Figuring Appliance Electricity Usage

by Gary Foreman


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Dear Dollar Stretcher,
Can you tell me approximately how much it costs to run two box fans for 24 hours a day, 7 days a week? Also, what about a small high velocity floor fan and long shop style fluorescent bulbs? The barn where we have our injured horse has its electricity connected to the rental house. I paid $30 for the extra use of electricity for the fans for July. They said their bill went from $70 to $170. Of course we have been under a major heat wave and they have two or three window A/C units. But they seem to think it is our fault. Please help.
Thank you!
Carl

A landlord, tenant spat! They sure can get nasty. Fortunately, Carl can use some basic math to help find a reasonable solution to this one.

Let's begin by understanding the question. Carl will need to know two things. How many kilowatt hours each item uses and how much does a kilowatt hour cost where he lives. Once he knows that Carl can calculate how much each item will cost to operate. His answers won't be exact, but should be good enough to prevent a shouting match with his tenant.

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 a kilowatt used for one hour.

On most appliances you can find the wattage on it's nameplate. To calculate the kilowatts used by an appliance, divide the wattage by 1,000. So a 200 watt appliance would be 200 divided by 1,000 or 0.2 kilowatts. A 1,500 watt hair dryer would use 1.5 kilowatts.

Now let's see if we can figure out how much each item is using and what it costs. To do that we'll need to know the price for a kilowatt hour of electricity. The U.S. average runs about 12 7 cents per kilowatt hour. Carl can check his electric bill. It will show how much he's paying per kWh.

We'll start with the box fans. According to the Central Iowa Power Cooperative the typical box fan is rated at 200 watts. So if a kilowatt costs 12 cents per hour, the fan would cost 20% of that or 2.4 cents per hour. Extend that out to a month and it works out to $17.28 per month if it runs round the clock (2.4 cents x 24 hours x 30 days). Two fans would be about $35 per month.

Now for the high velocity floor fan. Carl will need to check the wattage. We found one that consumed 135 watts. So at 0.135 kilowatts per hour that would cost $11.66 per month if used continuously.

Carl might find that the fan is rated in horsepower (appropriate in this case!). If so, he can convert. One horsepower is equal to 0.75 kilowatts.

Next the lighting. Pacific Gas and Electric estimates that the fluorescent bulbs run about 1 cent per hour for a 4 foot bulb and 2 cents per hour for an 8 foot bulb. So if Carl has an 8 foot bulb he'd consume $24 each month. Of course, once he knows the wattage and his electric rates he can do his own calculation.

Let's total Carl's electric usage. We've got two box fans at $36, the high velocity fan at $11.66 and $24 for the fluorescent lights. Or a total of $71.66 per month.

How does that stack up to the tenant's electric usage? The Nebraska Public Power District estimates a window air conditioner (12,000 BTU size) will cost an average of $29.50 per month to operate. So three of them could easily consume $90 in a month. And perhaps much more in a 'heat wave'.

One problem with measuring the air conditioner is that it's not continually on. A 1,000 watt unit might only be running 15 minutes per hour on some days. So it's only consuming 250 kilowatts per hour. Of course in a heat wave it might be on almost continuously. And the older and less efficient the unit is, the longer it will stay each hour. So the only way to know for sure would be to watch the unit for an hour or two and notice how many minutes per hour that it's really running and then do the calculation.

The bottom line is that both Carl and his tenant are contributing to the $100 increase. Hopefully a little math can lead them to a reasonable compromise.

Updated September 2014


Gary Foreman

Gary Foreman is a former financial planner and purchasing manager who founded The Dollar Stretcher.com website and newsletters in 1996. He's been featured in MSN Money, Yahoo Finance, Fox Business, The Nightly Business Report, US News Money and he's a regular contributor to CreditCards.com. You can follow Gary on Twitter or visit Gary Foreman on Google+. Gary is also available for audio, video or print interviews. For more info see his media page.

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