How to Build a Solar Panel Calculator

interface SolarInputs {
  dailyKwh: number;          // average daily electricity usage
  electricityRate: number;    // $/kWh
  annualRateIncrease: number; // e.g., 0.03 for 3%/year
  peakSunHours: number;       // location-dependent (3-7 hours)
  panelWattage: number;       // typically 370-430W
  costPerWatt: number;        // $2.50-3.50 installed
  federalITC: number;         // 0.30 for 30%
  stateIncentive: number;     // flat dollar amount
  roofSqFt?: number;          // available roof space
  wantsBattery: boolean;
  batteryBackupKwh?: number;  // daily kWh to back up
  batteryAutonomyDays?: number;
  }

  interface SolarResult {
  systemSizeKw: number;
  numberOfPanels: number;
  roofSpaceNeeded: number;
  annualProductionKwh: number;
  inverterSizeKw: number;
  batterySizeKwh: number | null;
  grossCost: number;
  federalCredit: number;
  stateIncentive: number;
  netCost: number;
  yearOneAnnualSavings: number;
  simplePaybackYears: number;
  twentyFiveYearSavings: number;
  co2OffsetTonsPerYear: number;
  }

  const SYSTEM_EFFICIENCY = 0.80;   // accounts for inverter loss, wiring, temperature, shading
  const PANEL_SQFT = 17.5;          // average 400W panel footprint
  const DEGRADATION_RATE = 0.005;   // 0.5% per year
  const CO2_PER_KWH = 0.0004;      // metric tons CO2 per kWh (US grid average)
  const BATTERY_DOD = 0.80;         // depth of discharge for lithium batteries

  function calculateSolarSystem(inputs: SolarInputs): SolarResult {
  // System sizing
  const systemSizeKw = inputs.dailyKwh / inputs.peakSunHours / SYSTEM_EFFICIENCY;
  const systemSizeW = systemSizeKw * 1000;
  const numberOfPanels = Math.ceil(systemSizeW / inputs.panelWattage);
  const actualSystemKw = (numberOfPanels * inputs.panelWattage) / 1000;
  const roofSpaceNeeded = numberOfPanels * PANEL_SQFT;

  // Annual production
  const annualProductionKwh = actualSystemKw * inputs.peakSunHours * 365 * SYSTEM_EFFICIENCY;

  // Inverter sizing (10% over panel capacity for string inverter)
  const inverterSizeKw = Math.ceil(actualSystemKw * 1.1 * 10) / 10;

  // Battery sizing
  let batterySizeKwh: number | null = null;
  if (inputs.wantsBattery && inputs.batteryBackupKwh) {
    const autonomyDays = inputs.batteryAutonomyDays || 1;
    batterySizeKwh = Math.ceil(
      (inputs.batteryBackupKwh * autonomyDays) / BATTERY_DOD
    );
  }

  // Financial calculations
  const grossCost = actualSystemKw * 1000 * inputs.costPerWatt;
  const federalCredit = grossCost * inputs.federalITC;
  const netCost = grossCost - federalCredit - inputs.stateIncentive;
  const yearOneAnnualSavings = annualProductionKwh * inputs.electricityRate;

  // 25-year savings with rate increases and panel degradation
  let cumulativeSavings = 0;
  for (let year = 1; year <= 25; year++) {
    const degradedOutput = annualProductionKwh * Math.pow(1 - DEGRADATION_RATE, year);
    const escalatedRate = inputs.electricityRate * Math.pow(1 + inputs.annualRateIncrease, year);
    cumulativeSavings += degradedOutput * escalatedRate;
  }
  const twentyFiveYearSavings = cumulativeSavings - netCost;
  const simplePaybackYears = netCost / yearOneAnnualSavings;
  const co2OffsetTonsPerYear = annualProductionKwh * CO2_PER_KWH;

  return {
    systemSizeKw: actualSystemKw,
    numberOfPanels,
    roofSpaceNeeded,
    annualProductionKwh: Math.round(annualProductionKwh),
    inverterSizeKw,
    batterySizeKwh,
    grossCost: Math.round(grossCost),
    federalCredit: Math.round(federalCredit),
    stateIncentive: inputs.stateIncentive,
    netCost: Math.round(netCost),
    yearOneAnnualSavings: Math.round(yearOneAnnualSavings),
    simplePaybackYears: Math.round(simplePaybackYears * 10) / 10,
    twentyFiveYearSavings: Math.round(twentyFiveYearSavings),
    co2OffsetTonsPerYear: Math.round(co2OffsetTonsPerYear * 10) / 10,
  },
  }

  // Peak sun hours by region (annual average)
  const PEAK_SUN_HOURS: Record<string, number> = {
  "Southwest US (AZ, NV, NM)": 6.5,
  "Southeast US (FL, GA, TX)": 5.5,
  "Midwest US (IL, OH, MN)": 4.5,
  "Northeast US (NY, MA, PA)": 4.0,
  "Pacific NW (WA, OR)": 3.5,
  "UK": 3.0,
  "Australia": 5.5,
  "Germany": 3.0,
  "India": 5.5,
  };