Example 1
0.8 years
Seals or a cheap thermostat: 400 and 40 a month, no rebate.
Energy savings calculator
The card divides investment cost by yearly saving. 400 and 40 a month come back in 0.8 years. 2,500 and 80 in 2.6. A rebate you type comes off the cost. Not Clean Heat and not an official grant.
This is not a subsidy form and not a utility bill. House CO₂: home energy. Carbon: carbon footprint.
Result
Cost and monthly saving. The result will show here.
How it works
Payback years = cost × (1 − rebate/100) / (saving × 12). 400 and 40 with no rebate is 400 / 480 ≈ 0.8 years. 2,500 and 80 is 2,500 / 960 ≈ 2.6. 35,000, 450 and a 20% rebate is 28,000 / 5,400 ≈ 5.2.
Cost must be above 0. Saving cannot be negative. A blank rebate counts as 0. A rebate of 100 or more will not start the sum, because there is nothing left to divide.
Here you compute years from your money. Home energy computes CO₂ from power and gas, not payback. Carbon footprint stacks car, flight and power. Business payback divides outlay by profit, not by a power saving.
The rebate is your percent. We do not insert a Clean Heat or official grant amount. House type and floor area stay out of the formula.
The result shows years, the year, cost after rebate and the month. The example button fills cost, saving and rebate.
Rebate is the percent you type. A comma in 40.5 works. Do not put spaces in 35,000: type 35000. The example button fills cost, saving and rebate together.
How to use
- Type 400 and 40. Leave rebate blank or 0.
- Click Calculate. Payback is 0.8 years, year 480.
- 2,500 and 80 give 2.6 years. 35,000, 450 and 20% rebate give 5.2.
- Take the saving from a bill difference, not from a shutter ad.
- We will not add an official grant. You type the percent.
Formula
years = cost × (1 − rebate/100) / (saving × 12). cost > 0. rebate < 100. saving ≥ 0.
Cost, rebate and payback
400 and 40 with no rebate is 0.8 years. 2,500 and 80 is 2.6.
- Cost
- Outlay on seals, a thermostat, shutters or a heat pump. 400 at 40/month is 0.8 years.
- Rebate
- A percent you type. 20% of 35,000 leaves 28,000. Not a grant amount.
- Payback
- Net cost / yearly saving. 2,500 / 960 ≈ 2.6.
- Saving
- Month × 12. The card does not guess it from floor area.
Worked examples
Example 2
2.6 years
Shutters at 2,500 and 80 a month, no rebate.
Example 3
5.2 years
35,000, 450 a month and a 20% rebate you typed.
Suggested uses
- You check whether 400 on seals comes back in 0.8 years at 40 saved a month.
- Blinds at 2,500 and 80 a month take 2.6 years. A 35,000 job at 450 a month is about 5.2 years.
Frequently asked questions
How many years from 400 and 40 with no rebate?
0.8. The year is 480. 400 / 480 ≈ 0.83; the calculator shows 0.8.
What does cost after rebate mean?
Cost × (1 − rebate/100). At 0% the full cost stays. At 20% of 35,000 you keep 28,000.
What about 2,500 and 80?
2.6 years. 80 × 12 = 960. 2,500 / 960 ≈ 2.6.
Is 20% an official grant?
No. It is a percent you typed. The office and Clean Heat stay outside this calculator.
Why is floor area unused?
Because the formula divides cost by a saving you type. Floor area does not invent money.
What if saving is 0?
There is no year and payback disappears. The calculator shows n/a, not infinity.
How is this different from home energy?
Here you compute years from your money. There you stack CO₂ from power and gas.
Does a comma in 40.5 work?
Yes. Do not put spaces in 1,000: type 1000.
How is the result calculated?
The calculator computes energy savings calculator. years = cost × (1 − rebate/100) / (saving × 12). cost > 0. rebate < 100. saving ≥ 0. In the first example the result is 0.8. The year is 480. 400 / 480 ≈ 0.83; the calculator shows 0.8..
Knowledge sources
The factors on the card are a sketch. Below are EPA emissions terms, not a national inventory.
Page updated in 2026.