Home Kilowatt

Changelog

Changelog

Every change to a rate, constant or formula on Home Kilowatt, newest first. When a number on the site changes, it's recorded here with the date and the pages it affects.

  1. Formula

    Formula corrections in the topic calculators

    Pages: /heating/furnace-size, /solar/panels-needed, /solar/lease-vs-buy, /efficiency/attic-insulation, /efficiency/window-replacement

    • Furnace size: the result now rounds the furnace input up to the sizes sold (40,000, 60,000, 80,000, 100,000 or 120,000 BTU/h), as the method describes (R2-B8).
    • Solar panels needed: the roof area uses the whole number of panels, e.g. 20 panels × 18 sq ft = 360 sq ft (R2-D4).
    • Solar lease vs buy: yearly savings now apply the state's net metering factor, as the payback and output calculators do. In net-billing and utility-dependent states the 25-year figures are lower than before (R2-D10).
    • Attic insulation and window replacement: the cooling saving is kWh = BTU ÷ (SEER 14 × 1,000). The earlier figure also divided by 3,412 and was about 3.4 times too low (X14).
    • The reference formulas, the test cases and the specification were corrected together; the site passes all 210 test cases.

  2. Formula

    How the topic calculators present results

    Pages: /heating, /cooling, /solar, /generators, /ev, /efficiency

    • Sizing: the BTU, mini split and multi-zone calculators keep the nearest standard size and add the next size up; a load below the smallest unit reads "smaller than the smallest unit", never "0 BTU" (R2-C1, R2-C2, R2-C4).
    • Heat pump payback and heat pump water heater: a payback over 30 years reads "More than 30 years"; zero or less reads "Paid back at install" (R2-B5, R2-B13).
    • Heat pump and resistance options have no efficiency input, since the calculation uses the seasonal COP or 100% (R2-B1). The thermostat setback calculator has an AFUE input (default 0.90) and the full fuel list (R2-B11).
    • Heat pump vs propane or oil has a propane / oil toggle (R2-B2). Propane vs electric and gas vs electric show the heat pump as a third line (R2-B3). The wood stove comparison shows its fixed 0.90 gas furnace AFUE (R2-B9).
    • Tankless vs tank shows yearly costs only; there is no installation-cost input, so no payback (R2-B12). Net metering savings takes the export credit as your input, default 0.25 (R2-D3).
    • No state data, no table: state rebates link to DSIRE (R2-B7, R2-D2); outage hours (R2-D7) and peak sun hours (R2-D11, with the NREL map) are inputs; there is no per-state SEER2 minimum (R2-C7); charger rebates say "check your utility" (X9).
    • Values that follow the reference model are labelled "typical" or "our estimate": the peak pellet month (R2-B10), portable vs window gap (R2-C6), dehumidifier watts (R2-C10), duct table (R2-C11), panel direction factors (R2-D5), battery with no running cost (R2-D9), the 12-month curve (E3), and the ×1.08 preconditioning range (X11).
    • Generators: diesel is priced as heating oil + $0.30/gal and fuel prices read "EIA/AAA, 2025–26 reference (data as of 2026-09-26)" (X1, X2); standby exercise fuel is priced as propane (X5). The 3,500 W page keeps the default checklist (X19).
    • EV: the charger calculator shows 7.2 kW typed and 7.7 kW for 32 A Level 2 (X12); charging loss 1.12 and charge-time efficiency 0.9 are kept as specified (X13). The model list has the 8 vehicles with EPA figures; more are coming (X8).
    • Efficiency: LED and thermostat prices are inputs with the reference defaults, $3 and $200 (X18); smart thermostat COP and cooling hours by zone are cited as Home Kilowatt model values (X16).
    • The 30C notice ships on the charger installation page and the EV hub: the federal credit for home charger installation ended for property placed in service after June 30, 2026 (X10).
    • One page with a state or model picker for now: gas vs electric by state (R2-B4), solar payback and net metering by state, EV vs gas by state (X6) and charging cost by model.

  3. New page

    50 topic calculators

    Pages: /heating, /cooling, /solar, /generators, /ev, /efficiency

    • Heating (13), cooling and sizing (9), solar and batteries (9), generators (7, with four "what can it run" sizes), electric vehicles (6) and home efficiency (6), each with state prices and climate data, the formula and its sources.

  4. Formula

    Calculator inputs aligned with the reference formulas

    Pages: /cost-to-run/window-ac, /cost-to-run/central-ac, /cost-to-run/mini-split, /cost-to-run/hot-tub, /cost-to-run/dryer, /cost-to-run/dishwasher, /cost-to-run/refrigerator, /cost-to-run/well-pump

    • Each calculator shows only the inputs its formula uses; every result matches the three worked test cases for that page.
    • Window AC and Central air: watts come from BTU ÷ CEER and tons × 12,000 ÷ SEER2. A separate "enter watts directly" field is not offered.
    • Mini split: the average-watts line is labelled with the modulation factor you enter (for example 70%), not a fixed 60%.
    • Hot tub: the result covers the heater (watts × heater hours). Circulation-pump watts and a climate picker are not inputs; the climate appears as typical heater hours (warm 0.8, mild 1.2, cold 1.8). Add a pump as its own row with the Appliance calculator.
    • Clothes dryer: electric (3.0 kWh/load) and heat pump (1.2 kWh/load). A gas option is not offered until the gas-price formula is added.
    • Dishwasher: "Loads per month" sets the number of cycles; a separate loads-per-week field is not used.
    • Refrigerator & chest freezer: yearly kWh from the EnergyGuide label only.
    • Well pump: pump size (HP) and gallons per day only; the month is 30 days.

  5. Constant

    Appliance presets from energy.gov

    Pages: /cost-to-run/appliance

    • The Appliance calculator lists typical wattages from energy.gov, "Estimating Appliance and Home Electronic Energy Use". Where energy.gov gives a range, the preset is its midpoint rounded to 10 W: Clock radio 10 W (10 W); Coffee maker 1050 W (900–1,200 W); Clothes iron 1400 W (1,000–1,800 W); Hair dryer 1540 W (1,200–1,875 W); Microwave oven 930 W (750–1,100 W); Toaster 1100 W (800–1,400 W); Toaster oven 1225 W (1,225 W); Vacuum cleaner 1220 W (1,000–1,440 W); Laptop 50 W (50 W); Desktop computer (CPU, awake) 120 W (120 W); Computer monitor (awake) 150 W (150 W); Flat-screen TV 120 W (120 W); Stereo radio 240 W (70–400 W); DVD player 20 W (20–25 W); Window fan 150 W (55–250 W); Furnace fan 750 W (750 W); Whole-house fan 500 W (240–750 W); Water bed (heater, no cover) 250 W (120–380 W).
    • Appliances that have their own Home Kilowatt calculator link to it instead of presetting a wattage.

  6. Constant

    Electric bill estimator: example rows named

    Pages: /cost-to-run/electric-bill-estimator

    • The example list uses the reference defaults: space heater 1,500 W × 6 h, refrigerator 150 W × 8 h (the Refrigerator calculator default in the reference formulas), water heater element 4,500 W × 2.6 h.

  7. Formula

    Seasonal curves use a typical US pattern

    Pages: /cost-to-run/space-heater, /cost-to-run/electric-blanket, /cost-to-run/pool-pump, /cost-to-run/dehumidifier

    • The 12-month curve spreads the yearly cost with one typical monthly share of heating or cooling degree-days (NOAA 1991–2020 normals) for every state. Per-state monthly patterns are planned.

  8. New page

    Constants reviewed; first release

    Pages: all

    • Home Kilowatt launches with 20 cost-to-run calculators, the Electric Bill Estimator, and the Methodology and Data pages. Electricity and gas prices start from 2025–26 reference values and switch to the monthly EIA series with the first update. All incentive notes follow 2026 rules: the federal 25C and 25D tax credits ended for installations after Dec 31, 2025.
    • Reference data pack dated 2026-09-26: rates, gas prices and average bills for 50 states, DC and the US average are shown as "EIA reference 2025–26". Every calculator page carries "Last reviewed: 2026-09-26".

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