Aircon Calculator — Methodology
Every formula and assumption the aircon ROI calculator uses, in the order it fires.
In one paragraph
For each room, we compute the cooling energy delivered per year from your capacity × hours × days × load factor. We divide that by each option's effective COP (or IEER for 3-phase) to get electricity drawn from the wall. Multiply by tariff (with annual escalation and GST) to get yearly cost. The savings between Option A and Option B, accumulated year-by-year, hit the upfront cost difference at the break-even year.
Inputs
| Variable | Symbol | Default |
|---|---|---|
| Per-room cooling capacity (kW) | capi | BR 2.6 / Master 3.5 / Living 5.0 |
| Per-room hours per day | hi | BR 8 / Master 9 / Living 4 |
| Per-room days per year | di | 350 |
| Inverter load factor | α | 0.65 |
| Diversity factor (system-wide) | δ | 0.7 |
| Multi-split COP penalty | π | 10% |
| Existing aircon COP degradation per year | θ | 1%/yr |
| Option's nominal Weighted COP / IEER | COP | (per option) |
| Option's age (years) | a | (per option) |
| Electricity tariff (SGD/kWh, before GST) | P | 0.2727 |
| Annual tariff escalation | r | 0.03 |
| GST | g | 0.09 |
| Lifespan horizon (years) | L | 10 |
| Per-option upfront: price + install − rebate − salvage | UA, UB | (per option) |
1Cooling demand per year
Sum of per-room capacity × hours × days, scaled by load factor and diversity. Load factor captures the fact that an inverter compressor rarely runs at 100% of nameplate; diversity captures the fact that not all rooms run simultaneously.
Q = α × δ × Σi (capi × hi × di) [kWh-cooling / year]
Default 3-system (Living + Master + BR), 350 days, α=0.65, δ=0.7: Q = 0.65 × 0.7 × (5.0×4 + 3.5×9 + 2.6×8) × 350 ≈ 11,790 kWh-cooling/yr.
2Effective COP per option
Start from the option's nominal Weighted COP (1-phase) or IEER (3-phase), then apply two adjustments:
COPeff = COPnominal × (1 − π if multi/vrf else 1) × (1 − θ)a
- The multi-split penalty π reflects matched-system COP loss vs the single-split rating.
- The degradation θa term applies when an existing aircon has age > 0.
- For 3-phase VRV, IEER is the part-load-weighted efficiency — we treat it identically to Weighted COP for energy estimation.
3Annual electricity drawn
E = Q ÷ COPeff [kWh-electricity / year]
Done independently for Option A and Option B. EA, EB.
4Year-N tariffs & annual cost
P(N) = P × (1 + r)N−1 × (1 + g) CostA(N) = EA × P(N) CostB(N) = EB × P(N) Savings(N) = CostA(N) − CostB(N)
5Net upfront, cumulative savings, break-even
UA = priceA + installA − rebateA − salvageA UB = priceB + installB − rebateB − salvageB Net = UB − UA [extra you pay to switch to B] Cum(N) = Σk=1..N Savings(k) Net pos(N) = Cum(N) − Net Break-even year = first N where Cum(N) ≥ Net
We refine the break-even with linear interpolation inside that year so the headline reads as a fractional year (e.g. "4.2 yrs"). Special results: Net ≤ 0 → "already paid back"; Savings(1) ≤ 0 → "no payback"; doesn't break even within the lifespan horizon → "> L yrs".
Notes on the NEA dataset
- The Weighted COP column blends full-load and part-load efficiency — it's a more realistic real-world figure than full-load COP alone.
- The Annual Energy Consumption (kWh) column NEA publishes assumes a fixed standard test usage profile, not yours. We ignore it and compute energy directly from your capacity, hours, and COP.
- 3-phase entries report IEER (Integrated Energy Efficiency Ratio) instead. IEER weights performance at 25/50/75/100% load — structurally the same role as Weighted COP for a yearly energy estimate.
- The tick band 1, 2, 4, 5 maps directly to NEA's current scheme (3 ticks is unused under the active labelling rules).
Assumptions & Limitations
- Constant load factor. α=0.65 is a single number for the whole year; real cycling depends on outdoor heat load (peak afternoon >> nighttime).
- Diversity is system-wide. δ multiplies the whole sum — it doesn't model "kids' bedroom always on, study sometimes" non-uniform diversity.
- Constant COP. COP rated at 27°C indoor / 35°C outdoor; SG runs hotter at peak so real COP can drop. We don't model thermal lift derating.
- Tariffs compound annually at a constant rate; real tariffs revise quarterly.
- No ambient-temperature seasonality. SG is roughly constant year-round so this is acceptable; not transferable to non-tropical climates without changes.
- Servicing cost not included. Both options are assumed to incur similar annual maintenance; if a 5★ inverter has higher servicing cost, fold it into "install" up-front.
- 30-day months / fractional break-even via linear interpolation. Good to ~1 month of error.