What the calculator does
An electricity bill counts kilowatt-hours, not watts. The calculator turns an appliance's rated power into the energy it actually uses by allowing for how much of its running time it really draws that power (the duty cycle), how many hours a day it is in use and how many days a month, and adds the standby draw for the hours it is "off". It multiplies the units by your tariff to give the cost per day, month and year, reports the standby share, compares the average draw with 10 W LED bulbs, and warns when the appliance alone adds enough units in a month to move a residential bill into a higher slab.
Formula
Energy per day (kWh) = n × (Prated × duty × huse + Pstandby × (24 − huse)) ÷ 1 000
Energy per month = per day × days Energy per year = per day × 365
Cost = energy × tariff
Standby share = standby energy ÷ total energy
where n is the number of identical appliances, duty the fraction of the time in use during which the appliance draws its rated power, and tariff the all-in price of one unit in PKR/kWh. One unit on the bill is one kilowatt-hour: 1 000 W for one hour, or 100 W for ten.
Worked example
A 1.5-ton fixed-speed split air conditioner in Lahore, 1 800 W input on the nameplate, its compressor running about 70 % of the time, used 8 hours a day for 30 days, on an all-in tariff of PKR 45 per unit, with no standby draw.
- Average draw while in use = 1 800 × 0.7 = 1 260 W, the same draw as 126 LED bulbs of 10 W.
- Energy per day = 1 260 × 8 ÷ 1 000 = 10.08 kWh.
- Per month: 10.08 × 30 = 302.4 units. Per year, if every day were like this one: 10.08 × 365 = 3 679 units.
- Cost: 10.08 × 45 = PKR 453.60 a day, PKR 13 608 a month and PKR 165 564 a year.
The 302 units from this one appliance exceed the whole monthly consumption of a household that stays in the lower slabs, which is why the calculator flags it: on the residential tariff the extra units may also lift the rate charged on every unit. In practice the air conditioner runs four or five months a year, so the annual figure is an upper bound for it; for a fridge or a router, which run every day, it is the real number.
Typical input power and duty of common appliances
| Appliance | Input power | Duty while in use | Notes |
|---|---|---|---|
| Ceiling fan, AC induction motor | 60–80 W | 100 % | at full speed; less on a low regulator setting |
| Ceiling fan, DC inverter (BLDC) | 25–35 W | 100 % | at full speed |
| LED bulb | 9–12 W | 100 % | gives about the light of a 60 W incandescent |
| LED tube | 18–20 W | 100 % | |
| Refrigerator | 150–250 W | 30–40 % | runs 24 h a day; duty rises in summer and with frequent door opening |
| 1.5-ton fixed-speed AC | 1 700–2 000 W | 50–80 % on a hot day | compressor cycles on the thermostat; 100 % when the room never reaches the set point |
| 1.5-ton inverter AC | 300–1 500 W, varying | continuous | full power to cool the room down, then settles to 30–60 % of rating; enter the settled average and 100 % |
| Electric iron | 1 000–1 500 W | thermostat cycles | the element is on only part of the time once the plate is hot |
| Electric storage geyser | 1 500–2 000 W | 100 % while heating from cold | cycles on the thermostat once hot; left on all day it reheats standing losses |
| Water pump, 0.5 hp | 500–600 W | 100 % while running | electrical input, not the 373 W shaft rating |
| Washing machine | 300–500 W | varies through the cycle | add 1 500–2 000 W while it heats water |
| Microwave oven | 1 000–1 500 W | 100 % on full power | cycles the magnetron at lower settings; input is well above the stated cooking power |
| Desktop PC | 150–300 W | varies with the work | monitor extra |
| Laptop | 40–65 W | varies | charger rating; less once the battery is full |
| 40-inch LED TV | 60–100 W | 100 % | standby from under 1 W on recent sets to several watts on older ones and set-top boxes |
| Wi-Fi router | 8–12 W | 100 % | runs 24 h a day: 6–9 units a month on its own |
Assumptions and limitations
- Nameplate is not measurement. The rated power is a maximum under test conditions. A fan on a low speed, a laptop with a full battery or an air conditioner on a mild evening draw less; a compressor in a 45 °C loft, or a pump lifting further than it was sized for, can draw more. A plug-in energy meter over a day is the honest figure.
- Duty cycle is a guess unless metered. It is the largest uncertainty for anything with a thermostat. Room size, insulation, set point, outside temperature and how often doors open all change it, and it is different in May and in September.
- Tariff slabs and time-of-use are not modelled. The calculator applies one flat rate. Residential slabs and time-of-use peak rates mean the marginal unit can cost far more than the average; use the rate for the slab you are in, or the peak rate if the appliance runs in peak hours.
- Power factor does not matter here. A domestic kWh meter records real energy, so the reactive current of a motor or an old fluorescent ballast does not appear on the bill. Industrial and commercial consumers are metered for it separately.
- No seasonal variation. The yearly figure multiplies one day by 365. For an air conditioner, a geyser or a heater, scale it by the months the appliance really runs.
- Fixed charges are extra. Fixed monthly charges, meter rent and fees on the bill are not per unit and are not included.
Frequently asked questions
Why is my bill higher than the sum of my appliances?
Usually because the duty cycles were guessed low, because loads were missed (the fridge, the water pump, the UPS or inverter keeping its battery on float, standby), because the bill's units are metered over a reading period that is not the calendar month, or because the taxes and adjustments on the bill are not in the per-unit rate you used. Take the all-in rate as the bill total divided by its units, and compare the appliance list with the meter reading rather than with the bill.
What is a "unit"?
One kilowatt-hour (kWh): a kilowatt drawn for an hour. A 1 000 W iron on for an hour, ten 100 W bulbs for an hour, or a 10 W LED for 100 hours each use one unit. The meter counts them; the bill prices them.
Does a bigger air conditioner cost more to run?
Not necessarily. The energy goes on removing the heat that leaks into the room, which is set by the room, the sun on its walls and the set point, not by the size of the machine. A larger fixed-speed unit draws more while its compressor runs but runs it for a smaller fraction of the time; a badly oversized one short-cycles, which is inefficient and uncomfortable, and an undersized one runs at 100 % without reaching the set point. What decides the units is the efficiency (EER, or a seasonal figure for inverter units) and the hours, so size it to the room and buy the more efficient one.
Inverter or fixed-speed?
A fixed-speed compressor is either fully on or off and drags the room temperature up and down around the set point. An inverter compressor varies its speed and, once the room is cool, runs continuously at a fraction of its rating, where a compressor is more efficient, and without the current surge of each restart. The saving is largest in a room that is cooled for many hours a day and small for a short burst. To model one, enter the settled average draw and 100 % duty. Inverter units also start far more gracefully on a generator or a solar inverter.
How do I measure instead of estimating?
For anything on a plug, a plug-in energy meter records kWh over a day or a week and gives the true duty cycle for free. For hard-wired loads such as an air conditioner, a pump or a geyser, switch everything else off, read the electricity meter, run the appliance for an hour and read it again; the difference is its units per hour under that day's conditions. A clamp meter gives amps, and amps × 230 V is volt-amperes, not watts; for a motor multiply by about 0.8 for a rough wattage. Where the connection has a smart meter or a net-metering meter that logs interval data, the hourly profile shows each large appliance switching.
References
- IEC 62301:2011, Household electrical appliances — Measurement of standby power — how standby power is defined and measured
- IEC 62552-1, -2 and -3:2015, Household refrigerating appliances — Characteristics and test methods — energy consumption test for refrigerators and freezers
- ISO 5151:2017, Non-ducted air conditioners and heat pumps — Testing and rating for performance — rated cooling capacity and power input of split air conditioners
- NEPRA, consumer-end tariff schedules of the distribution companies — residential slabs, protected and unprotected categories and time-of-use rates; the schedule in force governs