DEV Community: Moarij0

Writing an Appliance Energy Calculator in JavaScript

Moarij0 — Thu, 05 Nov 2026 12:52:02 +0000

Appliance energy calculators answer one simple question: how many units does this device use in a month? It is a great small project for learning about units and clean input handling.

The formula

One unit of electricity is one kilowatt-hour (kWh).

function monthlyUnits({ watts, hoursPerDay, days = 30, quantity = 1 }) {
  if (watts <= 0 || hoursPerDay < 0 || hoursPerDay > 24) {
    throw new Error('Check wattage and hours');
  }
  return +((watts * hoursPerDay * days * quantity) / 1000).toFixed(1);
}

// Example: two 75 W ceiling fans running 12 hours a day
console.log(monthlyUnits({ watts: 75, hoursPerDay: 12, quantity: 2 })); // 54

Make it more realistic

Duty cycle: fridges and ACs do not draw full power all the time. Let users enter a percentage
Summing a list: let users add several appliances and show a total
Units to money: tariffs are usually slab-based, so pass the total units into a separate slab function rather than multiplying by one rate

See it in a real tool

BaKhabar Pakistan's appliance calculator shows units per appliance, and its solar calculator uses total usage to suggest a system size. Comparing your output with a working tool is a good way to check your maths.

Build a Road Trip Fuel Cost Calculator in JavaScript

Moarij0 — Thu, 05 Nov 2026 12:19:22 +0000

A road trip fuel calculator is one of the friendliest beginner projects: the maths is simple, but it teaches input validation, units and rounding.

The core formula

function tripFuelCost({ distanceKm, kmPerLitre, pricePerLitre, roundTrip = false }) {
  if (distanceKm <= 0 || kmPerLitre <= 0 || pricePerLitre <= 0) {
    throw new Error('All values must be positive');
  }
  const km = roundTrip ? distanceKm * 2 : distanceKm;
  const litres = km / kmPerLitre;
  return { litres: +litres.toFixed(1), cost: Math.round(litres * pricePerLitre) };
}

// Example values only
console.log(tripFuelCost({ distanceKm: 600, kmPerLitre: 12, pricePerLitre: 250, roundTrip: true }));

Details that make it useful

Round trip toggle: most people forget the drive back
A safety margin: hills, traffic and AC lower your real average, so add 10 to 15 percent
Live fuel price: hard-coding the price makes the tool wrong after the next revision. Fetch it or let users edit it
Clear rounding: show litres to one decimal and money as whole numbers

Compare with a live tool

In Pakistan, fuel prices are revised regularly, so a real tool needs the current rate. BaKhabar Pakistan's trip planner does this kind of calculation, and its fuel prices page lists the latest petrol and diesel rates.

Try building the function, then add tests for zero and negative inputs. Small projects like this are a good way to practise writing code that real people can use.

How Progressive Slab Calculators Work (with a JavaScript Example)

Moarij0 — Thu, 05 Nov 2026 12:03:06 +0000

Tax and utility calculators look simple from the outside, but most of them share one idea: progressive slabs. Pakistan's income tax and electricity tariffs both work this way, and it is a nice small problem to code.

The idea

A slab table says: income up to A is taxed at 0%, the part between A and B at r1, the part between B and C at r2, and so on. The common mistake is applying one rate to the whole amount. You have to tax each slice separately.

A minimal JavaScript version

The numbers below are example slabs only, not official rates:

const slabs = [
  { upTo: 600000, rate: 0 },
  { upTo: 1200000, rate: 0.05 },
  { upTo: 2200000, rate: 0.15 },
  { upTo: Infinity, rate: 0.25 },
];

function progressiveTax(income) {
  let tax = 0, lower = 0;
  for (const { upTo, rate } of slabs) {
    if (income <= lower) break;
    const slice = Math.min(income, upTo) - lower;
    tax += slice * rate;
    lower = upTo;
  }
  return Math.round(tax);
}

console.log(progressiveTax(1800000));

Things real calculators add

Monthly vs yearly input: convert monthly salary to annual before applying slabs
Fixed amounts per slab: many official tables express each slab as a fixed amount plus a percentage of the excess, which is the same maths written differently
Rate tables per year: budgets change the slabs, so keep them in data, not code
Rounding rules that match the official method

Electricity bills use the same slice-by-slice idea on units instead of rupees, plus fixed charges and taxes on top.

See it in a real tool

If you want to compare your output with a working calculator, BaKhabar Pakistan's income tax calculator uses this kind of slab logic for salaried income, and its electricity bill calculator applies it to units.

It is a good beginner exercise: write the function, then write tests for the slab edges (exactly at A, just above A), because that is where bugs hide.