All The Time Is A Long Time (And a
lotta money)
Or
Hell, it's just one light
It costs
about a dollar per watt for the electricity to operate an electrical device all
year.
So:
It costs
roughly $75 to burn a 75 watt light bulb all year.
It costs
about $150 to leave a 150 watt computer turned on all year.

If you got the message above you got the
reason for the page.
No matter what I do, this
article comes out dull, confusing and ponderous. I've
rewritten it about 5 times trying to make it read easier, and
every time it's become more ponderous. So, if you got the
message in the box above you can stop here.
If you're a glutton for punishment
plow on.

The amount of money pouring into
the air while you leave electrical gadgets on creeps up on you over time. For making
tradeoff decisions on the spur of the
moment I find it useful to be able to quickly estimate how much
electrical devices cost to operate. I
use a quick and dirty rule of thumb to figure out how much I'm paying to operate
these devices. This article also discusses & illustrates shortcuts I
use to make quick cost tradeoffs.
A device costs roughly $1 per watt if it's left
on all year. So, it costs $100 to leave a 100 watt bulb
on for a year. (Plus the cost of light bulbs  see Why
Bulbs Burn Out So Quickly). If you turn it off in the daytime it will
cost you about half that, or $50 a year.
Several of my neighbors have 2 or 3 dual 150 watt
flood lights they leave on all night. This costs them roughly 34 hundred dollars
a year.
2 bulbs x 150 watts = 300
watts. 300 watts x 2 fixtures = 600 watts
600 watts x $1 per watt = $600
Night only ==> $600 ÷ 2 =
$300 a year for 2 dual floods
left on all night
I paid an extra $10 apiece and got the motion
activated kind that only operate a fraction of the time even with the neighbor's
cat triggering them occasionally. I estimate it costs me about $30$50 a year
for both my dual fixtures (4 150 watt bulbs).
Say they're on for an hour a
day. That's onetwelfth the neighbor's on time. Say
onetenth for easy arithmetic. So, each of mine costs about onetenth of
my neighbor's $300 (or $600 for both), or about $30 a year. I saved the extra cost of
the motion activated feature in the first month of operation.
[This does not consider the issue of whether motion activated or
continuouslyon security lights give greater security. I think the
motion activated give more security, others believe the opposite.]
One of these neighbors said, "I
bought 3 because
they were so cheap. I got all three for only $36. At that price I'm
thinking about getting a couple more for my daughter." If she does
this,
they'll be
paying about $1500 a year to operate them all She'll be giving her daughter an
expensive gift  expense for the daughter, that is.
My 2 computer's are turned on 16 hours
a day, every day. Including printers and other peripherals, together they use about
500 watts. This costs me about $333 a year:
500
watts = $500
16 hours is 2/3 of the day (16 ÷
24).
2/3s of $500 is $333
per year
So, to repeat, the devices' watt rating
is a rough estimate of annual operating cost.
Part of the trick of doing these quicklook estimates is to use numbers that are easy to
do in your head. Don't try to work with 97 in your head, make it a 100. So,
97 divided by 23 becomes 100 divided by 25 or a dollar has how many
quarters  answer, 4. The whole idea here is to get in the ballpark and go from there.
I'm sure you've got the ideal
by now, but if you are having so much fun you can't stop, here are some
more examples of the way I use this.
Example A
We burn a hall light almost
continuously. I replaced a 60 watt bulb with a 15 watt compact fluorescent:
60 watts  15 watts = 45 watts savings. 45 watts times $1 = $45.
I save about $45 a year.
If you prefer to think of it in dollars rather than watts it's $60  $15 = $45.
Because it's a dollar a watt it works either way.
To continue the analysis beyond the cost of electricity:
The bulb cost about $12 and will last about a yearandahalf, so
the bulb costs about $8 a year.
(1 1/2 years x $8 = $12)
So, I save about $37 ($45$8) a year even without considering the amount I would have spent on ordinary bulbs.
The same use would have required about 13 ordinary light bulbs. That's obviously a good deal.
Usually things aren't turned on all the time so, I use the one year cost and ratio it down to estimated
time on. Using the above compact example:
If I used the light only at night I'd estimate the annual savings and divide by 2. So, my
annual savings is about $18 ($36/2)still a good deal, unless I'm planning to move soon.
AND. . .<drumroll>, I avoid changing the bulb 13 times during the life of the fluorescent.
If instead I only burn it about 4 hours a day, that's about onesixth of a year (24hrs
÷ 4hrs = 6) so I'd only save about $6 ($36/6) the first year. Since the bulb costs $12, it will take me 2 years to break even. This
is too close to call with this gross estimation method and, given all the other vagaries of life  moving, breaking the bulb, premature bulb failurethis isn't enough to warrant the extra cost of the bulb.
But, if I were still interested, I'd need to do more precise calculations.
If it's a major purchase and the rough numbers suggest it's close, I
delay the purchase 'till I've
gotten home, and taken pencil to paper (actually
Excel).
Caution About
Compact fluorescents
Caution:
Compact
fluorescents have other considerations. For example, they aren't likely to give the light output they claim. That
is, a compact fluorescent that claims 100 watt equivalence probably
doesn't give as much light as a 100 watt incandescent (regular light
bulb). If you take this reduced light output into account they may
not produce the expected savings.
Here's an article that covers this issue in
more detail. 
Example B
Assume my printer uses 60 watts when idling. If I leave it on
half the day it costs about $30 (onehalf of $60) a year for electricity.
On some days I don't print anything. Usually I only print once or twice a day for about 1015 minutes. I can save
almost the full $60 a year by turning the printer on only when I need it.
Is it worth the trouble? That's another kind of tradeoff, but at least I know
what the convenience tradeoff costs. Note: the 60 watts is a
number I pulled out of the air for the example. If your printer
has a "sleep" feature it may use very few watts when idle.
A More Controversial Example
Assume your TV uses 140 watts. If you
operate it 12 hours a day it costs you $70 a year
($140 ÷ 2) for electricity.
If by turning it offandon you cut your use to 4 hours a day, you'd save
twothirds of the use/electricity or about $47 a year ($70 X 2/3).
But, some people think turning the set onandoff shortens it's life. Assume you reduced the life of the set from 8 years to
6. Assume the set cost $400. You reduce the life by
oneforth (2 years / 8 years). Oneforth of $400 is $100.
You will have increased your cost of buying television sets by about $100
every 6 years, or about $20 a year. You will still save
$27 a
year. ($47 per year saved by turning it onandoff minus the
$20
increased cost of buying TVs.)
And, nobody seems sure
whether turning a set onanoff really shortens its life And, if
you leave it off for several hours at a time you probably lengthen it's
life.
Disclaimer 1: Instead of a dollar, the actual number in my area when
I wrote this was about 8.2 cents (Note: it's now 11 cents). My $1 is simply an easy number to remember and use for a quick and dirty assessment.
If you know your exact rate, say $1.20 or 80 cents and want to, you can windage your result more or less, based on that. I just consider close results uncertain and don't use them.
Disclaimer 2: The above calculations use deliberate
shortcuts that introduce errors in order to make them simple enough to
do quickly in your head, rather than doing nothing at all.
Some purists may say that the
varying electric rates across the nation make this inaccurate.
They're free to spend their weekends getting super accurate numbers
while you and I figure out while standing at the light bulb shelf at the
store whether that compact fluorescent will really pay for itself.
