Where your electricity units go: what appliances really cost to run

The bill arrives, the units have doubled, and everyone in the house blames a different appliance. The argument can be settled with arithmetic, because a unit is a kilowatt-hour and every appliance's share of the bill is its power multiplied by the time it actually draws it. That second factor is where intuition fails: the biggest number on a nameplate is not the biggest number on the bill, and a 10 W router can out-consume a 1 500 W iron. This guide works through the usual suspects in a Pakistani home, explains why one extra unit can raise the price of all the others, and shows how to measure instead of guess.

A unit is a kilowatt-hour

Power, in watts, is the rate at which an appliance uses energy. Energy, in kilowatt-hours, is power multiplied by time, and it is energy that the meter counts and the bill prices. One unit is one kilowatt-hour: 1 000 W for an hour, 100 W for ten hours, 10 W for a hundred hours. So a 2 000 W geyser that heats for half an hour uses one unit, and a 10 W router uses one unit every four days, about 7 a month, without anyone touching it.

Two things follow. A rating tells you nothing about cost until you multiply it by hours, and the hours must be the hours the appliance is actually drawing that power, which for anything with a thermostat or a compressor is less than the hours it is switched on.

Power is not energy: why the duty cycle matters most

A fixed-speed 1.5-ton air conditioner draws about 1 800 W while its compressor runs. Once the room reaches the set point the thermostat stops the compressor, leaving only the indoor fan at a few tens of watts, and restarts it when the room has warmed a degree or so. On a hot Lahore afternoon the compressor might run 40 minutes in every hour; at night, or in a shaded and insulated room, 20 or 30. The fraction of time it runs is the duty cycle, and it, not the 1 800 W, decides the units: at 70 % duty the average draw is 1 260 W and eight hours cost 10 units; at 40 % the same eight hours cost under 6.

The refrigerator is the same machine in miniature and runs all day: 150–250 W when the compressor is on, on for roughly a third of the time, so 40–60 units a month depending on the model, the kitchen temperature and how often the door opens. An inverter air conditioner takes the idea further. Instead of switching the compressor on and off it slows it down, so once the room is cool it runs continuously at 30–60 % of its rated input, where the compressor is more efficient and without the current surge of each restart.

The rule, then, is to think of every appliance as an average draw over the hours it is in use. Fans, lights, pumps and routers draw their rated power the whole time; anything with a thermostat draws a fraction of it, and the fraction is a guess unless it is measured.

The usual suspects in a Pakistani home

A summer month with the fixed-speed AC, the pump, fridge, AC fans, lighting, TV and router comes to about 507 units, three-fifths of it the air conditioner. Swap in the inverter AC and the DC fans and it is about 315. The geyser belongs to the winter bill, where a 2 000 W fan heater for an hour a day would add another 60.

The slab trap

Residential electricity in Pakistan is priced in slabs by the number of units used in the month, with the rate per unit rising from slab to slab. The rates are on the schedule of tariff that NEPRA approves for each distribution company and they change often enough that none are quoted here; two features of the schedule matter more than the rates themselves.

First, not every slab is incremental. In an incremental (telescopic) tariff the higher rate applies only to the units above the threshold, as income tax does. In the residential schedule some slabs instead apply the rate of the slab you land in to every unit of the month, so that one unit over the threshold re-prices all the others, and the difference between the last unit below the line and the first above it can be a large sum. Which slabs are incremental and which are not is stated in the notes to the schedule and has changed between determinations, so read the notes on the current one.

Second, there are protected and unprotected categories. A consumer whose consumption stays at or below a threshold for a run of consecutive months is billed on the protected schedule at lower rates; one month over the threshold moves the connection to the unprotected schedule, and it takes months of low consumption to get back. A single summer of air conditioning can therefore raise the price of the winter's units too.

Connections metered on time-of-use pay a peak rate for a few evening hours, set seasonally, and an off-peak rate the rest of the day. For those, when an appliance runs matters as much as how long: the same pump on a timer at two in the morning costs less than at eight in the evening.

This is why the electricity cost calculator warns when a single appliance adds more than 200 units a month: an appliance of that size can move the whole bill across a threshold on its own.

Standby and phantom loads

An appliance that is "off" but still plugged in often is not: a television waiting for its remote, a set-top box keeping its clock and tuner alive, a microwave with a display, phone chargers left in the socket, a desktop PC asleep, the doorbell transformer. IEC 62301 defines how standby power is measured, and regulated products made in the last decade usually draw under a watt in standby; older televisions, set-top boxes, audio systems and printers can draw several watts each, and a home UPS or inverter keeping itself alive and its battery on float draws tens of watts around the clock.

The arithmetic is small per device and large in aggregate: 1 W for a month is 0.72 units, 5 W is 3.6, and a house with 30 W of assorted standby pays for 21.6 units a month, about two weeks of the refrigerator. The calculator's standby input puts a number on any device; a switched socket strip deals with the cluster under the television.

How to measure instead of guess

• A plug-in energy meter. It sits between the socket and the plug and totals kilowatt-hours, so a day on the fridge or a week on the television gives the true consumption, duty cycle included. It is rated for an ordinary socket, which covers any plug-in appliance.
• The meter test. For hard-wired loads (air conditioner, pump, geyser), switch everything else off, note the meter reading, run the appliance for an hour and note it again. The difference is its units per hour under that day's conditions. On a digital meter, read the kWh register; on an old disc meter, count revolutions against the revolutions-per-kWh constant printed on its face.
• A clamp meter, with care. It reads current, and amps × 230 V is volt-amperes, not watts. For heaters and lights the two are the same; for a motor multiply by roughly 0.8, and for a duty-cycled appliance the reading changes every minute, so it tells you the draw, not the energy.
• Interval data. Where the connection has a smart meter or a net-metering meter that logs consumption by the hour or the half-hour, the profile shows each large appliance switching on and off and its duty cycle can be read straight off the plot. Ahmedonics builds monitoring and control systems that log sub-circuits in the same way for houses, offices and plants, which is the starting point for any serious saving.

The example in numbers

The calculator's default is the 1.5-ton fixed-speed air conditioner: 1 800 W, 70 % duty, 8 hours a day, 30 days, PKR 45 a unit.

1. Average draw while in use: 1 800 × 0.7 = 1 260 W, the same draw as 126 LED bulbs of 10 W.
2. Energy: 1 260 × 8 ÷ 1 000 = 10.08 kWh a day; × 30 = 302.4 units a month; × 365 = 3 679 units if it ran every day of the year.
3. Cost at PKR 45 a unit: PKR 453.60 a day, PKR 13 608 a month, PKR 165 564 a year. For four summer months the real figure is closer to PKR 54 000.

Replace it with an inverter unit that settles at about 700 W: 700 × 8 = 5.6 kWh a day, 168 units and PKR 7 560 a month, 134 units fewer, which may also keep the bill in a lower slab. That is the arithmetic a purchase should rest on, and the calculator does it for any appliance.

What actually reduces the bill

• The air conditioner first. An inverter unit sized to the room, a higher set point (each degree is commonly reckoned at a few per cent of the energy, a rule of thumb), a clean filter, and keeping the sun off the room by shading the window, closing doors and insulating a roof under the sun. Everything that cuts the heat the machine must remove cuts the units, because that is where the energy goes.
• Fans. A DC inverter fan draws 25–35 W against 60–80 W; with four fans running twelve hours a day that is nearly 60 units a month.
• Lighting. If anything is still incandescent or CFL, replace it with LED; a 10 W LED gives the light of a 60 W bulb.
• The pump on a float switch or a timer, so that it fills the tank once instead of cycling whenever a tap opens, and a pump matched to the head it has to lift, since an oversized pump is a bigger motor running the same hours.
• The geyser on a timer for the hour before it is needed, with the thermostat at 50–55 °C rather than the maximum and the tank and pipes insulated.
• Solar for the daytime load. The fridge, fans, pump and a daytime air conditioner run while the sun is up; a grid-tied or hybrid system sized to that load removes those units from the bill and, under net metering, exports the surplus. The solar sizing guide and calculator start from the daily units this guide has just counted.

References

• IEC 62301:2011, Household electrical appliances — Measurement of standby power
• IEC 62552-1, -2 and -3:2015, Household refrigerating appliances — Characteristics and test methods — energy consumption of refrigerators
• ISO 5151:2017, Non-ducted air conditioners and heat pumps — Testing and rating for performance — rated 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