Appliance running cost:
what a heater, bulb or AC costs to run

From a label rating to a rupee figure

Every appliance has a power rating printed on its plate, usually in watts. The rating tells you how fast it draws energy while it runs. To price it you also need to know how long it runs and what each unit of electricity costs you.

Energy is power multiplied by time. A kilowatt-hour is the amount a 1 kW load uses in one hour, and your tariff quotes a price per kWh. Put the three together and any device, from a phone charger to a geyser, can be costed in the same way.

Voltage and power factor sometimes confuse the rating. For most household resistive devices, such as heaters, kettles and bulbs, the watt figure on the plate is what you pay for. Motors and electronics can have a power factor below one, but residential meters bill the real energy, so the watt rating or a plug-in meter reading remains the right starting point.

The formula, with the unit conversion

The common slip is skipping the division by 1,000. A 1,500 W heater is 1.5 kW, not 1,500 kW, and that error inflates the answer a thousandfold.

For a quick conversion, an appliance of W watts running h hours costs W × h ÷ 1,000 units. A 750 W iron used 20 minutes is 0.75 kW × 0.333 h = 0.25 kWh, or ₹2 at ₹8 per unit, so ironing a week's clothes in one go is a small cost compared with space heating.

Worked example: a room heater for a month

Whether the heater really draws its full rating for all four hours is a different matter, and the next sections cover that gap.

Put another way, one hour of that heater costs ₹12, the same as a 9 W LED bulb left on for about 167 hours, or a week of non-stop use, which is why heating, cooling and water heating dominate most bills while lighting is rarely the problem.

To check the result against a bill, add 180 kWh to your normal usage for the month and apply your slab rates to the new total. Because the extra units sit in the top slab, the true increase can be larger than 180 × ₹8, so use the marginal rate of the highest slab you reach.

Why the rated figure can overstate

The plate shows the maximum draw. Thermostat-controlled devices such as heaters, geysers, refrigerators and air conditioners switch off once they reach the set point, so their average power over a day is lower than the label. A heater that cycles on for 60% of the time costs closer to ₹864 than ₹1,440, using the same inputs.

Other devices go the opposite way. Motors have a start-up surge, and an old fridge or a dirty AC filter can draw more than its rating when it is struggling. Where accuracy matters, a plug-in energy meter reading in kWh over a week beats any calculation.

Always-on devices and the small numbers that add up

Some devices run without anyone thinking about them. A refrigerator draws far less than its peak rating averaged over the day, but it runs around the clock. A router at 10 W for 24 hours uses 0.24 kWh a day, which is 7.2 kWh a month and about ₹58 at ₹8 per unit.

The same multiplication works for standby loads. A television that sips 2 W on standby for the 20 hours a day it is not watched uses 1.2 kWh a month. It is tiny alone, and it matters only because homes have many such devices. Add them up with the same formula and decide where the effort is worth it.

Where small loads matter

Lighting shows how the same sum scales with a change of bulb. Swapping a 60 W incandescent bulb for a 9 W LED saves 51 W. Used 5 hours a day for a year, that is 51 × 5 × 365 ÷ 1,000 = 93.075 kWh, worth about ₹745 at ₹8 per unit, from one bulb.

Comparing two ways of doing the same job

The formula is most useful for choosing between alternatives. A convection heater rated 2 kW and an oil-filled radiator rated 1.5 kW may both warm the same room, but the first runs more of the time and the second cycles. Estimate the hours each needs, multiply, and compare the monthly rupees instead of the ratings.

For a purchase decision, compare the saving with the price difference. If a more efficient air conditioner costs ₹6,000 more and saves 25 kWh a month at ₹8, the saving is ₹200 a month, so the extra price returns in 30 months. Higher usage shortens that payback and a low-use bedroom unit lengthens it.

Using the numbers to cut cost

• Rank devices by monthly kWh before replacing any; the top two or three usually decide most of the total.
• Check the standby draw of set-top boxes, chargers and routers, which can run for 24 hours.
• Use your top slab rate rather than the average rate when judging a saving, because the marginal unit is the one you stop buying.
• Shift heavy loads, where the tariff has time-of-day rates, to the cheaper window.

Efficiency labels, where they exist, state annual consumption under test conditions. Treat them as a way to rank models rather than as a prediction of your own bill, and always run the formula with your own hours of use.