Utility Bill Baseload and Weather-Sensitive Split (Energy Signature)

Plot monthly energy against monthly degree-days and the points fall on a line: the intercept is everything the weather does not drive -- water heating, refrigeration, lighting, plugs, standby -- and the slope is everything it does. That single split reorders an audit before anyone opens a wall. A house with a large intercept and a modest slope does not need insulation; it needs someone to find out what is running all the time, and no amount of air sealing will touch it. A house with the opposite shape is an envelope job. Getting that backwards is how a homeowner spends a great deal of money and sees their bill barely move. The base temperature is a fitted parameter rather than 65 degF by convention, and treating it as a convention is what makes the fit wrong. A well-insulated house with large internal gains stops needing heat at a much lower outdoor temperature, so its degree-days accumulate differently -- and fitting the base temperature is what makes the intercept and the slope mean what they claim to. A fit done at the wrong base temperature will still produce a line, which is why the quality of the fit is worth looking at rather than the coefficients alone. The implied heat loss coefficient is the bridge to the rest of an audit. The slope is energy per degree-day at the meter; correcting it for the equipment's efficiency and dividing by twenty-four hours gives a whole-building UA in the same units a heat loss calculation produces -- so a bill-derived UA and a modelled one can be compared directly, and a large disagreement means one of them is wrong. That comparison catches a bad model, a wrong efficiency assumption, and a house that is not being heated the way anyone thinks it is. This takes the fit as ENTERED: it does not fit the regression, choose the base temperature, assess the fit quality, separate heating from cooling in a bill that contains both, correct for occupancy changes or a partial-year record, or account for a second fuel. The utility records, a proper regression, and the auditor's own judgment govern.

a two-parameter energy signature: annual consumption = baseload + slope x heating degree days at the fitted balance point; the weather-sensitive share is slope x degree days over the total, and the implied building UA is slope x equipment efficiency x BTU per unit / 24.

Degree-day regression of utility bills (the energy signature method) as ASHRAE Fundamentals and PRISM-style bill analysis describe it, with the baseload, slope and balance point ENTERED as fitted parameters rather than derived here. The regression itself is not performed by this calculation.

One linear relation and one unit conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: baseload_per_year, slope_per_degree_day, degree_days, equipment_efficiency, btu_per_unit, balance_point_f, weather_units, total_units, weather_share_pct, baseload_share_pct, implied_ua

• Baseload, slope and balance point are fitted parameters entered here, not regresseda bill regression
• Degree days at the fitted base not the conventional 65 degF basethe fitted signature
• Whole-building UA includes infiltration and duct losses, not a takeoff U-valuea modelled load calculation