Compares indoor and outdoor air by total heat content (temperature and moisture), then prices the difference with your own equipment.
Inside
°F
%
Outside
°F
%
Weather apps usually report dew point; enter whichever you have.
Equipment, house, and pricing
Seasonal Energy Efficiency Ratio: cooling delivered per unit of electricity, in Btu per watt-hour. Higher is better. Printed on the yellow EnergyGuide sticker on the outdoor unit; if the sticker is gone, search the model number on the outdoor unit's data plate. Systems from the 1990s are typically 10–12, 2006–2022 installs 13–16, and anything sold since 2023 is at least 14–15 (SEER2 ratings are ~5% lower than SEER; either is fine here).
Yellow EnergyGuide sticker or model-number lookup. Old units 10–12, recent 14–16. More
Integrated Energy Factor: liters of water removed per kilowatt-hour of electricity. Higher is better. Listed on the ENERGY STAR label, the spec sheet, or the retailer's product page. If you only know pints/day and watts: IEF ≈ pints × 0.473 ÷ (watts × 24 ÷ 1000). Federal minimum is 1.30 (≤25 pints) to 1.60 (25–50 pints); ENERGY STAR units are about 1.6–2.0.
Spec sheet or ENERGY STAR label. Typical 1.3–2.0. From pints/day and watts: pints × 0.473 ÷ (W × 0.024). Lookup
Use your all-in rate: total bill divided by kWh used, so fuel charges, fees, and taxes are included. The 'energy charge' line alone understates it.
Total bill ÷ kWh used on the bill.
Sets barometric pressure. Small effect.
Conditioned floor area times average ceiling height. Only count the space the open windows actually flush; if you open one bedroom, use that room's volume. Volume scales the dollar figures but does not change the verdict.
Floor area × ceiling height, for the space you are ventilating.
How many times per hour the outdoor air replaces the room's air. Hard to know precisely; use a bracket. Windows cracked, still air: about 0.5–1. Several windows open, light breeze: 1–3. Cross-breeze through opposite windows: 3–6. Window fan or whole-house fan: fan CFM × 60 ÷ volume in ft³ (a 20-inch box fan is roughly 1,000–2,000 CFM). ACH scales the dollar figures but does not change the verdict.
Estimate: cracked 0.5–1, open 1–3, cross-breeze 3–6, fan CFM×60÷ft³.
Scales the dollar figures.
How close outdoor and indoor total heat content (enthalpy) must be before the verdict says 'marginal' instead of committing. Consumer humidity sensors are typically off by 3–5% RH, which is roughly 1–2 kJ/kg at room conditions, so 1.5 is a reasonable default. Set to 0 to always get a firm answer.
Sensor slop. 1.5 ≈ a ±4% RH sensor error.
Where this outdoor air sits
Windows winWindows loseMarginal / trade-offNot physically possible (dew point above temperature)
Air properties
Inside
Outside
Outside − inside
Energy for this session
Method: saturation vapor pressure from the Buck (1981) equation; humidity ratio W = 0.62198·Pw/(P−Pw); moist-air enthalpy h = 1.006·T + W·(2501 + 1.86·T) kJ/kg; latent heat ≈ 2465 kJ/kg. In AC mode, the total load through the coil is m·Δh and electricity is load ÷ COP. In dehumidifier mode, imported moisture costs (liters ÷ IEF) kWh, and all of that electricity plus the latent heat of the condensed water is released into the room as heat. Everything here is a steady-state estimate that ignores thermal mass, fan energy, and infiltration you'd have anyway.