Evidence

How we know the numbers are honest

Watt Matters is a model, not a meter. So we check it two ways: against physics that anyone can do on paper, and against public data from the U.S. Energy Information Administration and the Department of Energy. Those checks run in the test suite before every release, and they just ran again in your browser.

Running checks…

How the model works

Every number is built bottom-up from your answers in chapter one. We never scale a national average; we compute each end use from the physics of the job and the efficiency of the machine, with coefficients tuned so a typical home lands near the RECS 2020 averages.

Space heating and cooling

Delivered heat needed per square foot per year, by climate: cold 7, mixed 4.5, mild 3, hot-humid 1.5, hot-dry 1.6 kWh (thermal). That’s divided by the equipment’s efficiency: 1.0 for resistance heat, a seasonal COP of 2.2 (cold), 2.7 (mixed), 3 (mild), 3.2 (hot-humid), 3.1 (hot-dry) for heat pumps, 85% AFUE for gas and propane furnaces, 83% for oil. Cooling electricity per square foot: cold 0.45, mixed 1, mild 0.3, hot-humid 2.2, hot-dry 1.8 kWh, times 0.55 for window units.

Hot water

18 gallons of hot water per person per day, heated 45–65°F depending on climate (inlet water is colder up north), at 8.34 BTU per gallon-degree and 3,412 BTU per kWh. Divided by 0.92 for an electric tank, a COP of 3.0 for a heat pump water heater, or 0.62 for gas, plus tank standby losses. Laundry and dishwasher hot water are added on top from loads per year.

Everything else

Refrigeration, lighting, cooking, electronics, and standby are per-household constants scaled by size or occupants, taken from ENERGY STAR product data and RECS. Optional big loads (EV, pool pump, hot tub) use typical annual figures. “Everything else” is a flat 300 kWh so the total isn’t flattered.

Savings in the action plan

Each action applies a published percentage to the end uses it touches (for example, DOE’s 10% for thermostat setbacks, EPA’s ~10% each for air sealing and insulation), or re-runs the whole model with the upgrade in place (LED, cold wash, heat pump water heater, heat pump) and takes the difference. No action can claim more than the affected line items contain.

What this can’t tell you

  • Your actual bill. Insulation quality, thermostat habits, window area, and equipment age all move the real number, sometimes a lot. The proportions are the point.
  • Time-of-use rates, demand charges, solar, or net metering. We use a single flat rate.
  • Gas cooking and gas dryers are approximated; gas ranges are treated as electric for simplicity.
  • Window units are priced by how many rooms they cool, and charged 30% more electricity than central air for the same cooling. That is the softest number here. Room units and central systems are rated on scales that cannot be compared directly — CEER against SEER — and working it through seasonally gives a defensible range of roughly 15% to 40%, depending on how old the units are and whether the ducts run through an attic. We use 30%, toward the older-units end, because that is what tends to be in the window. If your units are new and your ducts are bad, the real gap is smaller than we show.
  • Prices move. State rates are 2025 approximations; the rate from your bill always wins.

Sources

Found something wrong?
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