{
  "about": "The load eras are the atlas's second key. Each names a technology that arrived and asked something new of the grid. The dates are the period in which that load was the one the system was being rebuilt around in eastern Massachusetts — they overlap, because the grid never finishes one before the next arrives. An era's end date is not the end of the load; lighting did not stop in 1900.",
  "note_on_dates": "Dates are editorial judgements over the sourced record, not measurements. Where a date is contested the field says so. They exist to order entries on a time axis, and no claim in the atlas rests on one.",
  "eras": [
    {
      "id": "lighting",
      "label": "Lighting",
      "start": 1882,
      "end": 1900,
      "asked_of_the_grid": "A few hours of high demand after dark, in a dense downtown, with no daytime load to pay for the plant.",
      "shape": "Sharp evening peak, winter-heavy, tiny load factor.",
      "date_note": "Boston's first central-station service dates from the mid-1880s; the Board of Gas Commissioners was extended to electric light in 1887."
    },
    {
      "id": "traction",
      "label": "Streetcars",
      "start": 1889,
      "end": 1930,
      "asked_of_the_grid": "All-day power at a scale lighting never needed, on a schedule set by commuting rather than by darkness.",
      "shape": "Twin morning and evening peaks; the first load with a daytime base.",
      "date_note": "The West End Street Railway electrified from 1889; the Boston Elevated Railway was incorporated in 1894."
    },
    {
      "id": "appliance",
      "label": "Appliances",
      "start": 1920,
      "end": 1960,
      "asked_of_the_grid": "Household load, spread across the whole system, sold by the utility that also sold the appliance.",
      "shape": "Rising base load; the beginning of a residential evening peak.",
      "date_note": "The utility appliance business is visible in Boston Edison's rate filings from the early 1920s."
    },
    {
      "id": "cooling",
      "label": "Air conditioning",
      "start": 1950,
      "end": 1995,
      "asked_of_the_grid": "A summer afternoon peak that eventually exceeded the winter one, coincident across the entire region on the same few days.",
      "shape": "Weather-driven, highly coincident, low load factor at the top.",
      "date_note": "New England's seasonal peak crossed from winter to summer around 1990 — summer-peaking by July 1991 on the primary record, with Boston Edison itself summer-peaking by 1986. The specific year is the atlas's best-supported estimate, not a measurement."
    },
    {
      "id": "gas-heat",
      "label": "Gas heat",
      "start": 1950,
      "end": 2020,
      "asked_of_the_grid": "Nothing — and that is the point. Heating went to a second network, built in parallel, and the electric system stopped being the winter system.",
      "shape": "Load taken off the electric peak and put on a pipe that is now leaking and being retired.",
      "date_note": "Included as an era because its unwinding is what makes the heat-pump era a load event at all."
    },
    {
      "id": "colocation",
      "label": "Colocation",
      "start": 1995,
      "end": 2015,
      "asked_of_the_grid": "Flat, continuous, high-density load in buildings that had been warehouses and telephone exchanges, concentrated on a handful of downtown circuits.",
      "shape": "Near-constant; a load factor close to 1, which is the opposite of everything before it.",
      "date_note": "Dated from the carrier-hotel conversions of the late 1990s."
    },
    {
      "id": "distributed-energy",
      "label": "Distributed energy",
      "start": 2008,
      "end": null,
      "asked_of_the_grid": "To run backwards. Generation on the customer's side of the meter, interconnecting under a tariff written for it — the one kind of new connection Massachusetts does publish capacity for.",
      "shape": "Midday export, a falling net peak that masks a rising gross one, and a summer peak pushed later into the evening."
    },
    {
      "id": "heat-pump",
      "label": "Heat pumps",
      "start": 2018,
      "end": null,
      "asked_of_the_grid": "The winter peak back, on the coldest mornings, in the same neighbourhoods whose gas pipe is being retired.",
      "shape": "A second, cold-morning peak that may eventually retake the annual maximum from summer."
    },
    {
      "id": "compute",
      "label": "Compute",
      "start": 2020,
      "end": null,
      "asked_of_the_grid": "Interconnections measured in hundreds of megawatts, requested faster than distribution can be built, by customers who publish nothing about their load.",
      "shape": "Flat and very large; a single request can exceed a substation."
    }
  ]
}