Gadget Replacement Calculator

“Should I replace this device?” sounds like a price comparison, but a cheap upgrade can still be a poor decision if the current device is inexpensive to keep or the new device carries a large manufacturing footprint. I built this calculator for a more explicit comparison: put the financial break-even point beside a rough carbon payback point, then see whether both axes agree.

The result is not a universal upgrade rule. It is a transparent model whose assumptions you can change: device type, resale value, repair cost, satisfaction score, expected lifespan, manufacturing CO₂, power draw, and daily usage.

The Current Device Is a Monthly Cost Model

The form supports smartphones, laptops, tablets, and desktop PCs. Selecting a type updates two defaults:

const CO2_DEFAULTS = {
  smartphone: 70,
  laptop: 300,
  tablet: 90,
  desktop: 700,
};
const POWER_DEFAULTS = {
  smartphone: 5,
  laptop: 15,
  tablet: 8,
  desktop: 80,
};

function setDeviceType(key) {
  deviceType.value = key;
  mfgCo2.value = CO2_DEFAULTS[key];
  powerWatts.value = POWER_DEFAULTS[key];
}

If no positive resale value is entered, the calculator estimates one with straight-line depreciation from the purchase price:

const resale = currentResale.value > 0
  ? currentResale.value
  : Math.max(
      0,
      purchasePrice.value *
        (1 - ageMonths.value / (newLifespan.value * 12))
    );

That fallback is intentionally visible in the source: it uses the new device's expected lifespan as the time scale for the estimate. The keep-cost model then adjusts remaining months by the satisfaction score, never allowing less than one month:

const remainingMonths = Math.max(
  (expectedLifeMonths - ageMonths.value) * (perfScore.value / 10),
  1
);
const monthlyDepreciation = resale / remainingMonths;
const monthlyCostKeep = monthlyRepair.value + monthlyDepreciation;

The new device's monthly cost is simply its price divided by its expected lifespan in months. If keeping the old device is cheaper per month, there is no financial benefit and moneyBreakEven becomes -1. Otherwise the upfront amount is the new price minus the resale value, divided by monthly savings and rounded up.

Carbon Payback Uses an Explicit Assumption

The carbon side estimates annual savings from reduced power use:

const co2PerKwh = 0.5;
const extraPowerWatts =
  powerWatts.value * (efficiencyLoss.value / 100);
const annualCarbonSaved =
  (extraPowerWatts / 1000) *
  usageHours.value *
  365 *
  co2PerKwh;
const carbonBreakEven = annualCarbonSaved > 0
  ? Math.ceil((mfgCo2.value / annualCarbonSaved) * 12)
  : 9999;

The 0.5 value is treated as kilograms of CO₂e per kilowatt-hour, described in the interface as a world-average estimate. It is not a measurement of your local grid. Manufacturing emissions are editable because the built-in reference values—roughly 70 kg for a phone, 300 for a laptop, 90 for a tablet, and 700 for a desktop—are estimates rather than certified product data.

The model asks a narrow question: if the current device consumes efficiencyLoss percent more power than a replacement, how long would the saved electricity take to offset manufacturing emissions? It does not count transport, recycling, repair parts, battery degradation, or the rebound effect of using a more powerful device.

The Verdict Is a Two-Axis Policy

The calculator labels a result replace only when financial break-even is positive and at most 24 months and carbon payback is at most 36 months. It labels a result keep when the money calculation does not favor replacement or takes more than 36 months, and carbon payback is over 48 months. Everything else is borderline:

const moneyFavorsReplace =
  moneyBreakEven > 0 && moneyBreakEven <= 24;
const carbonFavorsReplace =
  annualCarbonSaved > 0 && carbonBreakEven <= 36;
const moneyFavorsKeep =
  moneyBreakEven < 0 || moneyBreakEven > 36;
const carbonFavorsKeep = carbonBreakEven > 48;

if (moneyFavorsReplace && carbonFavorsReplace) {
  verdict = "replace";
} else if (moneyFavorsKeep && carbonFavorsKeep) {
  verdict = "keep";
}

Those thresholds are policy choices, not laws of economics or climate science. The UI also shows both monthly costs, the calculated break-even months, manufacturing carbon, annual carbon saved, and the two small bar comparisons, so you can disagree with the label without losing the underlying numbers.

The interface keeps the two questions visible instead of collapsing them into one score. The financial card reports the monthly cost to keep and the monthly amortized cost of the new device, while the carbon card reports manufacturing emissions and annual savings. A device can therefore be financially attractive but environmentally slow to pay back, or the reverse. The borderline state is not a calculation failure; it is the expected output when the chosen thresholds disagree or neither side is decisive.

Where the Model Can Mislead

Straight-line depreciation is a convenient approximation, not a resale-market forecast. A device can have a sudden battery failure, retain value unusually well, or be worth nothing while still working. The satisfaction score changes the remaining-life estimate, which makes it a subjective input rather than a measured performance benchmark. A device older than the assumed lifespan is clamped to one remaining month instead of producing a special “already beyond life” state.

Carbon payback becomes infinite when annual savings are zero, and all carbon values are estimates. Treat the result as a conversation starter and sensitivity test: change the resale value, repair cost, efficiency loss, and usage hours to see which assumptions actually control the recommendation.

I turned this model into a small free tool: Gadget Replacement Calculator. It is useful when a purchase decision needs more than “the new one has a lower monthly payment.”