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Saturday, July 9, 2016

Calculating energy cost of lamps and appliances

Conserving energy benefits the planet, while also conserving costs.  With this in mind it is helpful to be able to calculate how much money one saves by replacing an inefficient appliance or lamp with an energy efficient one.  For this example we will look at replacing an old 100 Watt incandescent lamp (which are being phased out around the world) with an equivalent 13Watt 800 Lumen CFL (Compact Florescent Lamp) and another equivalent brightness 10W LED lamp.

First, let's review the three salient electrical properties we are looking at here.  Volts which for the sake of this discussion is the available Voltage at a standard electrical outlet in the US which is 120 V.  Amps is the amount of current, and Watts is the product of Volts and Amps (W = V x A).


13W 800 Lumen CFL
In order to calculate cost, we first need to figure out how much power we will be billed for on a monthly basis.  Electrical power is billed by the kilowatt hour (kWh) which means 1000 W per hour.  Or another way of looking at this would be the equivalent of a 1000 W heater turned on for one hour.  The average cost per kWh in the US is approximately $.10, however that rate varies significantly from state to state and region to region.  Here in Maine we are paying approximately $.15/kWh.  It is also worth noting that the percentage of renewable energy sourced electricity varies significantly.  In New England we have the RGGI (Regional Green house Gas Initiative) that mandates a minimum amount of renewably sourced electrical power.  In Maine about 30% or our power is renewable sourced much of which comes from Canadian hydro, with the remainder sourced from biomass, wind, and solar in-state.

So let's start with an old-fashioned 100 W light bulb and assume that it is on in your living room for 6 hours a day.  To calculate how many Watt hours (Wh) are used per day we simply multiply:

100 Watts X 6 hours = 600 Wh

Which can be expressed as .6 kWh
Since we are billed by the month, we can multiply this by 30 to determine how many kilowatt hours per month we will be billed for:

.6kWh X 30days = 18kWh/month

To determine our monthly cost for this lightbulb, we simply multiply by the electric rate, so let's assume the US average of $.10:

18kWh X $.10 = $1.80/month

That doesn't look too bad does it?  But if you run the math for an energy efficient compact fluorescent lamp you get the following:

13W X 6hrs = 78Wh

.078kWh X 30 days = 2.34kWh/month

2.34kWh X $.10 = $.23/month

So by replacing that old style lamp with a compact fluorescent you would be saving $1.57 every month while also reducing fossil fuel emissions required to generate power for this electricity.  If you do the math for a 10 W LED lamp it comes out at $.18 per month with a net savings of $1.62 every month.  These lamps are so inexpensive now that you will recoup the investment within 5 months and since LED lamps last significantly longer than incandescent lamps it is a gift that keeps giving both to your wallet and to the planet.  Also, LED lamps contain no mercury.
9.99W 800 Lumen LED lamp
If like me, you have been using CFLs for years and are now in the process of upgrading to slightly more efficient LED lamps, please be sure to recycle the CFLs responsibly since they contain an average of about 4mg of mercury.  Lowe's stores have bins where you can return CFL's and also rechargeable batteries and plastic bags right next to their returns counter.
Recycling bins at Lowe's - next ro returns counter
So next time you go shopping for any electrical device from a lightbulb to an appliance.  You should start by looking at the label on the appliance you are replacing to see how much power it uses and do the math to figure out what it is costing you now and how much energy the new one will save.  If the nameplate on your appliance only shows the power in Amps, then just multiply it by 120 V to get Watts.  

Newer appliances are often more efficient.  For instance, it is generally assumed that if your refrigerator was made before 2000 that newer ones will be significantly more efficient.  So you will not only be saving money, you will also be doing the planet a favor by reducing CO2 emissions from the fossil fuels used to generate electricity.