{
  "_doc": "Modelling assumptions. Every one of these is exposed as a slider in the UI and published on /methodology/. Nothing here is a secret.",
  "_verified_on": "2026-09-04",
  "system": {
    "panel_watts": 400,
    "sqft_per_panel": 17.5,
    "offset_target": 1,
    "degradation_per_year": 0.005,
    "om_cost_per_year": 150,
    "inverter_replacement_cost": 1800,
    "inverter_replacement_year": 12,
    "analysis_years": 25,
    "min_installed_kw": 5,
    "_min_installed_kw_note": "The smallest system a household is realistically sold. LBNL Tracking the Sun (2024 edition, systems installed in 2023) reports a median residential size of 7.4 kW with the 20th-to-80th percentile band at 5-11 kW, so four in five installed systems are at least 5 kW. It is not a hard floor - one in five is smaller - but below it a buyer is outside what most installers routinely quote, and the per-watt price rises because permitting, design and a day of labour do not scale down. The engine applies this only when a caller asks for it (overrides.minSystemKw); it is not imposed on the default sizing.",
    "_min_installed_kw_source": "https://escholarship.org/uc/item/2c50c61d",
    "_min_installed_kw_verified_on": "2026-09-06",
    "median_installed_kw": 7.4,
    "_median_installed_kw_note": "Median US residential system size, LBNL Tracking the Sun 2024 edition, systems installed in 2023. Recorded so the page can quote it rather than restate it.",
    "p80_installed_kw": 11,
    "_p80_installed_kw_note": "The 80th percentile of US residential system size. Same source and edition as the median and the floor: LBNL Tracking the Sun reports the 20th-to-80th percentile band at 5-11 kW for systems installed in 2023, so three in five installed systems sit between min_installed_kw and this value. Recorded as a figure rather than left inside a note so a page can place a size in the real distribution instead of asserting one.",
    "_p80_installed_kw_source": "https://escholarship.org/uc/item/2c50c61d",
    "_p80_installed_kw_verified_on": "2026-09-06"
  },
  "bill": {
    "fixed_monthly_fee": 12,
    "_note": "Non-bypassable customer charge; solar does not remove it."
  },
  "market": {
    "rate_escalation_per_year": 0.025,
    "_escalation_note": "The single most contested assumption in any solar calculator. Always show it as an adjustable slider."
  },
  "self_consumption": {
    "without_battery": 0.4,
    "with_battery": 0.75,
    "_note": "Share of generation consumed on site rather than exported. Irrelevant under 1:1 net metering, decisive under NEM 3.0."
  },
  "loan": {
    "apr": 0.0699,
    "term_years": 20,
    "down_payment": 0
  },
  "lease": {
    "ppa_rate_vs_retail": 0.7,
    "annual_escalator": 0.029,
    "_note": "Third-party owner keeps the 48E credit and prices below the retail rate. Zero down, lower savings."
  },
  "battery": {
    "usable_kwh": 13.5,
    "installed_cost": 12000
  },
  "display": {
    "immediate_min_share_of_bill": 0.1,
    "_note": "A zero-upfront scenario is called immediate only if its year-1 net cash flow is at least this share of the annual bill. Below it: cash-flow neutral. A one-dollar margin must never read as a win."
  }
}