METHODS · U.S. EDITION
How the U.S. calculators work.
Cool Cost Lab turns public sizing guidance and visible product-label values into a shortlist. It does not perform a Manual J load calculation or predict a utility bill.
1. Room size estimate
The starting point is 20 BTU per square foot. We show a ±10% band rather than a single precise-looking answer, then apply the same adjustments published by ENERGY STAR: add 10% for strong sun, 4,000 BTU for a kitchen and 600 BTU for each regular occupant above two. Product suggestions move up to the next sold label size that covers the estimate.
Worked example: 250 sq ft. With two people and no sun or kitchen adjustment, the tool shows 4,500–5,500 BTU/hr. The next common window label is 6,000 BTU. Portable units are shown one label step higher as a conservative comparison convention; that step is our rule, not an ENERGY STAR rule.
2. Running-cost estimate
Monthly cost is watts ÷ 1,000 × hours/day × 30 × electricity price/kWh. The watt values are reference draws for each label class, not measurements of a particular model. For a 520 W, 6,000 BTU window unit used 8 hours daily at 17¢/kWh, the result is about $21.22 per 30-day month. If a product's EnergyGuide label or specification sheet gives a different wattage, use that product document as the better source.
3. Inverter-payback estimate
The comparison uses a 30% energy-saving scenario for an inverter model, then divides the purchase-price gap by annual estimated savings. At a $150 gap, 8 hours/day, 17¢/kWh and a four-month season, the default 6,000 BTU example breaks even in about 5.9 years. The 30% is a scenario, not a promise: compare the two models' annual kWh or CEER values before buying.
Where the method stops
Ceiling height, insulation, air leakage, window area, local design temperature and humidity can outweigh a floor-area shortcut. The calculator stops above its single-room product range. Multi-room, central and unusual-load projects need a qualified HVAC load calculation.
Primary sources
- ENERGY STAR: Room Air Conditioners — capacity chart and sun, kitchen and occupant adjustments.
- U.S. Department of Energy: Purchasing Energy-Efficient Room Air Conditioners — right-sizing, efficiency and humidity-removal context.
- ENERGY STAR: Room Air Conditioner Key Product Criteria — CEER and current efficiency criteria.
Method and links reviewed September 10, 2026. Product reference values change over time.