LED energy savings:
watts saved, hours used and the kWh they add up to

The saving is watts removed times time

An electric lamp uses energy in proportion to its power and the time it is on. When you replace it with a more efficient one that gives the same light, the difference in power, multiplied by the running time, is the energy you stop using.

The same lamp saves 80% of its energy in this case, since the LED draws 20 W compared with 100 W, and the saving scales exactly with the hours.

Turning kWh into money

To put a price on the saving, multiply by the cost of a unit of electricity. Tariffs differ by state, supplier and slab, and change over time, so use the figure from your latest bill instead of an assumed one. The example below uses ₹8 per unit only to show the working.

For a household, the figure depends on how many bulbs are in heavy use. Lights that run for hours, such as in a living room, a shop or a corridor, give most of the benefit, while a store room lit for minutes gives almost none.

Adding up several lamps

Because the sum is linear, you can add the savings of different groups of lamps together. The table lists a few groups with different usage.

The shop front dominates, with four lamps at ten hours a day, even though each lamp saves less per hour than the living-room bulb. That is why running time is as important as the wattage drop when you decide what to replace first.

Match the light, not the old wattage

Old wattage labels measured how much power a lamp drew, and people learned to treat them as brightness. LEDs break that link, so replacements should be chosen by lumens. A 100 W incandescent lamp gives roughly 1,500 lumens, and an LED of about 15 to 20 W can give a similar output, depending on its efficacy.

Always check the lumens on the pack and the colour temperature, since a lamp with matching wattage but lower output saves energy only because it gives less light. Using the lumens figure also shows how much the efficacy has improved, which is the true source of the saving.

Payback on the purchase price

An LED costs more than a basic incandescent lamp, so a sensible check is how long the saving takes to repay the difference. Divide the extra cost by the monthly saving.

Prices change, and a lamp that is used for only a half-hour a day will pay back much more slowly. LED lamps also last far longer than filament lamps, so there is a replacement saving on top of the energy one, though lifetimes claimed on packaging depend on the temperature and the quality of the driver.

A whole-home estimate

To get an overall figure for a home, list the lamps in each room with their wattage, the replacement wattage and the hours. Multiply each line, add them up, and then apply the tariff.

A saving of that size is visible on a bill, but it is the upper end for a single home because few households run ten 100 W lamps for six hours a day. Use your own counts and hours, and treat the answer as an estimate that depends on habits as well as hardware.

What can reduce the saving

The formula assumes that the new lamp delivers about the same light as the old one and that the hours do not change. Neither is always true.

• Choose an LED of matching lumens, not just matching base; a dim replacement may lead people to add more lamps.
• People tend to leave efficient lights on longer; if hours double, the saving doubles too, but the bill does not fall as much as expected.
• Dimmer switches and control gear designed for filament lamps may not suit LEDs and can shorten their life.
• Heat given off by an old lamp helped to warm a room in winter, which matters in cold climates but increases the cooling load in hot ones.
• Standby power of smart bulbs is small but not zero.

An energy-use tool for LED lamps and an appliance cost tool can speed up the comparison of several candidates, but the underlying sum is only wattage difference times hours.