How we calculate everything
Every figure on this site comes from a calculation with assumptions in it. They are all listed here, with their values, so you can check them or disagree with them. A number you cannot interrogate is one you should not act on.
Where the data comes from
Energy prices
US Energy Information Administration
Residential sector, per state, refreshed monthly. EIA publishes about two months in arrears, so a figure dated May appears in July. Every page states the month it refers to.
Solar production
NREL PVWatts v8
Modelled from real weather station data. Fixed roof mount, 20° tilt, 180° azimuth, 14% system losses — the defaults a homeowner quote assumes unless told otherwise.
ZIP coordinates
US Census Bureau gazetteer
33,642 ZIP codes with latitude, longitude and state. Resolved locally, so no geocoding service sees your ZIP.
Climate
NOAA normals, summarised per state
Heating and cooling degree days to base 65°F, and freezing nights, as thirty-year normals rounded to the nearest hundred.
Heating
Annual heat demand is the degree-day method: heating degree days × 24 × a whole-house loss coefficient. That coefficient is 0.35 BTU per hour per °F per square foot, which reflects a typical existing US home — insulated, but not a new build. It is the single largest assumption on this site.
Gas furnace efficiency
80% AFUE
The default; the calculator accepts your own.
Heat pump rating
HSPF2 8.5
A mainstream unit. HSPF2 is BTU per watt-hour, so the result is divided by 1,000 before meeting a per-kWh price.
Electric resistance
100% efficient at the appliance
The losses are upstream at the power station, and already inside the price you pay.
Heat pump install premium
$3,200
What a heat pump costs over replacing a furnace like for like. Payback is measured against this, not the full price — someone replacing a dying furnace is choosing between two purchases, not one.
Cooling
Cooling load uses the same degree-day method with a lower coefficient, 0.30, plus a humidity adjustment — because a compressor also wrings water out of the air, work no outdoor temperature reading captures.
Humidity adjustment
×1.25 / ×1.12 / ×1.00
Applied above 2,000 cooling degree days, above 1,000, and below that. A Louisiana summer costs more than an Arizona one at the same temperature.
Sizing
750 − (CDD ÷ 20) sq ft per ton
Clamped between 450 and 750, then rounded to the half-ton sizes actually sold. Hot climates genuinely need more capacity per square foot than the old flat 500 rule allows.
Efficiency baseline
SEER2 14.3
The federal minimum, and the honest baseline for payback: the choice is which unit to buy, not whether to buy one.
SEER to SEER2
×0.95
SEER2 is measured against higher static pressure, so the same machine scores about 5% lower. Old units are labelled in SEER and converted before comparison.
Installed cost
$2,800 + $1,150 per ton
Labour, lineset, permit and disposal do not scale with size; equipment does. Each SEER2 point above the minimum adds 8% of equipment cost. Shown as a ±20% range.
Solar
System lifetime
25 years
Run year by year rather than as a flat multiple, because output falls while the price of what it displaces rises, and those two do not cancel.
Panel degradation
0.5% a year
Roughly what manufacturer warranties assume.
Electricity price rise
2% a year, real
Deliberately conservative. Payback is enormously sensitive to this, and assuming 5% is the easiest way for a solar calculator to make any system look like a winner.
Federal tax credit
30%
A credit against tax owed, not a rebate. Anyone without the liability to absorb it will not see all of it.
Export credit
Your choice
What the utility pays for power you export rather than use. States have been cutting this, and it now decides the answer more often than sunshine does.
What none of this can see
Insulation, air sealing, window area and orientation, ductwork condition and leakage, the age and state of your existing equipment, shading on your roof, what you actually set the thermostat to, and what contractors in your town charge this month.
Any of these can move a real figure by a third or more. These estimates are built to tell you whether something is worth pursuing and to give you a number to hold a quote against — not to replace the quote.
Where a number is missing
When a state has not reported a price, we say so and show nothing rather than substituting a national average. A national figure dressed up as a local one is the failure this site exists to avoid, and it is invisible to the reader, which is what makes it worth refusing.
State pages describe a state, not a house. State-level climate is coarse — Texas spans El Paso and Amarillo — and the solar figures use the centroid of each state's ZIP codes, which in states split by mountains can sit in the sunnier half.
Found something wrong?
We would rather know. Email homeenergycost@gmail.com with the page and the figure. Corrections to assumptions are as welcome as corrections to arithmetic — the values above are judgements, and judgements can be argued with.