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How Much Does Your Old AC Really Cost to Run vs. a Modern SEER2 Unit?
A healthy 15-year-old AC was built around SEER 10–13 and has usually degraded from there; today's units start at SEER2 13.4 and high-efficiency runs 16–17. That's real percentage savings — but across Wisconsin's short cooling season it's helpful money, not life-changing money, and it should tip close replacement decisions rather than drive them alone.
The quick answer
The quick answer
A healthy 15-year-old AC was built around SEER 10–13 and has usually degraded from there; today's units start at SEER2 13.4 and high-efficiency runs 16–17. That's real percentage savings — but across Wisconsin's short cooling season it's helpful money, not life-changing money, and it should tip close replacement decisions rather than drive them alone.
Definition
Operating cost = cooling load ÷ efficiency × electric rate. Efficiency is the only lever a new unit changes — which is why honest math starts with how bad (or fine) your current unit actually is.
What is your old AC's real efficiency today?
Start from its era: units installed around 2006–2015 were typically SEER 10–13 by mandate and market. Then subtract for condition — years of coil grime, marginal charge, and worn compressors pull effective efficiency below nameplate; a neglected old unit can run meaningfully worse than its sticker while a maintained one holds close. That spread is why two identical houses replace identical ACs and report different savings. If you want your number rather than folklore: your summer We Energies bills minus your winter baseline isolates cooling kWh — a five-minute exercise that turns this whole comparison from brochure-speak into your actual data.
What does the percentage math say — honestly?
Efficiency scales by ratio. Old effective SEER ~10 to modern SEER2 15.2: roughly a third less cooling energy. To SEER2 17: closer to 40% less. Now the Wisconsin reality check: our cooling season is short — hundreds of meaningful hours, not the Sun Belt's thousands — so 'a third less' applies to one of your smaller annual energy lines. For a typical home spending a few hundred dollars a summer on cooling, high-double-digit to low-hundreds in annual savings is the honest ballpark, scaling with home size and setpoints. Real, pleasant, and worth counting — just not a payback engine on its own.
So when does efficiency actually decide the replacement?
When it's the tiebreaker on a decision already in motion. The unit is 15 and needs a compressor: efficiency savings join the repair-avoidance math and the A2L-era refrigerant costs, and replacement wins clearly. You're replacing the furnace anyway: the combo install ($5,400–$16,000+ in our market) makes the AC's efficiency upgrade nearly free at the margin. You're considering a heat pump: now the same efficient compressor also displaces winter gas — the savings pool triples and the calculator earns its keep. What efficiency alone rarely justifies: scrapping a healthy 8-year-old unit. The greenest and cheapest AC is often the one already installed, maintained properly.
What recovers efficiency without replacing anything?
Your existing unit is probably running below its own potential, and the recovery list is cheap: a genuinely clean outdoor coil (cottonwood season here packs them solid), correct refrigerant charge verified by subcooling — not by 'feels cold' — a clean filter and open returns for airflow, and sealed duct runs through hot attics or garages. That's the honest content of a $169 tune-up, and it's why we're comfortable telling someone their nine-year-old unit just needs service, not a sales pitch. Do the recovery first; measure a summer; then run the replacement math on real numbers.
Why you can trust this guidance
- 5.0★ Google rating across 500+ reviews
- Best of Waukesha 2025
- WI License #1482483 — On Time Heating & Cooling, LLC
- EPA Universal certified · owner-operated in Waukesha since 2019
Written and reviewed by Shane Herro (Owner & Lead Tech). Last updated 2026-07-14. Reviewed at least every 12 months and whenever a material code, program, or pricing change lands.
Common questions
What SEER2 should I buy if I do replace?
In our climate: buy the right size and install quality first, then the tier your comfort priorities justify — SEER2 15–16 is the pragmatic band, higher when variable-speed comfort features are the real motivation. The staging guide covers what each tier feels like.
Do old-unit rebates exist?
Focus on Energy's discounts target the new equipment's tier (heat pumps currently $400–$700) rather than paying for disposal. The federal 25C credit ended for equipment placed in service after 2025 — count only rebates that exist.
How much does a new AC cost installed here?
$8,400–$12,400 in our market — condenser and coil matched to your furnace, permitted and commissioned. Where you land depends on size, tier, and line-set/electrical condition. The cost guides break it down further.
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