Double Glazing Payback
Calculator
Results
- Break-even (years)
- 12.322624 yr
- Net position at the horizon
- $11,906.76
- Total savings
- $19,906.76
- First-year saving
- $546.00
- Energy saved per year (kWh)
- 5,459.999999 kwh
Cumulative cash flow crossing zero at break-even
- Cumulative cash flow
- Cumulative cost of doing nothing
- Cumulative cost with the upgrade
Year-by-year cash flow until break-even
| 1 | 546.00 | 546.00 | -7,454.00 | 748.80 | 8,202.80 |
| 2 | 562.38 | 1,108.38 | -6,891.62 | 1,520.06 | 8,411.68 |
| 3 | 579.25 | 1,687.63 | -6,312.37 | 2,314.47 | 8,626.83 |
| 4 | 596.63 | 2,284.26 | -5,715.74 | 3,132.70 | 8,848.44 |
| 5 | 614.53 | 2,898.79 | -5,101.21 | 3,975.48 | 9,076.69 |
| 6 | 632.96 | 3,531.75 | -4,468.25 | 4,843.55 | 9,311.79 |
| 7 | 651.95 | 4,183.70 | -3,816.30 | 5,737.65 | 9,553.95 |
| 8 | 671.51 | 4,855.22 | -3,144.78 | 6,658.58 | 9,803.37 |
| 9 | 691.66 | 5,546.87 | -2,453.13 | 7,607.14 | 10,060.27 |
| 10 | 712.41 | 6,259.28 | -1,740.72 | 8,584.15 | 10,324.87 |
| 11 | 733.78 | 6,993.06 | -1,006.94 | 9,590.48 | 10,597.42 |
| 12 | 755.79 | 7,748.85 | -251.15 | 10,626.99 | 10,878.14 |
| 13 | 778.47 | 8,527.31 | 527.31 | 11,694.60 | 11,167.29 |
| 14 | 801.82 | 9,329.13 | 1,329.13 | 12,794.24 | 11,465.11 |
| 15 | 825.87 | 10,155.01 | 2,155.01 | 13,926.87 | 11,771.86 |
| 16 | 850.65 | 11,005.66 | 3,005.66 | 15,093.47 | 12,087.82 |
| 17 | 876.17 | 11,881.83 | 3,881.83 | 16,295.08 | 12,413.25 |
| 18 | 902.45 | 12,784.28 | 4,784.28 | 17,532.73 | 12,748.45 |
| 19 | 929.53 | 13,713.81 | 5,713.81 | 18,807.51 | 13,093.70 |
| 20 | 957.41 | 14,671.22 | 6,671.22 | 20,120.54 | 13,449.31 |
| 21 | 986.14 | 15,657.36 | 7,657.36 | 21,472.95 | 13,815.59 |
| 22 | 1,015.72 | 16,673.08 | 8,673.08 | 22,865.94 | 14,192.86 |
| 23 | 1,046.19 | 17,719.27 | 9,719.27 | 24,300.72 | 14,581.44 |
| 24 | 1,077.58 | 18,796.85 | 10,796.85 | 25,778.54 | 14,981.69 |
| 25 | 1,109.91 | 19,906.76 | 11,906.76 | 27,300.70 | 15,393.94 |
Comparison
| Scenario | Total cost over the horizon | Cumulative energy cost | Net position at the horizon |
|---|---|---|---|
| Do nothing | 27,300.70 | 27,300.70 | 0.00 |
| With the upgrade | 15,393.94 | 15,393.94 | 11,906.76 |
Formula
Q = U·A·DD·24/1000 ÷ η; saving(n) = (Q_old − Q_new) × price × (1+g)^(n−1)= 12.32
Note
This is a simplified cash-flow model. It projects the prices, escalation rate and equipment costs you entered with a single geometric escalation and no discounting, no inflation adjustment, no financing costs and no tax treatment; savings are assumed to accrue evenly within each year, which is what the fractional break-even interpolates. Real energy prices, tariff structures, grants, weather, occupancy and equipment performance vary widely and change over time. A break-even of zero means the cumulative cash flow never crosses into positive territory within the horizon you chose. Get a professional energy assessment and a written quotation before committing to any of these measures.
More in Energy payback
See all →Frequently asked questions
What's the difference between the U-value and the g-value shown in results?+
The U-value measures how much heat escapes through the glass (lower is better for insulation), while the g-value (solar heat gain coefficient) measures how much solar heat passes through (relevant for cooling-dominated climates). Payback calculations for heating savings rely primarily on the U-value improvement between your old single or double glazing and the new units.
Why is my payback estimate so much longer than for insulation or LED upgrades?+
Windows are one of the most expensive envelope upgrades per unit of heat loss reduction avoided, often 400-1,000+ per window installed, while the heat loss they prevent is smaller in total than what walls or roofs lose. Paybacks of 15-30+ years are common and normal for window replacement done purely for energy savings.
Does the calculator separate window replacement from a whole-house energy upgrade?+
It should, because windows are rarely cost-justified on energy savings alone; the payback calculation isolates the energy-cost side of the decision from other reasons to replace windows, such as condensation, drafts, noise reduction, failed seals, or curb appeal. If those non-energy factors matter to you, the true value of the upgrade is higher than the payback number suggests.
How does climate affect the result?+
Heating degree days drive the savings side of the equation directly: the same window upgrade in a cold climate with a long heating season saves far more energy per year than the identical upgrade in a mild climate. Entering an accurate regional heating degree-day or climate zone value is one of the most important inputs for a realistic payback.
Should I replace all windows at once or prioritize certain ones?+
Prioritizing the largest, most poorly-oriented, or worst-condition windows (north-facing in cold climates, or any with failed seals/visible fogging) gives the fastest marginal payback per dollar spent. Replacing a small, already-decent window last, or not at all, is usually the financially smarter sequencing even if it means the project takes longer.