## Wednesday, August 19, 2009

### How bright are we?

Today we ask "How many light bulbs cost how much to run?" I read an article about saving energy by changing light bulbs. It said, "First step is to count your bulbs".

Since I have no idea how many bulbs are in my house, I'll go check while you count yours. Remember, count the bulbs, not the fixtures!

Fill in the numbers below. Add or take away rooms as necessary. Don't forget the bulbs in closets, stoves, refrigerator, freezer, microwave, dryer, garage door opener, etc. Ignore any battery-operated lights.

Here's the count for my 50-year-old house:

02 Entry
12 Kitchen
08 Dining room
08 Living room
12 Family room / Office
03 Bedroom 1
03 Bedroom 2
04 Bedroom 3
04 Bathroom 1
04 Bathroom 2
04 Bathroom 3
02 Laundry
08 Garage
03 Porch
04 Patio
01 Garden / Yard

82 Wow!

In my house, about half are fluorescent lamps. But we'll ignore that and just multiply each bulb by 60 to get average wattage. Let's imagine they are all on (yes, unlikely, but this is just for fun).

60 x 82 = 4920 watts ≈ 5000 watts or 5 kilowatts

If we ran the lights for an hour, it would consume about 5 kilowatt hours of electricity.
At the lowest rate in San Diego, the electricity cost would be calculated like this:

The baseline rate is 13¢ per kwh or 5 x \$.13 = \$.65
The highest rate is 34¢ per kwh or 5 x \$.34 = \$1.70

Let's calculate in our heads - come on, you can do it. First we round the numbers slightly for convenience since we just need a reasonable estimate. Besides, our meters may not be all that accurate (subject for another post!):

\$.65 = 2/3 of a dollar
\$1.70 = 1 dollar + 2/3 of a dollar, so 5/3 of a dollar

Daily
Low Rate (24 x 2/3) = (48/3) ≈ \$16
High Rate (24 x 5/3) = (120/3) ≈ \$40

[How easy was that? Next we'll use Associative & Distributive Properties to shift things around to make the math go faster.]

Weekly
Low rate (24 x 7 x 2/3) = (8 x 7 x 2) = (56 x 2) ≈ \$112
High rate (24 x 7 x 5/3) = (8 x 7 x 5) = (56 x 5) ≈ \$280

Monthly
Low (365 d x 24 hr ÷ 12 mo x 2/3) = (365 x 4/3) = (122 x 4) ≈ \$488

[Let's check our work (30 1/2 days x 16) = (30 x 16) + 8 = \$488 We're okay so far.]

High (365 x 24/12 x 5/3) =(365 x 2 x 5/3) = (365 x 10/3) = 3650/3 ≈ \$1217

[Checking ... (30 1/2 days x 40) = (30 x 40) + 20 = \$1220 Close enough.]

Annually
Low (365 x 24 x 2/3) = (365 x 16) = (3650 + 2190) ≈ \$5,840
High (365 x 24 x 5/3) = (365 x 40) ≈ \$14,600

Wow! Turn off those lights!

PS You can check with your calculator now. I haven't used mine til now. Honest.

My calculator says the annual amounts are \$5694 and \$14892
Mental math says the annual amounts are \$5840 and \$14600
If we add up our estimates and compare to the calculator, we have 20586 ÷20440 = 1.007
We are within one percent, overall. Isn't math fun?