{
  "id": "generator-size",
  "version": "4e0b1f569ee7",
  "status": "published",
  "name": "Generator Size Calculator",
  "question": "Generator sizing: how many watts?",
  "summary": "Generator sizing from your devices: the total running watts plus the highest additional starting watts gives the starting watts a portable or standby generator must supply, with an optional margin.",
  "category": "construction",
  "url": "https://www.acalculator.org/construction/generator-size-calculator",
  "markdown": "https://www.acalculator.org/construction/generator-size-calculator.md",
  "kind": "function",
  "method": "Running watts = sum of running watts × (1 + margin); starting watts = (sum of running watts + highest additional starting watts) × (1 + margin).",
  "assumptions": [
    "Only one motor starts at a time, so only the highest additional starting watts is added.",
    "Starting watts are the additional watts on top of running watts, as Champion and Firman list them.",
    "List only the devices that run at the same time."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "devices": {
        "title": "Devices you run at the same time",
        "description": "Each device’s running watts and additional starting watts, from its label or manual.",
        "type": "array",
        "items": {
          "type": "object",
          "properties": {
            "r": {
              "title": "Running watts",
              "description": "The watts the device draws while it runs (volts × amps when only those are listed).",
              "type": "number",
              "minimum": 0,
              "maximum": 1000000
            },
            "s": {
              "title": "Additional starting watts",
              "description": "The extra watts for the first seconds of a motor start; 0 for lights and electronics.",
              "type": "number",
              "minimum": 0,
              "maximum": 1000000
            }
          }
        }
      },
      "m": {
        "title": "Extra margin",
        "description": "A safety margin added to both totals, such as 20%. 0 adds none.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 100
      }
    }
  },
  "outputs": {
    "starting": {
      "label": "Starting watts needed",
      "description": "Total running watts + the highest additional starting watts, plus the margin.",
      "format": "number"
    },
    "running": {
      "label": "Running watts needed",
      "description": "The total running watts of every device, plus the margin.",
      "format": "number"
    },
    "totalRunning": {
      "label": "Total running watts",
      "description": "The sum of the running watts, with no margin.",
      "format": "number"
    },
    "surge": {
      "label": "Highest additional starting watts",
      "description": "The largest additional starting watts of one device.",
      "format": "number"
    },
    "kw": {
      "label": "Starting kilowatts",
      "description": "The starting watts needed ÷ 1,000.",
      "format": "number"
    },
    "largest": {
      "label": "Largest starting load",
      "description": "The device with the highest starting watts.",
      "format": "text"
    },
    "summary": {
      "label": "Answer",
      "description": "The generator rating to look for.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "devices": [
        {
          "running": 400,
          "starting": 1200
        },
        {
          "running": 1050,
          "starting": 2150
        },
        {
          "running": 1200,
          "starting": 2400
        }
      ],
      "m": 0
    },
    "outputs": {
      "starting": 5050,
      "running": 2650,
      "totalRunning": 2650,
      "surge": 2400,
      "kw": 5.05,
      "largest": "Device 3",
      "summary": "Look for a generator rated at least 2,650 W running and 5,050 W starting"
    },
    "text": "Look for a generator rated at least 2,650 W running and 5,050 W starting."
  },
  "examples": [
    {
      "given": {
        "devices": [
          {
            "running": 400,
            "starting": 1200
          },
          {
            "running": 1050,
            "starting": 2150
          },
          {
            "running": 1200,
            "starting": 2400
          }
        ],
        "m": 0
      },
      "expect": {
        "totalRunning": 2650,
        "surge": 2400,
        "starting": 5050,
        "largest": "Device 3"
      },
      "source": "Champion Power Equipment, What Size Generator Do I Need? (add all running watts; identify the highest starting wattage; add that starting wattage to the total running watts; starting watts are the additional burst a device needs to start; refrigerator 150–400 running, 800–1,200 starting; ½ HP sump pump 800–1,050 and 1,300–2,150; 10,000 BTU window AC 900–1,200 and 1,800–2,400), https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/ (retrieved 2026-10-02); hand calculation in content.mdx: 400 + 1,050 + 1,200 = 2,650 running; + 2,400 = 5,050 starting"
    },
    {
      "given": {
        "devices": [
          {
            "running": 400,
            "starting": 1200
          },
          {
            "running": 1050,
            "starting": 2150
          },
          {
            "running": 1200,
            "starting": 2400
          }
        ],
        "m": 20
      },
      "expect": {
        "running": 3180,
        "starting": 6060,
        "kw": 6.06
      },
      "source": "FIRMAN Power Equipment, Buying Guide (add the running watts of all items run at the same time; identify the device with the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required), https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02); hand calculation in content.mdx: 2,650 × 1.2 = 3,180; 5,050 × 1.2 = 6,060"
    },
    {
      "given": {
        "devices": [
          {
            "running": 60
          },
          {
            "running": 150,
            "starting": 0
          },
          {
            "running": 800,
            "starting": 1300
          }
        ],
        "m": 0
      },
      "expect": {
        "totalRunning": 1010,
        "starting": 2310,
        "largest": "Device 3"
      },
      "source": "FIRMAN Power Equipment, Buying Guide (add the running watts of all items run at the same time; identify the device with the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required), https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02); hand calculation in content.mdx: 60 + 150 + 800 = 1,010; + 1,300 = 2,310"
    }
  ],
  "sources": [
    "Champion Power Equipment, What Size Generator Do I Need? (list the devices; add all running watts; identify the highest starting wattage; add that starting wattage to the total running watts; starting watts are the additional burst certain devices need to start; refrigerator 150–400 running and 800–1,200 starting watts; ½ HP sump pump 800–1,050 and 1,300–2,150; 10,000 BTU window AC 900–1,200 and 1,800–2,400; choose a generator that exceeds the total by a safe margin). https://www.championpowerequipment.com/news-updates/what-size-generator-do-i-need/ (retrieved 2026-10-02)",
    "FIRMAN Power Equipment, Buying Guide (record the running and additional starting watts of the devices run at the same time; add the running watts; identify the highest additional starting watts; total running watts + highest additional starting watts = total starting watts required). https://firmanpowerequipment.com/pages/buying-guide (retrieved 2026-10-02)"
  ],
  "related": [
    "wattage",
    "electricity-cost",
    "btu"
  ],
  "changelog": []
}