{
  "slug": "off-grid-system-sizing",
  "title": "Off-Grid Solar System Sizing Calculator",
  "heading": "Off-Grid Solar System Sizing Calculator",
  "category": "other",
  "url": "https://www.revenuelab.fyi/toolbox/off-grid-system-sizing",
  "summary": "Panel, battery, and inverter sizing for a system with no grid backup.",
  "description": "Off-grid sizing is fundamentally different from grid-tied solar because there's no utility to fall back on during a cloudy stretch — the system must be sized for your worst realistic sun conditions, not the annual average. This calculator sizes panels for your worst-month sun hours (not annual average), sizes battery storage for a chosen number of autonomy days (how long the system runs with zero sun), and checks inverter capacity against your peak simultaneous load. Off-grid systems are typically oversized by 30-50% compared to grid-tied systems covering the same usage, precisely because of this need to handle worst-case weather without a grid safety net.",
  "formula": "Array size = daily kWh ÷ (worst-month sun hours × system efficiency); battery kWh = daily kWh × autonomy days ÷ max depth of discharge.",
  "dateModified": "2026-09-30",
  "run_url": "https://www.revenuelab.fyi/api/public/calc?tool=off-grid-system-sizing",
  "inputs": [
    {
      "id": "dailyKwh",
      "label": "Average daily energy use",
      "kind": "number",
      "hint": null,
      "default": 10,
      "unit": "kWh",
      "min": 1,
      "max": null
    },
    {
      "id": "worstMonthSunHours",
      "label": "Worst-month sun hours",
      "kind": "number",
      "hint": null,
      "default": 3,
      "unit": "hrs",
      "min": 1,
      "max": 6
    },
    {
      "id": "systemEfficiency",
      "label": "System efficiency",
      "kind": "number",
      "hint": null,
      "default": 78,
      "unit": "%",
      "min": 50,
      "max": 95
    },
    {
      "id": "autonomyDays",
      "label": "Days of battery autonomy needed",
      "kind": "number",
      "hint": null,
      "default": 3,
      "unit": null,
      "min": 1,
      "max": 10
    },
    {
      "id": "maxDod",
      "label": "Max depth of discharge",
      "kind": "number",
      "hint": null,
      "default": 80,
      "unit": "%",
      "min": 30,
      "max": 100
    },
    {
      "id": "peakLoadKw",
      "label": "Peak simultaneous load",
      "kind": "number",
      "hint": null,
      "default": 4,
      "unit": "kW",
      "min": 0.5,
      "max": null
    }
  ],
  "outputs": [
    {
      "id": "arrayKw",
      "label": "Recommended array size",
      "format": "decimal",
      "hint": null,
      "primary": true
    },
    {
      "id": "batteryKwh",
      "label": "Recommended battery capacity",
      "format": "decimal",
      "hint": null,
      "primary": false
    },
    {
      "id": "inverterKw",
      "label": "Recommended inverter size",
      "format": "decimal",
      "hint": null,
      "primary": false
    }
  ],
  "worked_example": {
    "inputs": [
      "Average daily energy use: 10 kWh",
      "Worst-month sun hours: 3 hrs",
      "System efficiency: 78 %",
      "Days of battery autonomy needed: 3",
      "Max depth of discharge: 80 %",
      "Peak simultaneous load: 4 kW"
    ],
    "outputs": [
      "Recommended array size: 4.27",
      "Recommended battery capacity: 37.5",
      "Recommended inverter size: 5"
    ]
  },
  "how_to": {
    "title": "How to use this",
    "steps": [
      "Enter average daily energy use (kWh).",
      "Enter worst-month sun hours (hrs).",
      "Enter system efficiency (%).",
      "Enter days of battery autonomy needed.",
      "Enter max depth of discharge (%).",
      "Enter peak simultaneous load (kW).",
      "Read your recommended array size 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": {
        "dailyKwh": 6,
        "worstMonthSunHours": 1.7999999999999998,
        "systemEfficiency": 50,
        "autonomyDays": 1.7999999999999998,
        "maxDod": 50,
        "peakLoadKw": 2.4
      }
    },
    {
      "name": "Typical",
      "description": "Defaults — the most common real-world setup.",
      "values": {
        "dailyKwh": 10,
        "worstMonthSunHours": 3,
        "systemEfficiency": 78,
        "autonomyDays": 3,
        "maxDod": 80,
        "peakLoadKw": 4
      }
    },
    {
      "name": "Ambitious",
      "description": "Higher-end numbers — what if things really pop?",
      "values": {
        "dailyKwh": 16,
        "worstMonthSunHours": 4.800000000000001,
        "systemEfficiency": 95,
        "autonomyDays": 4.800000000000001,
        "maxDod": 100,
        "peakLoadKw": 6.4
      }
    }
  ],
  "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": "Why size for worst-month sun hours instead of annual average?",
      "a": "A grid-tied system can borrow from the grid on a cloudy week and export surplus on a sunny one, netting out over a year. An off-grid system has nowhere to borrow from, so it must generate enough even during your location's worst month (typically December-January in most of the US) or you'll run the batteries down with no backup."
    },
    {
      "q": "How many days of autonomy should I plan for?",
      "a": "2-3 days covers typical multi-day cloudy stretches in most climates; remote cabins or critical medical-equipment applications sometimes plan for 5+ days. More autonomy days means a much bigger, more expensive battery bank, so this is a real cost-vs-risk tradeoff, not a 'more is always better' decision."
    },
    {
      "q": "Should I add a generator to an off-grid system?",
      "a": "Almost universally yes for anything beyond a small seasonal cabin — a backup generator lets you size the battery bank for typical conditions rather than worst-case-ever conditions, since the generator covers the rare extended low-sun stretch instead of requiring the battery to handle it alone. This usually cuts total system cost significantly."
    },
    {
      "q": "Why is the inverter sized above peak load, not equal to it?",
      "a": "The 1.25x multiplier gives headroom for motor startup surges (well pumps, refrigerator compressors) and avoids running the inverter at its absolute continuous limit constantly, which shortens its lifespan. Check the specific inverter's continuous vs surge rating against your largest motor's startup draw too."
    }
  ],
  "related": [
    "https://www.revenuelab.fyi/toolbox/solar-array-sizing",
    "https://www.revenuelab.fyi/toolbox/inverter-sizing",
    "https://www.revenuelab.fyi/toolbox/battery-backup-runtime"
  ],
  "license": "CC-BY-4.0",
  "citation": "RevenueLab — Off-Grid Solar System Sizing Calculator (https://www.revenuelab.fyi/toolbox/off-grid-system-sizing)"
}