Ceiling Fan Thermostat Offset Calculator
What this calculator estimates
A ceiling fan doesn’t lower the room’s air temperature the way an air conditioner does. Instead, it increases perceived comfort for people in the airflow. Many households use that comfort boost to raise the thermostat setpoint (for example, from 74°F to 76°F) while still feeling similar comfort. When the setpoint is higher, the air conditioner typically runs less, which can reduce cooling electricity use.
This calculator estimates your net savings by combining two effects:
- Reduced A/C energy from raising the thermostat by a chosen amount (°F).
- Added fan energy from running the ceiling fan for a chosen number of hours.
The result is a simple, practical estimate of daily dollar savings. You can then multiply by the number of cooling days to approximate monthly or seasonal impact.
Inputs (and what to enter)
- AC Power Draw (kW): Average electrical power while the A/C is running. If you only know amps and volts, kW ≈ (Volts × Amps × Power Factor) / 1000 (for many residential systems you may approximate PF near 1, but it varies).
- AC Runtime per Day (hours): Total compressor/on-time per day. If you’re unsure, use a typical hot-day estimate (e.g., 4–12 hours) and test a range.
- Thermostat Increase with Fan (°F): How many degrees higher you can set the thermostat while maintaining acceptable comfort with the fan running.
- Fan Power Draw (W): Fan wattage while operating at the chosen speed. Many ceiling fans are roughly 15–80 W depending on size, motor type, and speed.
- Fan Runtime per Day (hours): Hours the fan will run. For savings, this should generally align with occupancy; running fans in empty rooms adds cost without comfort benefit.
- Electricity Cost ($/kWh): Your all-in energy price (including delivery and taxes if you want closer-to-bill estimates).
Formulas used
The model starts with baseline daily A/C energy use:
Baseline A/C energy (kWh/day) = AC Power (kW) × AC Runtime (hours/day)
It then estimates the fractional cooling reduction from raising the setpoint. A commonly cited rule of thumb is about 3% cooling energy savings per 1°F increase (actual performance varies widely; see limitations below). That gives:
Estimated A/C energy saved (kWh/day) = Baseline A/C energy × (0.03 × Offset °F)
Fan energy is added back in:
Fan energy (kWh/day) = Fan Power (W) × Fan Runtime (hours/day) ÷ 1000
Net energy saved is:
Net kWh/day = (A/C kWh saved) − (Fan kWh used)
And net dollar savings is:
Net $/day = Net kWh/day × Electricity Rate ($/kWh)
Where S is net savings ($/day), r is electricity rate ($/kWh), Pac is A/C power (kW), tac is A/C runtime (h/day), d is thermostat increase (°F), and Pf, tf are fan power (W) and runtime (h/day).
Worked example (using the default values)
Inputs: A/C power = 3.5 kW, A/C runtime = 8 h/day, offset = 2°F, fan power = 50 W, fan runtime = 8 h/day, electricity rate = $0.15/kWh.
- Baseline A/C energy: 3.5 × 8 = 28 kWh/day
- Estimated A/C energy saved: 28 × (0.03 × 2) = 28 × 0.06 = 1.68 kWh/day
- Fan energy: 50 × 8 ÷ 1000 = 0.40 kWh/day
- Net energy saved: 1.68 − 0.40 = 1.28 kWh/day
- Net savings: 1.28 × $0.15 = $0.192/day (about $0.19/day)
If you experience similar conditions for 90 cooling days, a rough seasonal estimate would be 90 × $0.19 ≈ $17. If your A/C runs more than 8 hours/day during peak season, savings can scale up quickly.
Interpreting your results
- Positive net savings means the estimated reduction in A/C energy is larger than the fan’s added energy use.
- Near zero suggests either (a) the thermostat offset is small, (b) the fan runs many hours at higher wattage, or (c) baseline A/C use is already low.
- Negative net savings can happen if the fan runs long hours (especially at high speed) while the thermostat offset is minimal or the A/C isn’t running much. In that case, the fan may still improve comfort, but it may not reduce electricity cost.
To convert daily savings into longer timeframes:
- Monthly estimate: multiply by days in the month you typically use cooling.
- Seasonal estimate: multiply by number of cooling days (or sum over months).
Comparison table: how the thermostat offset changes savings
The table below uses the same default values except for the thermostat increase. It illustrates how larger offsets generally produce larger savings (until practical comfort limits are reached).
| Thermostat increase (°F) | A/C energy saved (kWh/day) | Fan energy used (kWh/day) | Net savings ($/day) |
|---|---|---|---|
| 1 | 0.84 | 0.40 | $0.07 |
| 2 | 1.68 | 0.40 | $0.19 |
| 3 | 2.52 | 0.40 | $0.32 |
| 4 | 3.36 | 0.40 | $0.44 |
Assumptions & limitations (important)
- “3% per °F” is a rule of thumb, not a guarantee. Actual savings depend on outdoor temperature, humidity, solar gains, building insulation/air leakage, duct losses, thermostat programming, internal loads, and A/C efficiency/controls (single-stage vs variable-speed).
- Fans cool people, not rooms. The best savings happen when fans are used only when rooms are occupied. Leaving a fan running in an empty room usually increases energy use.
- Runtime and power are averages. A/C power draw can vary with cycling, outdoor conditions, and equipment type. Fan wattage changes by speed and motor type (AC vs DC motor fans can be significantly lower wattage at comparable airflow).
- Comfort limits vary. Some people can raise the setpoint 1–4°F with a fan; others may find only 0–1°F acceptable, especially in high humidity or low air movement areas.
- Does not model humidity separately. In humid climates, raising the setpoint can sometimes reduce dehumidification and comfort even with airflow. Conversely, some systems with dedicated dehumidification may behave differently.
- Not a full HVAC simulation. This tool is intended for quick budgeting and comparison, not equipment sizing, code compliance, or guaranteed utility-bill predictions.