Inputs
Room & Conditions
ft²
Use units toggle to switch to m².
ft
°F
°F
Auto-fills from climate; editable.
% wall
persons
EER
A
$/kWh
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Assumptions
This tool applies a transparent, rules-based approximation (not a Manual‑J). Coefficients are adjustable in the script config. If your result straddles two sizes, prefer higher EER over simply higher BTU.
Show multipliers & formulas
- Baseline =
area × base_btu_per_ft² × (ΔT / 20°F) × (ceiling / 8ft) - Solar factor ≈ orientation × glazing × shading × (window% / 20) (capped at +30%)
- Infiltration sensible ≈
1.1 × CFM × ΔT, whereCFM = (ACH × Volume)/60 - People sensible: low 230, med 380, high 600 BTU/h per person
- Appliances sensible:
BTU/h = Watts × 3.412 - Latent (optional): none / 10% / 15% of sensible + people (200/300 BTU/h per person); pints/day ≈
latent × 0.024 - Portable penalty: single-hose +20%, dual-hose +5% capacity requirement
- Energy:
Watts = BTU ÷ EER,Amps = Watts ÷ Voltage
Recommended Capacity
— BTU/h
Good / Better / Best bins shown below. Includes solar, infiltration, internal, and (optional) latent loads.
Electrical & Cost
Watts
—
Amps
—
$/hr
—
Breakdown
Sizing Bins
| Bin | Capacity | Est. Watts | Notes |
|---|
Typical noise: Window 52–58 dB • Portable 55–65 dB (varies by model).
Fit & Window Notes
- Ensure the window opening and sill can support the chosen chassis. Casement/slider windows may require special kits.
- For single‑hose portables, minimize vent length and seal gaps to reduce capacity losses.
- If your space is open to adjacent rooms, consider a second unit rather than oversizing one.
FAQ
Window vs Portable — how much capacity do I lose?
Single‑hose portables can lose ~20% effective capacity due to depressurization; dual‑hose designs reduce that to ~5%. This calculator boosts recommended BTU accordingly.
Ceiling 10 ft — how should I adjust?
Capacity scales roughly with volume. We multiply by ceiling / 8 ft vs the baseline, so taller rooms need proportionally more BTU.
What if I’m between two sizes?
Prefer the higher EER/CEER at the lower BTU if comfort allows. If your space sees big solar swings or parties, stepping up one bin can help avoid long runtimes.