How central vs window AC cost is worked out
Central air and window units are priced on the same cooling load, so efficiency is the only energy difference. The annual energy is the cooling load times the cooling hours divided by the efficiency times 1,000, in kilowatt-hours, and the energy cost is the kilowatt-hours times the price a kWh. The total is the install cost plus the years times the energy cost plus the maintenance, plus the replacements: central is replaced at the end of its life, window units are all replaced at the end of theirs. The winner is the smaller total.
At the defaults — 36,000 BTU/h, 1,200 hours, $0.17 a kWh, horizon 10 years, central install $6,000, SEER 16, life 15 years, maintenance $150 a year, window units 3 at $300, EER 12, life 8 years, maintenance $0 — central uses 36,000 × 1,200 ÷ (16 × 1,000) = 2,700.00 kWh a year, which costs 2,700.00 × $0.17 = $459.00, and needs no replacement in 10 years, for a total of $6,000 + 10 × ($459.00 + $150) = $12,090.00. Window units use 36,000 × 1,200 ÷ (12 × 1,000) = 3,600.00 kWh a year, which costs 3,600.00 × $0.17 = $612.00, and need one replacement of all 3 units at year 8, which is $900.00, for a total of $900 + 10 × $612.00 + $900 = $7,920.00. Window wins by $12,090.00 − $7,920.00 = $4,170.00. A year costs $1,209.00 for central against $792.00 for window units.
| Case | Central total | Window total | Winner |
|---|
| Default (36,000 BTU/h, 10 years) | $12,090.00 | $7,920.00 | Window by $4,170.00 |
| 6 window units | $12,090.00 | $9,720.00 | Window by $2,370.00 |
| Central install $3,500 | $9,590.00 | $7,920.00 | Window by $1,670.00 |
| $0.30/kWh | $15,600.00 | $12,600.00 | Window by $3,000.00 |
| 2,000 h season | $15,150.00 | $12,000.00 | Window by $3,150.00 |
| Flip: $3,500, 2,000 h, $0.30/kWh | $18,500.00 | $19,800.00 | Central by $1,300.00 |
| 25-year horizon | $27,225.00 | $18,900.00 | Window by $8,325.00 |
The worked example, line by line
Setup: 36,000 BTU/h, 1,200 hours, $0.17 a kWh, 10 years, central $6,000 install, SEER 16, life 15 years, maintenance $150 a year, window 3 units at $300, EER 12, life 8 years, maintenance $0.
Line 1 — the central energy. 36,000 × 1,200 ÷ (16 × 1,000) = 2,700.00 kWh a year, and 2,700.00 × $0.17 = $459.00 a year.
Line 2 — the window energy. 36,000 × 1,200 ÷ (12 × 1,000) = 3,600.00 kWh a year, and 3,600.00 × $0.17 = $612.00 a year.
Line 3 — the central total. $6,000 + 10 × ($459.00 + $150) = $6,000 + $6,090.00 = $12,090.00, with no replacement in 10 years.
Line 4 — the window total. $900 + 10 × $612.00 + $900 replacement at year 8 = $900 + $6,120.00 + $900 = $7,920.00.
Line 5 — the winner. $12,090.00 − $7,920.00 = $4,170.00 to the window units. A year costs $1,209.00 for central against $792.00 for window units.
Every branch, checked
6 window units: the window install is 6 × $300 = $1,800.00, the replacement at year 8 is $1,800.00, and the energy is still $612.00 a year, because the load is the same, for a total of $1,800.00 + $6,120.00 + $1,800.00 = $9,720.00, while central stays $12,090.00. Window wins by $2,370.00. More units raise the window cost, because every unit is bought twice in 10 years, but the energy does not change, because the load does not change.
Central install $3,500 (existing ducts): central is $3,500 + 10 × ($459.00 + $150) = $3,500 + $6,090.00 = $9,590.00, while window stays $7,920.00. Window wins by $1,670.00. Cheap install narrows the gap a lot, because the install is most of what central costs, but it does not close it: the $150 a year of maintenance alone is $1,500.00 over 10 years.
$0.30 a kWh: central energy is 2,700.00 × $0.30 = $810.00 a year, for a total of $6,000 + 10 × ($810.00 + $150) = $15,600.00, and window energy is 3,600.00 × $0.30 = $1,080.00 a year, for a total of $900 + $10,800.00 + $900 = $12,600.00. Window wins by $3,000.00. Dear electricity helps central, because it burns 900 kWh less a year, but the saving is $270.00 a year, and 10 years of it is $2,700.00, which is less than the $5,100.00 install gap.
2,000-hour season: central energy is 36,000 × 2,000 ÷ (16 × 1,000) = 4,500.00 kWh a year, which costs $765.00, for a total of $6,000 + 10 × ($765.00 + $150) = $15,150.00, and window energy is 6,000.00 kWh a year, which costs $1,020.00, for a total of $900 + $10,200.00 + $900 = $12,000.00. Window wins by $3,150.00. A long season helps central the same way dear electricity does, and it is still not enough at these install prices.
Flip — central install $3,500, 2,000 hours and $0.30 a kWh: central energy is 4,500.00 × $0.30 = $1,350.00 a year, for a total of $3,500 + 10 × ($1,350.00 + $150) = $18,500.00, and window energy is 6,000.00 × $0.30 = $1,800.00 a year, for a total of $900 + $18,000.00 + $900 = $19,800.00. Central wins by $1,300.00. All three together flip it: cheap install, a long hot season and dear electricity, each worth a few thousand, add up to more than the window’s lead.
25-year horizon: central is $6,000 + 25 × ($459.00 + $150) + one $6,000.00 replacement at year 15 = $27,225.00, and window is $900 + 25 × $612.00 + $2,700.00 of replacements at years 8, 16 and 24 = $18,900.00. Window wins by $8,325.00. A longer horizon grows the window’s lead, because central’s $150 a year of maintenance and its $6,000.00 replacement cost more than the window’s $900.00 replacements. A year costs $1,089.00/yr for central against $756.00/yr for window units over 25 years.
Reverse — the break-evens: central breaks even on install at $1,830.00, because the install plus the $6,090.00 of 10-year running costs must equal the window total of $7,920.00, so $7,920.00 − $6,090.00 = $1,830.00. Electricity breaks even at about $0.63 a kWh (exactly $0.6333 a kWh): solve $7,500.00 + 27,000 × price = $1,800.00 + 36,000 × price, which gives price = $5,700.00 ÷ 9,000 = $0.6333 a kWh. Window units break even at 9.95 units: at 10 units the window total is $12,120.00, which is $30.00 more than central’s $12,090.00, so 9 units still win and 10 lose.
What the defaults assume
The model assumes both sides cool the same 36,000 BTU/h load for the same 1,200 hours, so efficiency is the only energy difference, and it assumes the electricity price stays flat for the whole horizon, with no price rises. It assumes SEER 16 and EER 12 are comparable annual efficiencies, which is a simplification: SEER is a seasonal rating and EER is a steady-state rating, so a real system may run a little better or worse than its rating. It assumes replacements cost the same as the original install, with no labour inflation, and it leaves out the ductwork, the aesthetics, the noise, the heating, and the effect on the home’s value, which are real but not priced here. Rounding convention: money is rounded to cents, energy to 2 decimals, and all figures are computed from unrounded values, rounded only for display.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Central energy | load × hours ÷ (SEER × 1000) | 2,700.00 kWh a year, costing $459.00. |
| Window energy | load × hours ÷ (EER × 1000) | 3,600.00 kWh a year, costing $612.00. |
| Central total | install + years × (energy + maintenance) + replacements | $12,090.00; replacements $0.00. |
| Window total | units × price + years × energy + replacements | $7,920.00; replacements $900.00. |
| Winner | smaller total | Window wins by $4,170.00. |
| Central per year | central total ÷ years | $1,209.00/yr. |
| Window per year | window total ÷ years | $792.00/yr. |
| 6 window units | same model, 6 units | Central $12,090.00 vs window $9,720.00, Window wins by $2,370.00. |
| Central install $3,500 | same model, $3,500 install | Central $9,590.00 vs window $7,920.00, Window wins by $1,670.00. |
| $0.30/kWh | same model, $0.30 | Central $15,600.00 vs window $12,600.00, Window wins by $3,000.00. |
| 2,000 h season | same model, 2,000 h | Central $15,150.00 vs window $12,000.00, Window wins by $3,150.00. |
| Flip: $3,500, 2,000 h, $0.30/kWh | same model, all three | Central $18,500.00 vs window $19,800.00, Central wins by $1,300.00. |
| 25-year horizon | same model, 25 years | Central vs window , . |
| Reverse: break-even central install | window total − central running costs | $1,830.00. |
| Reverse: break-even electricity price | tie point of the two totals | $0.63/kWh (exactly $0.6333/kWh). |
| Reverse: max window units | (central total − window energy) ÷ cost per unit | 9.95 units; at 10 units window costs $12,120.00, $30.00 more than central. |
Who uses this
- The homeowner replacing a dead central system: they learn window units cost $7,920.00 over 10 years against $12,090.00 for central — so they know the window route saves $4,170.00, and they check the $3,500-install case at $9,590.00 before they decide.
- The renter cooling a small flat: they learn 3 window units cost $792.00 a year against $1,209.00 for central — so they know the window units are cheaper every year, and they check the 6-unit case at $9,720.00 before they add a unit.
- The homeowner in a hot climate with dear electricity: they learn central wins by $1,300.00 when install is $3,500, the season is 2,000 hours and electricity is $0.30 a kWh — so they know the flip needs all three at once, and they check the $0.63 a kWh break-even before they pay for ducts.
- The landlord pricing a whole building: they learn 25 years of cooling costs $27,225.00 for central against $18,900.00 for window units — so they know the window route saves $8,325.00 a home, and they check the 10-unit limit before they add units to a big flat.
Accuracy & limitations
The math is exact for the numbers you enter: the energy is the load times the hours divided by the efficiency times 1,000, the energy cost is the kilowatt-hours times the price a kWh, the total is the install plus the years times the energy plus the maintenance plus the replacements, and the winner is the smaller total. What the model cannot do is read your electricity bill for you: check the price a kWh on the bill, because a $0.17 bill and a $0.30 bill give very different answers. It assumes the same load and the same hours for both sides, a flat electricity price, and replacements at the original price, and it treats SEER and EER as comparable annual efficiencies, which is a simplification. It leaves out the ductwork, the noise, the look of the units, the heating, and the effect on the home’s value. For the heat pump, see heat pump vs furnace calculator; for the oven that heats the kitchen, see gas vs electric oven calculator.
Cite this calculatorBright Side Kit. “Central AC vs Window AC: 10-Year Cost Calculator 2026.” brightoolbox.com, 2026. Method: Annual energy kWh = load × hours ÷ (SEER or EER × 1000); energy cost = kWh × $/kWh; central total = install + years × (energy + maintenance) + replacements at years 15, 30, ...; window total = units × unit price + years × energy + replacements of all units at years 8, 16, 24, ...; winner = lower total. Defaults: load 36,000 BTU/h, 1,200 h, $0.17/kWh, 10 yr, central install $6,000, SEER 16, life 15 yr, maintenance $150/yr, window 3 units at $300, EER 12, life 8 yr, maintenance $0/yr → central 2,700.00 kWh, $459.00/yr, total $12,090.00; window 3,600.00 kWh, $612.00/yr, total $7,920.00; window wins by $4,170.00; per year central $1,209.00 vs window $792.00; 6 window units → window $9,720.00, window by $2,370.00; central install $3,500 → central $9,590.00, window by $1,670.00; $0.30/kWh → central $15,600.00 vs window $12,600.00, window by $3,000.00; 2,000 h season → central $15,150.00 vs window $12,000.00, window by $3,150.00; flip ($3,500, 2,000 h, $0.30/kWh) → central $18,500.00 vs window $19,800.00, central by $1,300.00; 25-yr horizon → central $27,225.00 vs window $18,900.00, window by $8,325.00; reverses: break-even central install $1,830.00; break-even electricity price $0.6333/kWh (about $0.63/kWh); max window units 9.95, at 10 units window $12,120.00, $30.00 more than central. Bright Side Kit. "Central AC vs Window AC: 10-Year Cost Calculator 2026." https://brightoolbox.com/central-ac-vs-window-ac/ Accessed October 25, 2026.
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