How to calculate the energy cost of appliancesIt's easy once you know the formula, wattage, power cost, hours of useBy Gary Foreman
Dear New Frugal You,
I need to figure out how much it costs to run two box fans
for 24 hours a day, seven days a week. Also, what about a small high-velocity
floor fan and long, shop-style fluorescent bulbs? We've been stabling a horse
in a barn on property that we own but rent out. The tenants say that their
electric bill went from $70 to $170! I had agreed to give them $30 a month.
We're in a major heat wave, and I'm betting it's their air conditioning, but you
know how that goes. Can I prove how much electricity my fans and light are
using? -- Carl
Dear Carl,
So you want me to get in the middle of a landlord-tenant
spat? OK, but only if you leave my name out of it! Let's see if we can't help
you prove your point. I think that a few facts and a little math should help
clear things up.
Let's start by checking out how many kilowatt-hours each
item uses and how much a kilowatt-hour costs where you live.
A watt is the standard measure of how much electricity is
used. A kilowatt is simply 1,000 watts (kilo = 1,000). A kilowatt-hour (kwh) is
1,000 watts used for one hour.
Most appliances have the wattage listed on an attached
plate. You'll need to convert that wattage to kilowatts. It's simple. Just
divide the wattage by 1,000. So a 1,500-watt hair dryer used for one hour is
1.5 kwh. Burning a 250 watt lightbulb for an hour is .25 kwh.
If the serial plate doesn't include watts, you can multiply volts times amps to get
watts. Of, if the model is sold online, its specs, including wattage, can often be found there.
Now let's figure out what a kwh costs. According to the U.S.
Energy Information Administration in April 2011, the
average cost per kwh was 9.65 cents. You can find out what it costs in your area
by checking your most recent electric bill.
So how much electricity are you using? We went shopping online
and found typical box fans range from 40-100 watts. At the midpoint, 70 watts, if a
kilowatt costs 10 cents per hour, the fan would cost 0.7 cents per hour (0.07 kwh x 10 cents). Extend that out to a month and it works out to $5.04 per month if
it runs around the clock (0.7 cents per hour x 24 hours x 30 days). Two fans
would be just over $10 per month.
Now for the high-velocity floor fan. We found one rated at
135 watts. So at 0.135 kilowatts per hour, that would cost $9.72 per month if
used continuously. You can see how your fan compares.
The fluorescent bulbs are a similar calculation. To keep it
simple, let's suppose that he has a fixture that includes two 50-watt bulbs. So
they'd consume 100 watts per hour. That would work out to about $7.20 per month
(.1 kwh x 10 cents x 24 hours x 30 days).
So how much would you owe the tenants for "your" share of the power consumption? If you total them up,
it would come to $27 per month.
Now let's see what air
conditioners should cost. Depending on size, window units should cost between 7
cents and 12 cents per hour of use. Central air costs three to four times what an
individual unit does.
Let's assume that the ones that the tenant is using costs 9
cents per hour and run 12 hours per day. Each one would cost about $32.40 per
month. So it wouldn't be unreasonable for three of them to consume $100 of
electricity per month.
So who's right? I can't be sure because of all the assumptions in my examples, but if my estimates of power consumption
are close to the equipment you have, you are. Get
over there for a visit, take down the wattage of your equipment and crunch your
own numbers to be sure. My back-of-the-envelope estimate says it looks like
you're being fair, but now you have the tools to make a visit to find out for sure. By the way, as a goodwill gesture, I'd include bringing fresh
filters and a toolkit to make sure your air conditioner system is running as
efficiently as possible.
If you're interested, the U.S. Department of Energy has a page to
help you estimate the electric usage of various appliances.
Use it to help you estimate how much it costs to run some of your appliances.
One final thought: Many local power companies offer low-cost
or free energy audits to see where the electricity is going, and they suggest methods to stop it from being wasted. That sounds like
it could be a win for the electric company, you and your tenants. It lowers the
strain on the power supply, and it means less money out of the pockets of you
and your tenants. A triple-frugal play!
See related: How to reduce your electric bills, Want to cut electricity? Beware 'phantom loads'
For more than 35 years, Gary Foreman has worked to help people get the most for their money. Prior to founding The Dollar Stretcher.com, he was a financial planner and purchasing manager. Gary began The Dollar Stretcher website and newsletters in April 1996. Today the website features more than 6,000 articles on different ways to live better for less. Gary has been interviewed by The Wall Street Journal, The Nightly Business Report, USA Today, Reader's Digest and other newspapers and magazines. Gary answers a question about a budgeting or saving issue from a CreditCards.com reader each week.
Send your question to The New Frugal You.
Published: August 11, 2011
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