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How to Calculate Your Light Savings From Replacing Incandescent Bulbs

The news about money and light savings by replacing old bulbs is old news (unless you have been living under a rock). But the topic of light savings is not overdone yet. Suppose every household in the US would switch to high-efficiency light bulbs (such as compact fluorescent 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 the energy use in the US. That’s a lot of oil.

Still not sure about switching to high-efficiency light bulbs? Don’t buy the hype about light savings? Don’t believe the positive impacts on the pocketbook or the environment? Want to calculate and test the light savings for yourself? OK, let’s tackle the cost savings and simple payback below. (Simple payback refers to the amount of time it takes for you to make back the cost of the new bulbs from the savings).

To calculate the bottom line, here is the information needed:

  • The wattage rating (watts) of the existing bulb
  • The wattage 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 must remember to divide our answer by 1,000 to convert it to kilowatt-hours
  • The cost of the original bulb
  • The cost of the new bulb

As an example, let’s swap out a highly used light bulb in a fixture in a living room that is on continuously for 5 hours per day. The fixture has one 100 Watt incandescent bulb which costs $050. It is to be replaced with one 25 Watt compact fluorescent or CFL (provides the equivalent brightness of the incandescent), costing $2.50. Let’s assume $0.15 per Kilowatt Hour (kWh) for electricity rate, the national average in the US.

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

Yearly Cost of Energy ($) = 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:

Cost of Energy for 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

Cost of Energy for 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 is how to calculate the simple payback in years:

Simple Payback (Yrs): ( Cost of new bulb ($) – Cost of old bulb ($) ) / yearly savings ($)

For our example, the simple payback is:

Simple Payback (Yrs) = ($2.50 – $0.5) /$20.54 = 0.1 years or 1.2 months

That is not a bad return on investment for light savings. An average house has about 15-20 light bulbs. If all of them were 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 house, and add up all the room savings to get a total of your yearly savings.

You can check your savings by monitoring your utility bills from month to month, provided that your rates stay the same and you do not change the hours of operation on the bulb. Even with proven savings, there still seem 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 (light savings of 90%), and longer lifespans (25,000-50,000 hours) and will be the dominant technology of the intermediate future. They are also more environmentally friendly to produce, and are less susceptible to breakage or moisture. But at this point, their major drawbacks are their high price and lower light output (or lumens) as compared to incandescent bulbs. The technology is advancing very rapidly though, and once prices drop to reasonable levels, these issues will be a passing memory.

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

While the cost of CFLs is still higher than a $0.50 incandescent bulb, the prices have come down to affordable levels for replacements, typically on the order of $1.50-$4.50 per bulb, depending on the type. The average lifespan of CFLs is 8,000 hours (or roughly five years at four hours per day of usage), whereas incandescent bulbs are rated for 800-1,200 hours. One thing is worth noting for light savings calculations. The lifespan of CFLs decreases if they are of switched on and off frequently. If you plan on installing them in areas where they will be switched often, then lower their lifespan by 20% to 6,400 hours.

What about the mercury in CFLs? The amount of mercury in a CFL is 5 mg or about 1/100th of the amount of mercury in one tooth filling (500 mg in tooth filling). Even more to the point, the mercury used by a power plant to manufacture an incandescent bulb is 10 mg, whereas for a compact fluorescent it is roughly 2.5 mg. None-the-less, broken bulbs must be handled with care and burnt bulbs should be disposed of at home centers such as Home Depot and Ikea.

No matter how we look at it, light savings from replacing incandescents is one of the easiest and cheapest methods of incorporating energy efficiency and deriving home energy savings. Many countries have started to systematically phase out the production of incandescent bulbs. The economics is there, and the environmental benefits will only improve as technology advances.