Appliance running cost:
from watts and hours to rupees a month

From watts and hours to units

The meter on your wall counts kilowatt-hours, which most bills call 'units'. One unit is one kilowatt, or 1,000 watts, used for one hour. A 100-watt bulb left on for ten hours uses one unit; a 2,000-watt heater uses one in half an hour.

Every appliance has a power rating on its label, in watts or kilowatts. The cost of running it is therefore the power drawn, multiplied by the time on, multiplied by the price of a unit.

Many people look only at the rating and ignore how long the appliance runs. A fan at 75 W costs less per hour than a 1,500 W air conditioner, but a fan that runs for 14 hours may match some heavier things that run for 30 minutes.

The formula, the inputs and why tariffs vary

• Use the rated power, but remember that compressors in air conditioners and fridges cycle on and off, so average draw is lower than the label.
• Use your actual tariff. Domestic tariffs in India are usually slab-based, so the price per unit rises as consumption rises, and there are fixed charges and taxes on top.
• Add standby draw for TVs, chargers and set-top boxes that stay plugged in.

Many domestic tariffs charge less per unit for the first block of consumption and more as use rises. A household using 150 units a month and one using 450 units are therefore not paying the same average rate, and the extra ₹84 a day from an air conditioner may be billed at the highest slab. That makes the last unit more expensive than the first.

To estimate the marginal cost of an appliance, use the rate of the slab you fall into at the top of your usage, not the average on the bill. Fixed monthly charges, fuel adjustments and taxes then add a further percentage on top. For a quick estimate, divide the total bill by the total units to get the all-in rate; for the cost of a single appliance, the marginal rate is the better guide.

Worked example: a 1.5-ton air conditioner

Over a 30-day month that is 360 units and ₹2,520. A real AC does not draw its full rating continuously once the room is cool, so an inverter model, or one set to 24–26 °C, usually costs noticeably less. The calculation here is the upper bound at constant full power.

The point of the number is to compare. Cutting one hour of cooling a day saves 1.5 units, or ₹10.50 at ₹7; across a month that is ₹315.

Typical appliances side by side

Ratings are typical and vary by model; read your own label. The pattern holds regardless: lighting is tiny, and anything that makes heat or cold is large.

Always-on loads add up quietly

Devices that never switch off cost little per hour and plenty per month. They are also the easiest to overlook, because nobody 'uses' them.

The refrigerator's duty cycle depends on room temperature, door openings and age, so the 40% is an assumption to adjust. Even so, one fridge matches several fans in cost. If you want a measured number, a plug-in energy meter reads the true consumption over a day or a week.

Water: a similar running-cost sum

Water is billed by volume, usually per thousand litres or per kilolitre in municipal supply, and by the tanker where there is none. The logic matches electricity: litres used per day times price per litre.

The 135 litres per person is a commonly cited planning figure for urban supply; real use varies by season, household and fixtures. Rates here are placeholders for illustration, so substitute your own.

Check the sums against your meter

Meter readings give an independent total. Note the reading at the same time on two days a week apart; the difference is units used.

If your itemised appliance estimates add up to much less than this, something is missing: an old appliance that draws more than its label, a motor pump, or a heater left on. The comparison is the quickest way to find the bill's real driver.

An efficient replacement is judged the same way. An older 1,800 W air conditioner averaging full power for 8 hours uses 14.4 units a day. A newer inverter model averaging 1,000 W uses 8 units, saving 6.4 units, or ₹44.80 at ₹7. Over 120 cooling days a year that is ₹5,376, which you weigh against the price of the new unit.

Where the saving actually is

1. 1Rank appliances by units per day. The top two or three usually account for most of the bill.
2. 2Trim hours or adjust settings on those first: AC temperature, geyser timers, pump run time.
3. 3Replace very old fridges and ACs, which have worse efficiency ratings, when the savings pay back the cost within a few years.
4. 4Switch off standby loads, which add a few units a month with no benefit.
5. 5Check your slab: if one extra unit pushes you into a higher tariff, shifting use below the boundary matters.

Cross-check the sum against your bill. The appliance estimates should add up to roughly the units billed; a large unexplained gap suggests an old appliance drawing more than its label says, or a use you forgot.