Keep Above
Cheap to buy or cheap to run
A plug-in heater against a heat pump, a budget printer against a laser — some things are cheap at the till and dear at the wall. This calculator counts both options over the same years, buying each again whenever it wears out, and lands both on a cost per year. Name the options yourself; no winner is ever declared.
Not counted while this is $0
Not counted while this is $0
Put in what each one would cost to buy and to run. Nothing here is read from your budgets, and nothing is saved.
| Option A | Option B | |
|---|---|---|
| Paid up front (one unit) | — | — |
| Times it would be bought | — | — |
| Buying, all-up | — | — |
| Running, all-up | — | — |
| Total over 10 years | — | — |
| Cost a year | — | — |
When does a low purchase price stop deciding the total?
Each option here describes a line that starts at its purchase price and climbs at its running cost, so the two begin apart and close at a fixed rate. The one that starts lower is overtaken at whatever year the gap between the prices has been used up by the gap between the running costs, and the size of those two gaps is the only thing that sets it.
That is why the horizon does the deciding rather than either figure on its own. A keep shorter than the crossing year means the totals never meet at all and the purchase prices settle it; a keep well past it hands the answer to the running costs. A short life on the low-price side brings the crossing forward again, because each replacement puts a whole purchase price back into the total while the running-cost gap has never stopped widening.
This is one of the tools built into Keep Above
Keep Above is a budgeting app where you enter your bills and income and it works out the balance you need to keep — so you can see what's safe to spend. No bank connection, no ads on your data.
Which one costs less depends on how long it is kept, so the honest horizon is years — which budgeting apps forecast your balance, and how far ahead.
About this calculator
Why are replacements counted?
Because anything that wears out inside the horizon has to be bought again, and a second or third purchase can be larger than the entire running-cost gap it was set against. Each replacement enters at the price you typed, as a step in the cumulative line rather than money paid today. Set a life longer than the horizon and no replacement is counted at all.
What is the crossing year?
The first year at which the two running totals meet, with every replacement already counted in them. It is stated as a position in the arithmetic and names no side, because which side of it you would actually land on depends on how long the thing is kept — a figure only you can put in.
What happens if the two totals never cross?
Then the page says so instead of printing a year. Two options with the same running cost never meet, because the gap between their purchase prices stays exactly where it started, and where one option starts lower and climbs more slowly as well, the totals only drift further apart. Neither is a fault in the arithmetic; both are real answers about the figures you put in.