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

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How many kWh does my house use in a day?

Mantas Silvanavicius
Licensed Electrician
Mantas Silvanavicius is a Licensed Electrician and the Owner of M+S Electric based in Las Vegas, Nevada. With more than 20 years of experience, he specializes in home electrical installations, testing, and wiring. Mantas and his team have completed projects for companies such as Seiko and Springhill Suites by Marriott. M+S Electric is licensed, bonded, and insured.

Mantas Silvanavicius

Licensed Electrician

Expert Answer

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.

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Question

How much electrical energy is consumed by a 30 W bulb in 5 minutes?

Community Answer

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.

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Question

If 100V is applied across a 200V, 100W bulb, what will be the amount of power consumed?

Community Answer

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.

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Question

Is 1.5 volts more or less than 20 watts?

Community Answer

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.

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Question

How many KWH does it take to turn on a 60-watt light bulb?

Community Answer

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.

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How do I calculate and compare the energy use of two different watt bulbs over the same period of time?

Community Answer

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.

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Question

If I am using a 1.5 volt battery to power a 20 watt lightbulb, what is the amperage?

Community Answer

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.

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Question

What does it means when a light bulb says "9 watts 2000 hours"?

Community Answer

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

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How much energy does a 25-watt light bulb consume in a day?

Community Answer

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.

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