Q&A for How to Calculate Kilowatts Used by Light Bulbs

Take a reading of your electric meter and write down the numbers. The next day (at the same time), take another reading of your meter and record the numbers. Then, compare the difference increased between both days—that will give you an average of what your house used in the last 24 hours.

A one watt bulb consumes 1 joule of elecrical energy every second. To calculate the total amount of energy consumed in your example, multiply (1 joule/second) x (30 watts) x (5 minutes) x (60 seconds/minute) = 9000 joules. Joules are a very small unit of energy, useful for problems like this one with a small unit of energy. Energy companies use a unit called the kilowatt-hour instead. 9000 joules = 0.0025 kilowatt hours.

This formula is complicated for light bulbs, since the amount of heat produced changes the properties of the light bulb filament. From a practical standpoint, the bulb will be extremely dim. It's best to use a circuit as close to the bulb's rated voltage as possible. Using a higher voltage is even worse, since the bulb could overheat or explode.

They measure two different things, so you cannot compare them directly. The watts are a measure of power, and that's what you need to calculate kilowatt-hours and electricity costs. Voltage is a property of the battery or household electrical circuit. If "1.5 volts" is printed on a bulb, that bulb is designed to be run off a 1.5 volt battery.

A 60-watt incandescent lamp will require 60 watts : 120 volts = .5 amperes after turning it on, but at first, it will require 12 - 16 times that amount, about 12 x .5 amperes = 6 amperes for a fraction of a second. To convert it to KWH will be 6 x 120 = 720 watts x about .1 seconds, which will give you .072 KWH.

1 KWh = 1000 W over a period of one hour. A 60 W light bulb over a period of one hour = 0.06 KWh. 100 W light bulb over a period of one hour = 0.1 KWh.

Amps = Watts / Volts (or Current = Power / Voltage), so the current in your circuit is equal to (20 watts) / (1.5 volts) = about 13.3 amps. Note that you do not need to know the current for any of the calculations in this article.

The number of hours is an estimate of the bulb's lifespan. It does not affect the kilowatt-hour calculations.

Depends on how long it's on. It consumes no energy if it's not on. But if you have it on 24/7, it would consume 18kwh per month. or $1.98/mo if being charged 11 cents per kwh.