Calculator Inputs
For best results, use your own equipment specs and utility rate.
Example Data Table
Sample scenarios to show how inputs change savings.
| Scenario | Rate | Season days | AC tons | SEER | AC hours/day | Replaced % | Fan W | Fan hours/day | Install cost |
|---|---|---|---|---|---|---|---|---|---|
| Balanced | $0.18 | 120 | 3.0 | 14 | 6 | 40 | 350 | 6 | $1,800 |
| High rates | $0.30 | 150 | 3.5 | 15 | 7 | 45 | 420 | 7 | $2,400 |
| Efficient AC | $0.16 | 110 | 3.0 | 20 | 5 | 35 | 300 | 5 | $1,700 |
Formula Used
1) AC input power estimate
Convert cooling capacity to electrical input using SEER.
- Cooling output: BTU/h = tons × 12,000
- AC watts: W = (BTU/h) ÷ SEER
- Adjusted kW: kW = (W ÷ 1000) × load factor
2) Seasonal energy
- Total AC hours: hours/day × season days
- Replaced hours: total hours × replaced %
- Replaced AC kWh: replaced hours × AC kW
- Fan kWh: (fan W ÷ 1000) × fan hours/day × season days
3) Savings and payback
- Primary kWh savings: replaced AC kWh − fan kWh
- Extra kWh savings: seasonal AC kWh × extra %
- Net kWh savings: primary + extra
- Annual savings: net kWh × rate + maintenance delta
- Simple payback (years): install cost ÷ annual savings (only if savings > 0)
- NPV: discounts annual savings over your analysis horizon.
How to Use This Calculator
- Enter your electricity rate and the number of cooling-season days.
- Estimate typical AC runtime per cooling day and how much you can replace.
- Fill in AC capacity and SEER from your equipment label or manual.
- Enter fan wattage and expected nightly runtime for ventilation flushing.
- Add installation cost and any annual maintenance savings or added cost.
- Click Calculate Savings to see energy, payback, and NPV.
- Use Download CSV or Download PDF to save results.
Rate sensitivity and seasonal runtime
Electricity price and cooling-hours drive most of the result. If your rate rises from $0.15 to $0.30 per kWh, the same net kWh savings doubles the dollar savings. Seasonal hours matter similarly: 90 cooling days at 5 hours/day produces 450 AC hours, while 150 days at 7 hours/day produces 1,050 hours—more than twice the baseline energy exposure. For budgeting, multiply seasonal kWh savings by your marginal tiered rate, not only the average.
Estimating AC input power with SEER
The calculator converts cooling capacity to electrical input using SEER. A 3‑ton system delivers 36,000 BTU/h. At SEER 14, estimated input is about 2.57 kW before part‑load adjustment (36,000 ÷ 14 ÷ 1000). Applying a 0.65 load factor yields roughly 1.67 kW average draw during runtime, which anchors the seasonal kWh estimate. Higher SEER lowers kW, shrinking avoided kWh for each hour.
Replacement share versus fan energy
Savings occur when avoided AC kWh exceeds fan kWh. Replacing 40% of 720 seasonal AC hours avoids 288 hours. With 1.67 kW average draw, that is about 481 kWh avoided. If the fan runs 6 hours/day for 120 days at 350 W, fan use is about 252 kWh, leaving a positive primary savings window before any extra heat‑gain benefit. If you add 5% extra savings, improves by 36 kWh.
Payback, NPV, and planning horizon
Simple payback divides installation cost by annual savings, but NPV adds time value. With $250/year savings, 3% electricity growth, and a 6% discount rate, earlier years weigh more heavily than later ones. Extending the horizon from 5 to 10 years can materially change NPV, especially when savings grow faster than discounting. NPV clarifies long-term value better.
Quality checks for realistic inputs
Use measured fan wattage, not marketing airflow claims. Keep replaced-hours conservative if nights are humid or outdoor air is warm. If net kWh savings turns negative, reduce fan runtime or replacement share until the model reflects your comfort strategy. The chart helps validate whether cumulative savings crosses zero within your expected equipment life. Target quiet operation and sealing.
FAQs
1) What does “AC load factor” mean?
It approximates average draw during cycling and part‑load operation. A value below 1.0 reduces estimated kW and seasonal kWh. If you have measured usage, adjust load factor so the model matches your bill.
2) How do I estimate the “replaced %” realistically?
Start conservative: 20–35% if nights are only mildly cooler, 35–55% if nights are consistently cool and you can flush heat effectively. Use your thermostat history to estimate hours you can avoid.
3) Why can net kWh savings be negative?
If the fan runs long hours or draws high watts, it can consume more energy than the avoided AC use. Reduce fan runtime, improve whole‑house airflow path, or lower replaced % until the primary savings becomes positive.
4) Should I use my utility’s average or marginal rate?
If you have tiered pricing, marginal rate better reflects the cost of the last kWh you avoid. If pricing is flat, average works well. When unsure, run both to bracket expected savings.
5) What CO2 factor should I enter?
Use a value that matches your grid or supplier if you have one. If not, leave the default as a rough estimate. The CO2 result is optional and does not affect the financial outputs.
6) What’s the difference between payback and NPV?
Payback shows how long it takes for annual savings to equal installation cost. NPV discounts future savings and can include price growth, so it better compares long‑term value across different assumptions.