| Project cost | Incentives | Heating cost | Cooling cost | Heat save % | Cool save % | Years | Discount % | Energy change % |
|---|---|---|---|---|---|---|---|---|
| $8,500 | $500 | $900 | $700 | 12% | 8% | 20 | 6% | 3% |
- Net upfront cost:
netCost = projectCost − incentives - Year 1 energy savings:
heatCost × heatSave% + coolCost × coolSave% - Energy savings growth:
yearSavings = year1Savings × (1 + energyChange%)^(year−1) - Total annual benefit:
energySavings + otherSavings(plus resale value in the final year) - Discounted savings:
discounted = total / (1 + discountRate%)^year - NPV:
NPV = −netCost + Σ(discountedSavings) - ROI:
((totalBenefits − netCost) / netCost) × 100 - Payback: first year where cumulative benefits exceed net cost.
- Enter the installed project cost from your contractor quote.
- Add incentives, rebates, or credits you expect to receive.
- Use your last year’s heating and cooling costs from bills.
- Set savings percentages based on your climate and window specs.
- Choose an energy price change and a discount rate you believe.
- Pick an analysis period that matches your planned ownership.
- Press Calculate ROI to view payback, ROI, NPV, and a yearly schedule.
- Use CSV or PDF to save results for budgeting decisions.
Energy savings assumptions and baselines
Triple pane upgrades reduce heat transfer through glazing and frames. Use your last year’s heating and cooling spend as the baseline. If you spent $1,600 total, a 12% heating cut and 8% cooling cut yields about $164 in year‑one savings. The goal is a realistic starting point you can refine with your own utility history. In colder zones, heating savings often dominate the result.
Project cost, incentives, and net investment
Returns should be based on net upfront cost. Subtract rebates, utility incentives, or credits from the quote to estimate what you truly pay or finance. An $8,500 project with $500 incentives becomes an $8,000 investment. Because payback hinges on that first outflow, incentives can shift payback noticeably in mild climates.
Annual cashflows and energy price changes
Energy savings may grow if prices rise. The energy price change input scales savings each year to reflect utility trends. At 3% per year, $164 becomes about $220 by year ten. Run three scenarios—0%, 3%, and 6%—to see how sensitive total savings and payback are to price uncertainty. Inflation lifts later years but may not speed early payback.
Discounted value, NPV, and IRR interpretation
Discounting converts future benefits into today’s dollars using your discount rate. A higher rate values near‑term savings more, which is why NPV can differ from simple profit. Positive NPV means the upgrade exceeds your required return. IRR summarizes the annualized return implied by the cashflow stream, useful for comparing alternative uses of cash. It also helps compare upgrades with loans.
Reading payback, ROI, and decision boundaries
Simple payback is the first year cumulative benefits exceed net cost. ROI compares total benefits to net cost over the chosen period, including any resale value in the final year. If payback exceeds your expected ownership horizon, weigh comfort, condensation control, and noise reduction more heavily. The chart highlights when cumulative net crosses zero. Use discounted savings to judge long‑run value.
1) What savings percentages should I start with?
If you lack audit data, start near 10% heating and 8% cooling, then test a low and high case. Climate, leakage, and installation quality can move results significantly.
2) Why does NPV matter if ROI looks good?
ROI totals benefits over time, while NPV discounts future savings back to today. NPV helps compare upgrades with different timing, and it reflects your opportunity cost through the discount rate.
3) What does the discount rate represent?
It is your required return or alternative yield for the same money. Higher discount rates reduce the value of distant savings and can extend the “economic” payback even if bills decline.
4) Should I include comfort or noise benefits?
Yes, but keep them conservative. You can add a yearly “other savings” value to reflect avoided maintenance, fewer drafts, or improved comfort. If uncertain, set it to zero and review sensitivity.
5) How is payback calculated here?
Payback is the first year cumulative benefits exceed net upfront cost. The calculator also estimates months within that year using simple interpolation, which is useful for quick planning.
6) What if I move before the analysis period ends?
Shorten the analysis years to your expected ownership and add a resale value estimate for that final year. This keeps the cashflow aligned with how long you personally capture the savings.