Does Leaving Lights On Waste Electricity?

There’s a common wisdom that you should avoid turning on and off the lights in order to save on the electric bill.

Is that true? Or does leaving lights on waste electricity and cost more than turning them on and off as per your need?

And how much energy could you waste or save in each case.

These questions and more, I will be answering in this short article based on my study and my experience as an electric engineer and an Energy Manager.

Does Leaving Lights on Waste Electricity?

Yes, leaving the lights wastes electricity and costs more than what it should, compared to turning on and off the lights based on your needs.

Electric energy doesn’t come out of thin air.

As per the first law of thermodynamics, energy cannot be created or destroyed, but it can be converted from one form into another.

Therefore, you need energy in order to leave the lights on all the time, which means that you waste energy by leaving lights on when they are not in use.

How Much Electricity Is Wasted by Leaving Lights on?

The amount of electricity wasted by leaving lights on is related to the size and number of lighting units, their efficiency, and to the time during they are being left on.

To calculate how much energy is wasted by leaving the light turned on, you need to follow the following simple calculation:

The lighting units rating in kW X number of hours they are on per day X number of units = energy wasted per day in kWh

Energy wasted per day X 30 (or 31) = Energy wasted per month in kWh

For example, let’s say that you have one incandescent lamp that has the rating of 150 W (0.15 kW), and you leave on, when not in need for it, for two hours a day.

The energy that is lost per day on that lamp is 0.15 kW X 2 hour/day X 30 days/month = 9 kWh per month.

There is also the energy lost in the wiring that powers that lamp, but let’s neglect it compared to the energy waste of the lamp itself.

How Much Does it Cost to Leave Lights on?

The additional cost for leaving the lights on is related to the total energy wasted, and the rate of the electricity supplier.

Energy wasted per month in kWh X energy tariff in $/kWh = Wasted money per month in $

Continuing with the previous example, the money you lose for wasting this 9 kWh per month depends on how much the utility company charges you per kWh consumed, which is different from one place to another and from one utility company to another.

For example, the average rate in Texas is around 11.39 cents/kWh.

Therefore, the additional money wasted per month would be 9 kWh X 11.39 cents/kWh = 102.5 cents a month, or $1.02 a month.

While if we calculate the cost based on a different area, like NYC for example, where the average rate is around 18.27 cents per kWh, then the cost would be:

9 kWh X 18.27 cents/kWh = 164 cents a month, or $1.64 a month.

And while that might not seem too much, but if you have the habit of leaving the lights on when unnecessary, then you probably do this with many lighting units, and for several hours a day, without being conscious about it.

Few more lights inside the place, in addition to the lighting units outside that you might leave on for an extra two hours every morning, the lights in the attic or in the basement (which might be left on for days!), …etc.

The numbers can add up so quickly in this case.

Assuming that you have a total of 15 lights with the rating of 150 W, that you leave on for an extra two hours every day, and the cost would go up to reach $15-$25 or more per month, depending on the electricity rates.

Another factor that might affect the cost of wasting energy by keeping the lights on is the billing policy of the electricity company, as some companies charge different rates based on the timing of consumption (day/night tariff).

In such case, you will pay for the price of the wasted energy based on the rate at the hours during which you kept the lights switched on.

Does Turning off The Lights Save Money?

Sure, turning off the lights will save you money that you could use to spend on different stuff.

You should always try to turn off the lights when they are not in use.

How Much Energy Does Turning off Lights Save?

The amount of energy and money you save by turning off lights is the same amount that you would lose if you keep them on without being in need for them.

As per the example in the previous section, when you turn off the one lamp of 150 W when its not useful for you, then you save the whole 9 kWh per month, which means that you save around $1.02-$1.64 per month for that lamp alone.

And assuming that you have fifteen lights of the same size that you keep switched on for two extra hours per day, then you could save around $15-$25 per month by turning off those lights when not in use.

Does Turning Lights on and off Waste Electricity?

No, turning lights on and off does not waste electricity.

There’s a common misconception that makes many people think that you waste a certain amount of energy if you turn on and off the lights on frequent basis, due to the inrush current that goes high at the beginning of the operation of certain kinds of lights, like the incandescent lamps.

Which is why people think that they need to keep the lights on all the time.

But this is not the right thing to do, because the inrush current lasts for few milliseconds or even seconds, and the energy wasted by it is much less than the energy wasted by keeping the lights on all the time.

Therefore, switching lights on and off does not waste electric energy, and doesn’t cost more money.

6 Tips to Help You Avoid Leaving Lights on

Ok, now you understand that you should be turning the lights off when they are unnecessary, but sometimes that could be impractical.

For example, you could have a corridor that is used frequently by more than one person, and they keep forgetting the lights there on.

Or maybe, you have a workplace or a cafe where people leave the lights in the bathrooms on.

Or probably you don’t wake up early enough to turn off the outdoor lights when the sun is already up.

But there are some solutions for such cases, which I will brief here:

  1. Use a photocell sensor to automatically turn on and off the outdoor lights based on the availability of natural light.
  2. Use solar powered external lights with a built-in rechargeable battery and photocell.
  3. Install motion detectors to the lighting circuit in places with frequent, but short use, such as corridors, kitchen, bathrooms. (Image below)
  4. Use dimmers if you want to keep the lights on in a certain area, but no one will be there, or maybe you need the light, but not in full capacity.
  5. Switch to LED strip and bulb lights, which are much more efficient than incandescent and fluorescent lamps. (Some LED bulbs come with a built-in motion detector)
  6. Install a timer on a certain lighting circuit where you know that the lights will be necessary for a certain amount of time, like in stairwell, or in the basement or attic.

Turn Lights Off Energy Saving Devices

Conclusion – Should You Turn off The Lights?

Sure. Do your best in turning off the lights whenever you are done with them in order to save electric energy and money.

Ignore the myth the says that turning on and off the lights cost more money, that’s false information.

And use any of the tips regarding the devices that can lower your electric bill in the previous section above in case that you cannot fully control the lights because you keep forgetting them on, or because of others’ behavior.

I hope that you found here the answers you were looking for when you asked if leaving the lights on wastes electric energy.

If so, please, feel free to share this article with others who might benefit from reading it.

If you still have any question, or if you need help, then please, feel free to ask me in the comments’ section below, and I will do my best to help you out 🙂

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