Paste the improvements you have been quoted for — one per line, with the installed cost, any contribution you received, the annual saving you were told to expect, the expected life in years and any maintenance — and see whether the money actually comes back, and when. It gives the simple payback, the payback once your own expected energy price change is compounded year by year and maintenance is taken off, the net position over each measure's own life, and an optional discounted payback if you want one. Crucially, a measure whose payback runs past its own lifespan is reported as NEVER PAYING BACK rather than as a number larger than the lifespan that you have to notice for yourself. No cost, saving, lifespan, grant or energy price is built in — every number is yours, and the result is only as good as the saving estimate you were given. Nothing is recommended and nothing is claimed about carbon.
Worked example
HOME IMPROVEMENT PAYBACK — 3 measures, energy prices assumed to change 0.00% a year
Every figure below comes only from the numbers you pasted. The annual saving is the one you were given, and the whole result rests on it.
Measure A
quoted $1,000.00 − contribution $0.00 = $1,000.00 to get back · life 20 years
Saving you were given $100.00 in the first year
Simple payback 10 years
(that figure ignores the time value of money, any energy price change and any maintenance)
Adjusted payback 10 years
(prices changing 0.00% a year, compounded year by year)
Crosses over in year 10
Saved over 20 years $2,000.00
Net position over its life +$1,000.00 ($2,000.00 saved − $0.00 maintenance − $1,000.00 to get back)
year 1 saving $100.00 net $100.00 running $100.00
year 2 saving $100.00 net $100.00 running $200.00
year 3 saving $100.00 net $100.00 running $300.00
year 4 saving $100.00 net $100.00 running $400.00
year 5 saving $100.00 net $100.00 running $500.00
year 6 saving $100.00 net $100.00 running $600.00
…years 7–20 not shown
Measure B
quoted $4,000.00 − contribution $500.00 = $3,500.00 to get back · life 15 years · maintenance $40.00/yr
Saving you were given $250.00 in the first year
Simple payback 14 years
(that figure ignores the time value of money, any energy price change and any maintenance)
Adjusted payback NEVER PAYS BACK — it would take 16 years 8 months, and the life you entered is 15 years. The measure is expected to be gone before the money comes back.
(prices changing 0.00% a year, compounded year by year, maintenance taken off)
Would cross over in year 17 — after the life you entered ends
Saved over 15 years $3,750.00
Maintenance over 15 years $600.00
Net position over its life -$350.00 ($3,750.00 saved − $600.00 maintenance − $3,500.00 to get back)
** Measure B DOES NOT PAY BACK inside the 15 years you entered. Over that whole life it returns $3,150.00 against $3,500.00 to get back, leaving you $350.00 down. **
Cost per year of life $233.33 — some measures are still wanted for comfort, warmth or noise, which this tool does not measure and does not weigh
year 1 saving $250.00 net $210.00 running $210.00
year 2 saving $250.00 net $210.00 running $420.00
year 3 saving $250.00 net $210.00 running $630.00
year 4 saving $250.00 net $210.00 running $840.00
year 5 saving $250.00 net $210.00 running $1,050.00
year 6 saving $250.00 net $210.00 running $1,260.00
…years 7–15 not shown
Measure C
quoted $2,500.00 − contribution $0.00 = $2,500.00 to get back · life 12 years
Saving you were given $90.00 in the first year
Simple payback NEVER PAYS BACK — it would take 27 years 9 months, and the life you entered is 12 years. The measure is expected to be gone before the money comes back.
(that figure ignores the time value of money, any energy price change and any maintenance)
Adjusted payback NEVER PAYS BACK — it would take 27 years 9 months, and the life you entered is 12 years. The measure is expected to be gone before the money comes back.
(prices changing 0.00% a year, compounded year by year)
Would cross over in year 28 — after the life you entered ends
Saved over 12 years $1,080.00
Net position over its life -$1,420.00 ($1,080.00 saved − $0.00 maintenance − $2,500.00 to get back)
** Measure C DOES NOT PAY BACK inside the 12 years you entered. Over that whole life it returns $1,080.00 against $2,500.00 to get back, leaving you $1,420.00 down. **
Cost per year of life $208.33 — some measures are still wanted for comfort, warmth or noise, which this tool does not measure and does not weigh
year 1 saving $90.00 net $90.00 running $90.00
year 2 saving $90.00 net $90.00 running $180.00
year 3 saving $90.00 net $90.00 running $270.00
year 4 saving $90.00 net $90.00 running $360.00
year 5 saving $90.00 net $90.00 running $450.00
year 6 saving $90.00 net $90.00 running $540.00
…years 7–12 not shown
RANKED — by adjusted payback (soonest first)
1. Measure A 10 years
2. Measure B NEVER PAYS BACK (would need 16 years 8 months, life is 15 years)
3. Measure C NEVER PAYS BACK (would need 27 years 9 months, life is 12 years)
RANKED — by net position over each measure's own life (best first)
1. Measure A +$1,000.00 over 20 years
2. Measure B -$350.00 over 15 years
3. Measure C -$1,420.00 over 12 years
The two orders agree.
NOTES
· No figure above is a benchmark or a normal one. This tool carries no built-in prices, savings, contributions, installed costs or lifespans at all — they differ enormously by country, home, installer and year, so a stored number would be confidently wrong. Every figure above came from your own line.
· The annual saving is an estimate somebody gave you, and the entire result rests on it. Installer and supplier estimates are frequently optimistic. If you halve the saving and the answer changes, you have learned that the answer was never firm.
· Year 1 uses the saving exactly as you entered it. The price change is applied from year 2 onward, compounded a year at a time in whole cents. Maintenance is held flat — if you expect it to rise, run the line again with a higher figure.
· Whole years come from the running total; the months inside the crossing year assume that year's saving arrives evenly and are rounded to the nearest month.
· Discounted payback answers a different question from the other two and is only shown when you enter a discount rate.
· Projections stop at 100 years; if that bound is reached the line above says so.
· No measure is recommended here. This is arithmetic about money, on your numbers, and everything the arithmetic does not capture — how cold the house is, how long you will live in it, how much you trust the quote — is yours to weigh. The decision is yours.
Frequently asked
What format do the lines take?
label | installed cost | grant or contribution | expected annual saving | expected life in years | annual maintenance. Separate the fields with a pipe, semicolon or tab; the grant and maintenance fields are optional. A comma works too, but only on a line that uses no other separator — otherwise "$1,200" would split in two. Dollar and percent signs are ignored. Lines starting with # are skipped.
Why are no typical costs or savings built in?
Because they differ enormously by country, home, installer and year, and they change. A stored figure would be confidently wrong for most people who read it, and it would quietly become the answer instead of your own quote. Every number here comes from your line.
How much should I trust the annual saving?
Less than the precision of the output suggests. It is an estimate somebody gave you, and estimates from people selling the work are frequently optimistic. The honest way to use this tool is to run the line twice — once with the saving you were quoted and once with half of it — and see whether the conclusion survives.
What does 'never pays back' mean here?
That the money does not come back inside the life you entered. If a measure would take twenty-eight years and you told the tool it lasts twelve, printing 'twenty-eight years' would be true and useless — the measure is expected to be gone first. So it says NEVER PAYS BACK, shows what it would have taken, and shows how much down you end up over the whole life.
Then why show cost per year of life?
Because payback is not the only reason to do work on a house. Something that never returns its cost may still be wanted for comfort, warmth or quiet. Cost per year of life is what that costs you annually. The tool states the number and does not weigh it — it cannot see your house.
Why is simple payback different from adjusted payback?
Simple payback is the cost divided by the first year's saving and nothing else — no price change, no maintenance, no time value of money. Adjusted payback compounds your expected energy price change year by year and subtracts maintenance. Rising prices shorten payback; falling prices and maintenance lengthen it.
What is discounted payback for?
It answers a different question: when does the saving, valued in today's money at a rate you choose, cover the cost. It is always at least as long as the adjusted payback. It is only shown if you enter a discount rate, because most people do not want it.
Why are there two rankings?
Because they often disagree. A cheap measure with a short life can come back soonest and still be worth least over its life. Neither order is the answer; they answer different questions, so both are printed.
Is any of the arithmetic done in floating point?
No. Money is held in whole cents and rates in basis points, and the price change is compounded one integer multiply-and-round per year rather than raised to a power. The printed table and the totals reconcile exactly.
Does it say anything about carbon or the environment?
No. This is arithmetic about money on the numbers you entered. It makes no environmental claim and recommends nothing.