kWh usage:
from appliance watts to a monthly unit count

What a kilowatt-hour really is

An electricity bill is charged in units, and one unit is one kilowatt-hour (kWh). That is the energy used by a 1,000 watt appliance running for one hour, or by a 100 watt bulb running for ten hours. It measures energy, not the rate of use.

The confusion between kW and kWh causes many errors. A kilowatt is a rate, like speed in km/h. A kilowatt-hour is an amount, like the distance covered. A 2 kW heater is always a 2 kW device, but it consumes only 1 kWh if it runs for half an hour.

Appliance labels give power in watts, so the first step is to divide by 1,000 to get kilowatts.

The calculation in three steps

1. 1Read the wattage from the label, or use a plug-in meter for the real running figure.
2. 2Convert to kW by dividing by 1,000.
3. 3Multiply by the hours used per day and by the days in the billing period.

To turn units into money, multiply by the tariff per unit. Tariffs differ by state, by consumer category and by slab, with higher rates for higher consumption, so check your distribution company's current rate card rather than relying on a single number.

Worked example: four appliances over 30 days

A small flat has one fan, five LED lamps, a water heater and a refrigerator. The figures below use plausible label values.

The shares are instructive. The heater uses 32 percent of the total in only half an hour a day, the refrigerator 35 percent because it never stops, and the fan 24 percent. The five lamps together use only about 9 percent. Replacing bulbs rarely saves as much as shortening heater time.

Typical running watts and how much a day costs in units

Air-conditioners and heaters dominate any home's usage. An AC does not draw its full wattage continuously, because the compressor cycles on and off, so the true consumption of the example above could be lower. A star rating or a plug-in meter tells you more than the headline figure.

Reading a label correctly and allowing for duty cycle

The wattage on a label is the maximum or rated draw. Fridges, ACs and washing machines run at that level only part of the time, so their daily energy is better read from an energy label, which states annual kWh under test conditions. Dividing the annual figure by 365 gives a daily average.

Devices with motors or heaters controlled by a thermostat have a duty cycle. If a 1.2 kW compressor runs for 60 percent of the time, the average power is 0.72 kW. Over 8 hours this is 5.76 kWh rather than 9.6.

Standby draw adds up silently. A set-top box and TV using 5 W in standby 24 hours a day consume 5 ÷ 1,000 × 24 × 30 = 3.6 kWh a month, which is not nothing.

What an efficient appliance saves

Once you can price a unit, an upgrade becomes a simple comparison. Take the 75 W ceiling fan used 10 hours a day for 30 days: 0.075 × 300 = 22.5 kWh a month. A modern energy-saving fan of 28 W doing the same work uses 0.028 × 300 = 8.4 kWh, a monthly saving of 14.1 kWh.

Over a year that is 14.1 × 12 = 169 kWh, which at an assumed ₹7 per unit is about ₹1,184 on a single fan. Divide the extra purchase price by the yearly saving to get the payback period in years, and compare it with the expected life of the appliance.

• Savings are largest where the appliance runs the longest: fans, refrigerators, lighting that stays on, and always-on routers.
• For intermittent loads such as a mixer or iron, efficiency barely changes the bill.
• Star labels compare models of the same type; a 5-star AC still uses far more than a 1-star fan.

Behaviour changes cost nothing. Shortening geyser use by ten minutes a day trims 2 kW × 10 ÷ 60 × 30 = 10 kWh from a month's total, which is comparable to the saving from replacing the fan.

Using the total to check a bill and to plan

Add the appliance figures and compare them with the meter reading difference over the same period. A gap of 10 to 15 percent is normal because of appliances you forgot and estimates in the hours. A much larger gap points to a hidden load, an old appliance drawing more than its label, or a billing error worth asking about.

When planning solar or an inverter, the daily kWh figure is the starting point. Divide the monthly total by 30 to get an average of about 3.1 kWh per day for the example flat.

• Rank appliances by kWh per month, not by watts.
• Reduce the heaviest first: cut heater hours, set AC thermostats higher, defrost the fridge.
• Check the tariff slabs, because a lower total may drop you into a cheaper slab and the saving is larger than the units alone.