From the watt rating to kilowatt-hours
A watt rating measures power: how fast a device draws electricity at a given moment. Your bill charges for energy, which is power multiplied by time and measured in kilowatt-hours (kWh). Pakistani electricity bills call a kilowatt-hour a "unit". The US Energy Information Administration gives the basic example: a 40-watt bulb used for five hours consumes 200 watt-hours, or 0.2 kWh.
A 1,500 W air conditioner running 8 hours a day for 30 days uses 1,500 × 1 × 8 × 30 ÷ 1,000 = 360 kWh. Multiply by your price per kWh to get the energy charge.
The wattage is usually printed on the rating plate or in the manual. If the label lists only volts and amps, multiplying them gives an upper limit rather than the typical draw. Motors and compressors often draw less than that product while running normally.
A month of common appliances, compared
The ratings below are examples for the arithmetic, so check the labels on your own appliances. The cost column uses a flat 50 per kWh, a round figure chosen only to make the comparison easy to follow. It is not any utility's tariff.
| Appliance | Rated watts | Hours per day | kWh in 30 days | Cost at 50 per kWh |
|---|---|---|---|---|
| LED bulb | 10 | 6 | 1.8 | 90 |
| Laptop | 60 | 8 | 14.4 | 720 |
| LED television | 100 | 5 | 15 | 750 |
| Clothes iron | 1,000 | 0.5 | 15 | 750 |
| Water pump | 750 | 1 | 22.5 | 1,125 |
| Ceiling fan | 75 | 12 | 27 | 1,350 |
| Refrigerator, nameplate for 24 h | 150 | 24 | 108 | 5,400 |
| Air conditioner | 1,500 | 8 | 360 | 18,000 |
Two points stand out. A 1,000 W iron uses the same energy as a 100 W television because it runs for a tenth as long, so wattage alone does not rank appliances; watts × hours does. And one air conditioner uses more than every other row on the list combined.
The refrigerator row deliberately overstates its use. A fridge compressor switches on and off, so the nameplate figure is not its average draw across 24 hours. If you know it runs about half the time, enter 12 hours, which gives 54 kWh. Better still, divide the annual kWh figure on its energy label by 12, or measure it for a week with a plug-in meter. Inverter air conditioners also vary their draw with the cooling load, so their rated input is closer to a ceiling than an average.
Electricity Cost CalculatorEnter your appliance's watts, count, daily hours, days and per-unit rate to estimate its kWh and cost.Comparing a replacement: the payback question
Eight 60 W bulbs lit 6 hours a day use 86.4 kWh a month. Eight 10 W LED bulbs doing the same job use 14.4 kWh. That saves 72 kWh a month, or 3,600 at the illustrative rate. Divide the cost of the new bulbs by the monthly saving and you get the payback period in months. This is break-even analysis applied to a single purchase, the same logic as the break-even worked example. Value the saving at your marginal rate, which the next section explains.
Why your bill will not match a flat-rate estimate
The calculator multiplies kWh by one flat rate that you enter. Many utilities do not charge that way. The EIA notes that some utilities offer time-of-day pricing and that most consumers pay rates that follow the seasonal cost of electricity.
In Pakistan, the residential tariffs determined by NEPRA are built on slabs: the rate per unit depends on how many units the household uses in a month. There are separate consumer categories, such as lifeline and protected consumers, and time-of-use meters charge different rates at peak and off-peak hours. Depending on the category, a bill can also carry fixed charges, fuel price adjustments and taxes or duties on top of the per-unit energy rate. Tariffs are revised regularly, so take current rates from your latest bill or the distribution company's notified schedule rather than from any article.
- Use your marginal rate. The cost of adding one appliance is the rate for the slab your extra units fall into, not the average rate across your whole bill.
- Check how slabs are applied. Adding a heavy appliance can push the monthly total over a slab boundary. Find out whether your tariff charges each band of units at its own rate or applies the higher rate to more of your consumption, because the difference can be large.
- Split time-of-use hours. On a time-of-use meter, run the calculator twice, once for peak hours at the peak rate and once for off-peak hours at the off-peak rate, then add the two results.
- Know which charges scale. A percentage-based tax rises with the energy charge, while a fixed charge does not. If a per-unit rate rises 20%, the energy charge rises 20% but the whole bill rises by less. The percent change guide covers that distinction.
Limits of the watts-and-hours estimate
- Standby power. Televisions, chargers and set-top boxes draw a little power when idle. Model it as a separate run, using the standby watts for the standby hours.
- Seasonal hours. Fans and air conditioners run far more in summer. Estimate each season separately instead of using one annual average.
- The fixed charge field. The tool adds the fixed charge once for the period. Adding a whole household fixed charge to a single appliance overstates that appliance's extra cost.
- Solar and net metering. Units exported or offset by rooftop solar are not modeled.