": 14
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "usableKwh": 13.5,
        "efficiency": 90,
        "continuousLoad": 800,
        "surgeLoad": 2500,
        "batteryPowerRating": 5,
        "cyclesPerDay": 24
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "usableKwh": 21.6,
        "efficiency": 100,
        "continuousLoad": 1300,
        "surgeLoad": 4000,
        "batteryPowerRating": 8,
        "cyclesPerDay": 24
      }
    }
  ],
  "limitations": [
    "Estimates assume standard, average conditions — local rules, pricing, and materials vary.",
    "Results are rounded for readability; add a buffer before ordering, booking, or committing.",
    "Double-check anything with a real cost attached against a local quote."
  ],
  "faq": [
    {
      "q": "What loads should I actually back up?",
      "a": "Prioritize the refrigerator, well pump or sump pump, a few lights and outlets, internet/router, and one AC or heat zone if you have the capacity. Trying to back up an entire house's central AC and electric range usually needs more power than a single battery module provides — that's a whole-home generator's job, not a battery's."
    },
    {
      "q": "Why does surge load matter separately from continuous load?",
      "a": "Motors — well pumps, refrigerator compressors, AC units — draw 2-3x their running wattage for roughly a second at startup. If your battery's inverter can't cover that surge, the load will trip out even though the battery has plenty of stored energy. This is the most common battery-sizing mistake."
    },
    {
      "q": "How much does depth of discharge affect usable capacity?",
      "a": "Manufacturers already report 'usable kWh' net of DoD limits in most modern lithium batteries (Tesla Powerwall, Enphase, etc.), typically 95-100% usable. Older lead-acid systems only allow 50% DoD, so double the nameplate rating to find true usable energy."
    },
    {
      "q": "Can I stack multiple battery units?",
      "a": "Yes — most residential systems (Powerwall, Enphase IQ Battery, FranklinWH) support 2-4 unit stacks. Add usable kWh together and take the sum of continuous power ratings; this calculator's numbers scale linearly if you enter combined totals."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/generator-sizing-fuel",
    "https://www.revenuelab.fyi/toolbox/inverter-sizing",
    "https://www.revenuelab.fyi/toolbox/off-grid-system-sizing"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Battery Backup Runtime Calculator (https://www.revenuelab.fyi/toolbox/battery-backup-runtime)"
}