The news about saving money and electricity by replacing old light bulbs is old (unless you’ve been living under a rock). But the theme of saving electricity is not yet exaggerated. Suppose every home in the US would switch to high-efficiency light bulbs (such as compact fluorescent light bulbs). This would reduce the nation’s energy consumption by 10% in the residential sector. The residential sector, by the way, accounts for about 20% of all energy use in the US That’s a lot of oil.

Still not sure about switching to high-efficiency bulbs? Don’t believe the hype about saving electricity? Don’t believe in positive impacts on your pocketbook or the environment? Do you want to calculate and test the light savings yourself? Well, let’s get to cost savings and simple recovery next. (Simple payback refers to the amount of time it takes you to recoup the cost of new bulbs from savings.)

To calculate the final result, here is the necessary information:

• The power rating (watts) of the existing bulb
• The power rating (watts) of the new bulb
• The number of hours we use the bulb every day.
• The rate we pay for electricity in kilowatt-hours or kWh. You can find your electric rate by looking at the electricity portion of your utility bill.
• One kilowatt is 1,000 watts, so we need to remember to divide our answer by 1,000 to convert to kilowatt-hours.
• The cost of the original bulb.
• The cost of the new bulb.

As an example, let’s replace a heavily used light bulb in a fixture in a living room that is on continuously for 5 hours a day. The fixture has a 100-watt incandescent bulb that costs $050. It will be replaced with a 25-watt compact fluorescent or CFL (provides incandescent-equivalent brightness), costing $2.50. Let’s assume $0.15 per kilowatt hour (kWh) for the electricity rate, the national average in the US.

To calculate cost savings, first calculate the energy use of the existing bulb, then that of the replacement bulb. Hopefully, the energy use of the replacement bulb will be less than that of the existing bulb. The difference between the existing and the new is savings. Here is the formula to calculate the cost of energy used per year:

Annual energy cost ($) = number of bulbs X watts per bulb/1,000 watts X hours of use per day X 365 days X electric rate

So for our example:

Energy cost per existing bulb ($) = 1 bulb X 100 watts X 5 hours per day X 365 days X $0.15 per kWh/1,000 watts = $27.38 per year

Energy cost per replacement bulb ($) = 1 bulb X 25 watts X 5 hours per day X 365 days X $0.15 per kWh/1000 watts = $6.84 per year

Savings per year ($) = $27.38 – $6.84 = $20.54

Here’s how to calculate the simple refund in years:

Simple payback (years): (Cost of new bulb ($) – Cost of old bulb ($)) / annual savings ($)

For our example, the simple refund is:

One-Time Refund (years) = ($2.50 – $0.5) /$20.54 = 0.1 years or 1.2 months

That’s not a bad return on investment for light savings. An average house has around 15 to 20 light bulbs. If they were all the same as the example above, that would result in a savings of about $411 per year. You can use the same method to calculate the savings for each room in your home and add up all the room savings to get a total of your annual savings.

You can check your savings by monitoring your utility bills from month to month, as long as your rates stay the same and you don’t change your bulb hours. Even with proven savings, there still seems to be objections to replacing incandescent bulbs with compact fluorescent lights (or CFLs) or light-emitting diodes (or LEDs), otherwise this would be a “done deal.”

LEDs provide even greater savings (90% light savings) and longer lifespan (25,000-50,000 hours) and will be the dominant technology in the intervening future. They are also more environmentally friendly to produce and are less susceptible to breakage or moisture. But at this point, its main drawbacks are its high price and lower light output (or lumens) compared to incandescent bulbs. However, technology is advancing very quickly, and once prices drop to reasonable levels, these problems will be a passing memory.

CFLs, on the other hand, are much more accessible and affordable, and have come a long way to closely match the light output and utility of incandescent bulbs. A repeated complaint about them is that CFLs need to warm up to full brightness, but that’s usually on the order of seconds to a minute for specialty bulbs. They are also affected by moisture and humidity.

While the cost of CFLs is still higher than a $0.50 incandescent bulb, prices have come down to affordable levels for replacements, typically on the order of $1.50 to $4.50. per bulb, depending on type. The average lifespan of CFLs is 8,000 hours (or about five years with four hours per day of use), while incandescent bulbs are rated at 800 to 1,200 hours. One thing is worth noting for daylight savings calculations. The life of CFLs decreases if they are turned on and off frequently. If you plan to install them in areas where they will be changed frequently, reduce their lifespan by 20% to 6400 hours.

What about mercury in CFLs? The amount of mercury in a CFL is 5 mg or about 1/100 of the amount of mercury in a dental filling (500 mg in a dental filling). More specifically, the mercury used by a power plant to make an incandescent light bulb is 10 mg, while for a compact fluorescent it is about 2.5 mg. However, broken bulbs should be handled with care, and burnt out bulbs should be disposed of at home centers like Home Depot and Ikea.

No matter how we look at it, saving light by replacing incandescent bulbs is one of the easiest and cheapest methods to incorporate energy efficiency and realize energy savings in the home. Many countries have begun to systematically eliminate the production of incandescent light bulbs. The economics are there, and the environmental benefits will only improve as technology advances.