What this calculator leaves out
What this calculator leaves out
Every model is wrong somewhere. These are the places this one is wrong, stated so you can correct for them.
Tax, regional pricing and currency. You enter gross prices. VAT, sales tax, education pricing and exchange rates are outside the model.
Downtime and lost work. A machine that fails for two days costs more than the repair. That cost is set to zero here because any number would be invented.
Repair variance. A repair budget is an average. Real repairs arrive in lumps: nothing for two years, then a 400 screen. A five-year average hides that.
Resale friction. Private sale takes time and sometimes fails; trade-in is fast and pays 20 to 40 percent less. The model assumes you receive the number you enter.
A new machine can also need repairs. The buy side assumes zero running cost. Add them to the price if you want the pessimistic case.
Why keeping is charged for value loss
This is the part people push back on. Keeping a machine that sells for 650 today does not cost 650 in cash, so charging 130 a year for it can look like double counting.
It is not double counting, it is opportunity cost. That 650 is a real asset that is being consumed by time. If you would never sell it, the true value is lower than 650 and you should enter the honest number instead. Entering a high resale value while planning to keep the machine forever is the one way to make this model lie to you.
Why break-even can be empty
Break-even is defined as net cash divided by the annual saving from buying. When buying is more expensive per year, there is no saving to divide by, so the number does not exist. The page says "none" rather than printing an absurd horizon.
An empty break-even is information: it means the decision is about capability, warranty, or time, not about money. Those are legitimate reasons. They are just not the ones the calculator measures.
Where the defaults come from
The pre-filled values are round placeholders chosen so the form works before you touch it. They are not scraped from a price index, not inflation-adjusted, and not updated automatically. Replace all six.
How often this page changes
The formula is stable. When a field is added or a definition changes, this page and the worked example change in the same commit. When the model was corrected from version one to version two, the change and the reason were written on the how-it-works page rather than quietly shipped.