{
  "id": "solar-panel",
  "version": "4976926093c8",
  "status": "published",
  "name": "Solar Panel Calculator",
  "question": "How many solar panels do I need?",
  "summary": "Computes the solar system size in kilowatts and the number of panels that cover a share of a home’s electricity use, from the monthly use, the peak sun hours, the panel rating and the system losses, and the energy they make.",
  "category": "construction",
  "url": "https://www.acalculator.org/construction/solar-panel-calculator",
  "markdown": "https://www.acalculator.org/construction/solar-panel-calculator.md",
  "kind": "function",
  "method": "System kW = monthly kWh ÷ (365 ÷ 12) × share ÷ (sun hours × (1 − losses)); panels = system W ÷ panel W, rounded up; energy per month = panels × panel kW × sun hours × (1 − losses) × 365 ÷ 12.",
  "assumptions": [
    "Peak sun hours are the average daily solar energy on the panels, in kWh per m² per day, typed for your site and tilt; the calculator uses no location data.",
    "Panels make their rated power at 1,000 W/m² of sun, less the system losses (14% by default, the PVWatts default).",
    "An average month is 365 ÷ 12 days, and every month gets the average sun; real output is higher in summer and lower in winter.",
    "No prices, incentives or battery storage: the calculator sizes the panels only."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "kwh": {
        "title": "Electricity use per month (kWh)",
        "description": "The electricity the home uses in an average month, in kilowatt hours, from the power bills.",
        "type": "number",
        "minimum": 1,
        "maximum": 1000000
      },
      "sun": {
        "title": "Peak sun hours per day",
        "description": "The average daily solar energy at the site, in kWh per m² per day, which equals hours of full 1,000 W/m² sun.",
        "type": "number",
        "minimum": 0.5,
        "maximum": 12
      },
      "w": {
        "title": "Panel rating (W)",
        "description": "The rated power of one panel in watts, from its label (its output at standard test conditions).",
        "type": "number",
        "minimum": 10,
        "maximum": 1000
      },
      "loss": {
        "title": "System losses",
        "description": "Energy lost between the panels’ rating and the power the home gets: heat, wiring, the inverter, shade and dirt.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 0,
        "maximum": 90
      },
      "offset": {
        "title": "Share of use to cover",
        "description": "How much of the monthly use the panels should make, in percent: 100 for all of it.",
        "type": "number",
        "x-unit": "percent",
        "minimum": 1,
        "maximum": 200
      }
    }
  },
  "outputs": {
    "panels": {
      "label": "Solar panels",
      "description": "The system size ÷ the panel rating, rounded up to whole panels.",
      "format": "integer"
    },
    "systemKw": {
      "label": "System size needed (kW)",
      "description": "The panel power that covers the share of use: daily use × share ÷ (sun hours × (1 − losses)).",
      "format": "number"
    },
    "installedKw": {
      "label": "System size with whole panels (kW)",
      "description": "The number of panels × the panel rating.",
      "format": "number"
    },
    "monthlyKwh": {
      "label": "Energy made per month (kWh)",
      "description": "What the whole panels make in an average month: kW × sun hours × (1 − losses) × 365 ÷ 12.",
      "format": "number"
    },
    "yearlyKwh": {
      "label": "Energy made per year (kWh)",
      "description": "What the whole panels make in a year: kW × sun hours × (1 − losses) × 365.",
      "format": "number"
    },
    "covered": {
      "label": "Share of use covered",
      "description": "The energy made per month ÷ the monthly use, in percent.",
      "format": "percent"
    },
    "dailyKwh": {
      "label": "Electricity use per day (kWh)",
      "description": "The monthly use ÷ (365 ÷ 12) days.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "kwh": 900,
      "sun": 4.5,
      "w": 400,
      "loss": 14,
      "offset": 100
    },
    "outputs": {
      "panels": 20,
      "systemKw": 7.6457470532016565,
      "installedKw": 8,
      "monthlyKwh": 941.7,
      "yearlyKwh": 11300.4,
      "covered": 104.63333333333334,
      "dailyKwh": 29.589041095890412
    },
    "text": "To make 100% of 900 kWh a month at 4.5 peak sun hours you need 7.65 kW; 400 W panels to buy: 20, making 941.7 kWh a month."
  },
  "examples": [
    {
      "given": {
        "kwh": 900,
        "sun": 4.5,
        "w": 400,
        "loss": 14,
        "offset": 100
      },
      "expect": {
        "dailyKwh": 29.589041095890412,
        "systemKw": 7.6457470532016565,
        "panels": 20,
        "installedKw": 8,
        "monthlyKwh": 941.7,
        "yearlyKwh": 11300.4,
        "covered": 104.63333333333334
      },
      "source": "NREL PVWatts Version 5 Manual (Dobos 2014), 14% default system losses; hand calculation in content.mdx: 900 ÷ 30.4167 = 29.59 kWh a day ÷ (4.5 × 0.86) = 7.65 kW ÷ 0.4 = 19.11, so 20 panels"
    },
    {
      "given": {
        "kwh": 816.14,
        "sun": 4,
        "w": 300,
        "loss": 14,
        "offset": 100
      },
      "expect": {
        "dailyKwh": 26.832,
        "systemKw": 7.8,
        "panels": 26,
        "installedKw": 7.8,
        "monthlyKwh": 816.14,
        "covered": 100
      },
      "source": "NREL PVWatts Version 5 Manual (Dobos 2014); hand calculation in content.mdx: 816.14 ÷ (365 ÷ 12) = 26.832 kWh a day ÷ (4 × 0.86) = 7.8 kW ÷ 0.3 = 26 panels exactly"
    },
    {
      "given": {
        "kwh": 1200,
        "sun": 6,
        "w": 500,
        "loss": 20,
        "offset": 50
      },
      "expect": {
        "systemKw": 4.109589041095891,
        "panels": 9,
        "installedKw": 4.5,
        "monthlyKwh": 657
      },
      "source": "NREL PVWatts Version 5 Manual (Dobos 2014); hand calculation in content.mdx: 1,200 × 0.5 ÷ 30.4167 = 19.73 kWh a day ÷ (6 × 0.8) = 4.11 kW ÷ 0.5 = 8.2, so 9 panels; 4.5 × 4.8 × 30.4167 = 657 kWh"
    }
  ],
  "sources": [
    "Dobos, A. P. (2014). PVWatts Version 5 Manual. National Renewable Energy Laboratory, NREL/TP-6A20-62641 (default system losses of 14%). https://www.nrel.gov/docs/fy14osti/62641.pdf",
    "NREL, PVWatts Calculator (solar resource in kWh/m²/day for a location, tilt and azimuth). https://pvwatts.nrel.gov",
    "IEC 60904-3, Photovoltaic devices, Part 3: Measurement principles for terrestrial photovoltaic solar devices with reference spectral irradiance data (standard test conditions: 1,000 W/m², 25 °C cell temperature). https://webstore.iec.ch/publication/61084"
  ],
  "related": [
    "electricity-cost",
    "power"
  ],
  "changelog": []
}