Does a high-efficiency HVAC system pay for itself?
For heating, yes; for cooling, often not. Across the 10 markets we cover, a gas furnace moving from 80 to 96 AFUE repays a $1,500 quote difference in 7.3 years to 13.8 years, and a heat pump going from 7.5 to 9.5 HSPF2 repays $1,800 in 7.0 years to 11.1 years — every market, inside the equipment's life. A $1,200 cooling upgrade from 13.8 to 17 SEER2 takes 9.7 years in Birmingham and 19.4 years in Lexington, and in 5 of the 10 markets it never repays itself inside the 15-year life this is quoted over. Put your own quote difference in below.
Run it on your own numbers
| Energy now | 2,909 kWh a season | $385 a year |
|---|---|---|
| Energy upgraded | 2,361 kWh a season | $313 a year |
| You save | at 13.24¢/kWh | $73 a year |
| After 15 years | energy only, before any rebate | −$105 |
How the number is worked out
Nothing here is a rule of thumb. The seasonal energy figure comes from the same model the city pages use, so the answer on this page and the answer on a city page cannot disagree.
Cooling
A season's cooling is expressed as equivalent full-load hours — the hours the system would run at full output to deliver the same work. We anchor on 1,570 hours at 2,098 cooling degree-days and scale it by each city's own degree-day count, so Louisville works out at 1,115 hours.
season kWh = full-load hours × Btu capacity ÷ SEER2 ÷ 1,000
Heating
The heating load starts from the reference home in our US Heating Cost Index — 600 therms delivered at 4,000 heating degree-days — scaled by each city's degree-days. A furnace burns that load divided by its AFUE; a heat pump draws it at its HSPF2 rating.
furnace therms = load ÷ AFUE · heat-pump kWh = load × 100 ÷ HSPF2
What it deliberately leaves out
Rebates, tax credits, financing interest, maintenance, breakdown risk and the humidity benefit of a longer cycle. It prices energy, and only energy — that is what makes the comparison between cities honest. A utility rebate frequently repays more of the premium than the meter does, which is the point of showing the two separately.
Payback across every market we cover
The same three upgrades, priced on each market's own degree-days and utility rates, at a fixed quote difference of $1,200 for cooling, $1,500 for a furnace and $1,800 for a heat pump, so the markets are comparable.
| Market | CDD | HDD | Power | AC 13.8→17 | Furnace 80→96 | Heat pump 7.5→9.5 |
|---|---|---|---|---|---|---|
| Birmingham, AL | 2,098 | 2,394 | 16.1¢ | 9.7 | 11.5 | 11.1 |
| Huntsville, AL | 1,889 | 2,914 | 16.1¢ | 10.7 | 9.4 | 9.1 |
| Bowling Green, KY | 1,648 | 3,729 | 13.24¢ | 15.0 | 9.1 | 8.7 |
| Covington, KY | 1,274 | 4,627 | 13.24¢ | 19.4 | 7.3 | 7.0 |
| Lexington, KY | 1,270 | 4,333 | 13.24¢ | 19.4 | 7.8 | 7.5 |
| Louisville, KY | 1,490 | 4,150 | 13.24¢ | 16.4 | 8.2 | 7.8 |
| Owensboro, KY | 1,608 | 3,998 | 13.24¢ | 15.4 | 8.5 | 8.1 |
| Knoxville, TN | 1,606 | 3,408 | 13.18¢ | 15.4 | 12.0 | 9.5 |
| Memphis, TN | 2,109 | 2,949 | 13.18¢ | 11.8 | 13.8 | 11.0 |
| Nashville, TN | 1,783 | 3,366 | 13.18¢ | 14.0 | 12.1 | 9.6 |
Figures in red do not repay the quote difference inside 15 years. Download this table as CSV — reuse is welcome with credit.
The finding: the ranking inverts with the climate
Read the table by column and a pattern appears that a national calculator cannot show. Birmingham, the warmest market here at 2,098 cooling degree-days, repays a cooling upgrade in 9.7 years — and is among the slower markets for a heating upgrade. Lexington, at 1,270 cooling degree-days, takes 19.4 years on the same cooling upgrade while repaying a heat-pump upgrade in 7.5 years.
The practical consequence: in this region the efficiency money is better spent on the heating side. The heating load is larger than the cooling load in every market here, so the same percentage gain in efficiency moves more dollars. And where power is inexpensive the cooling case weakens further, because each kilowatt-hour the efficient unit saves is worth less — which is why Birmingham beats Lexington on cooling payback despite neither being a severe cooling climate.
Common questions
Does a higher SEER2 air conditioner pay for itself?
Usually not on the electricity bill alone in the markets we cover. Moving a 3-ton system from 13.8 to 17 SEER2 saves $124 a year in Birmingham and $62 a year in Lexington, so a $1,200 price difference takes 9.7 years to 19.4 years to repay. In 5 of the 10 markets that is longer than the 15-year life the comparison is quoted over, which means the upgrade is repaid by a rebate or by comfort, not by the meter.
Which pays back faster, a heating upgrade or a cooling upgrade?
Heating, in every market we cover. The heating load in this region is larger than the cooling load, so the same percentage efficiency gain moves more money: a heat pump going from 7.5 to 9.5 HSPF2 repays a $1,800 premium in 7.0 years in Covington, while the cooling upgrade in that same city takes 19.4 years.
Why does cheap electricity make an efficiency upgrade worse?
Because the saving is measured in money, not in kilowatt-hours. An efficient system saves the same energy whatever the tariff, but each saved kilowatt-hour is worth less where power is inexpensive, so the payback stretches. Alabama's higher residential rate is why a cooling upgrade repays sooner in Birmingham than in Lexington despite both being modest cooling climates.
Is the payback the only thing that matters?
No. A variable-speed or two-stage system also runs longer and quieter cycles and pulls more humidity out of the air, which is a comfort argument rather than a financial one. Rebates, financing terms and the age of the rest of the system change the maths too — this calculator prices the energy only.
What size system should I use in the calculator?
Three tons is the common size for an average single-family home in this region, and it is the default here. The correct figure for your house comes from a Manual J load calculation, not from square footage alone, and a contractor should perform one before quoting a replacement.
Does this include the cost of the repair I might avoid?
No. This compares the energy cost of two new systems at different efficiency ratings. It does not model breakdown risk, maintenance, or the repair bill on an ageing system — those belong in the repair-or-replace decision instead.
Where to go next
- AFUE ratings explained — what the furnace number actually measures.
- HSPF2 ratings explained — the heat-pump heating equivalent.
- Single-stage, two-stage and variable-speed — the comfort case this calculator does not price.
- Repair or replace — the decision this one sits inside.
- HVAC installation cost — where the quote difference comes from.
- HVAC financing — how interest changes a long payback.
Sources and vintage
- Degree-days — ERA5 reanalysis via the Open-Meteo archive, 10-year normals, base 65°F.
- Electricity prices — EIA residential average by state, 2025.
- Natural gas prices — EIA residential price by state, from the US Heating Cost Index 2026-07-16.
- Full-load-hour anchor — 1,570 hours at 2,098 CDD, as published on our Birmingham AC page.
- Reference heating home — 600 therms at 4,000 HDD, 29.3 kWh per therm.
Modeled estimates for comparison between markets, not a quote and not a bill. Your house, your usage and your contractor's pricing will differ.