# Money Saver: Zapping the Electric Bill

I am excited to have my husband guest posting today on electricity use, as he is the physics expert around here.

TSW and I were sparked into a conversation this past weekend about our electricity usage.  We were notified that, because Sears underestimated the power rating of the chest freezer we bought last summer, we get $11.52 per year in return for the extra power it consumes. On 13 June 2026, we will have officially received a free freezer from Sears. It’s like winning the lottery. Considering the extra 96 cents per month we were spending keeping our food cold, we were curious where the 356 kWh we bought last month went. While a quick search on our favorite engine revealed a few websites that tell you what your stuff typically uses, it seemed like a good opportunity to use our high school physics. (College physics should give the same results.) Besides, the black box of a website from the ’90s was too much for me, when our favorite vacuum cleaner clearly states “12 amps” in one inch numerals on the front. It was asking for a calculation, and TSW was curious if she should cut down on her vacuuming. So that physics I was talking about. (The lesson ends after a paragraph.) In electronics, the power delivered to a component is equal to the products of the electrical current flowing through and the voltage across that component. That is, $P=I \cdot V$. We knew the current going through our vacuum cleaner was 12 amps. The voltage at a standard household wall socket is regulated to be 120 volts (give or take 6 volts) RMS in the U.S. (For those of you in the E.U., that’ll be 230 volts RMS. Not surprisingly, Wiki has an excellent map of standard electric mains voltages for every country, if you’re curious.) The product of amps and volts is watts, which is a measure of energy usage per unit time: joules per second. In the case of our vacuum, this means that it is running at $P = 12 \textrm{A} \cdot 120 \textrm{V} = 2520 \textrm{W}$. If you aren’t into physics, you’ll at least notice that watts makes up a third of kilowatt-hours, which is what you see as your electricity usage from the power company. To get to kilowatts, we divide by 1000, so we’re vacuuming at 2.52 kW. To get the hour part, we simply multiply by however many hours the vacuum is being used. Thus, if TSW vacuums for one hour, she will have used up 2.52 kWh of electricity. (Note that the units of watts are energy per time and we multiply by a time, so the unit kilowatt-hour is simply energy. Also note that the unit kilowatt-hour is abbreviated kWh. Not kwh. Apparently kW h or kW-h are appropriate.) We supposed, in a conservative estimate, that TSW and I might vacuum a combined total of two hours per month, which puts us at 5.04 kWh used per month. Given the going rate from our provider of$0.116 per kWh, we use a quarter’s worth of electricity per month vacuuming.  That is,

$\text{Cost per month} = \frac{\text{amperage} \cdot 120}{1000} \cdot \left( \text{hours used per month} \right) \cdot \left(\text{cost of energy per kWh} \right)$.

For our vacuum, that becomes

$\text{Cost to vacuum, per month} = \frac{\text{12} \cdot 120}{1000} \cdot \left( \text{2} \right) \cdot \left(\text{0.116} \right) = \0.26$.

I convinced TSW that cutting down in this area was likely not necessary.  (Physics lesson ends here.)

The conversation quickly turned to curiosity about where else our energy costs goes.  The 356 kWh from our last month cost us $41.34. A good chuck of this is likely from our refrigerator, although we’re not sure how much it uses. Likewise, the chest freezer is perhaps several dollars ($5.01 if it runs half the day).  Given that the freezer started all this, and that Saturday happened to be the first day of this billing cycle, and that Saturday happened to be the day our freezer decided to stop freezing things, we decided to move all the food to the freezer above the fridge, turn off the chest freezer, and see how much we’d save in our electric bill. [Andrea would like to mention that we just recently got room in the upright freezer, since it is no longer full of baby food and milk.] Then we wondered what other things cost:

Thing Hours per Day Cost per Month
3 x 60 W light bulbs (our dining room light) 6 $3.76 75 W light bulb (our kitchen main light) 4$1.04
20 W light bulb (if we used a flourescent bulb in the kitchen) 4 $0.28 Our laptops (95 W at max power each, 45 W average) 5$1.57

We decided to keep using our computers, but the kicker was the savings for the flourescent bulbs.  For a light that we use often, we save about $0.75 per month by switching to CFL bulbs. On electricity alone, that saves us$8.00 over the year.  Not to mention that the CFLs I stuck in our side table lamps on 29 September 2009 are still burning strong, while I’ve replaced most of the incandescents in our apartment.  This year.  Twice.  With that, we traded as many incandescents for CFLs that we had bought but never installed.  (Note: CFLs are not compatable with dimmer switches, as is in our dining room.)

So with turning off our chest freezer and trading out several lightbulbs for CFLs, we’re anxious to see what sort of energy savings we’ll see in next months bill.  Being two scientists, we recognize the experiment is not terribly controlled.  But we’re hoping for a drop anyway.  Our predictions:

 TSH -$6.13 TSW -$8.00

We’ll report back in a month with the results.

What is the biggest electricity consumer in your home?  In what ways have you tried to cut costs or simply save energy?  Do you know how to center tables in WP?