By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
Dear Mike (aka J.A.M.),
We had our RV sitting in the driveway over the winter and ran a space heater during the really cold months so we didn’t have to totally winterize it. But I just looked at our home electric bill for those months, and YIKES it went up a lot. So just how much does is cost to run a portable electric space heater and how does the power company calculate it? – Wanda K.
Great question, so it’s time to bust out a little arithmetic. Now don’t worry as it’s really simple, just multiplication.
First, every appliance is rated for the amount of wattage it draws from the power line. So a 100-watt light bulb draws 100 watts, a 1,200-watt space heater draws 1,200 watts, and a 1,800-watt hair dryer draws 1,800 watts. Seems obvious, doesn’t it? For one of my expanded articles on the typical wattage draw of appliances, go HERE.
Now, the power company doesn’t charge you by watts, they charge you by something called kilowatts, which is simply thousands of watts (or kW for short). So a 100-watt bulb uses 0.1 kW of electricity, a 1,200-watt heater uses 1.2 kW, and an 1,800-watt hair dryer uses 1.8 kW. With me so far?
That power usage is what we see as a spinning dial in an old-school electric meter. So it should be obvious that if you run an appliance 10 times longer than you did last month, the dial is going to spin 10 times as many revolutions, and the power company is going to charge you 10 times as much. And indeed they do. They charge you not only by kW (kilowatts) of usage (how fast the dial spins), but also by how many hours you use it (the total number of revolutions). What they’re actually metering is something called kilowatt hours (commonly abbreviated as kWh most of the time). You should find a notification on your monthly electric bill for a kWh rate, which let’s assume is 10 cents, or 0.10 dollars per kWh. The more hours you run a space heater, the more the power company is going to charge you.
Now let’s put it all together and see what a space heater could cost to run. Let’s assume it’s a 1,200-watt heater, so that’s 1.2 kW. And let’s assume it runs 50% of the time to keep your RV up to the desired temperature (that’s what we call a 50% duty cycle). Since there’s 24 hours in the day, that’s 12 hours of actual running time per day. And if you multiply 12 hours (of running time) times 1.2 kW (1,200 watts) you come up with 14.4 kilowatt hours per day. If we then multiply 14.4 kWh per day times 30 days in a month, that works out to 432 kWh of electrical usage per month. Now all we have to do is multiply 432 kWh times the 0.10 dollars per kWh the power company is charging you. That’s 14.4 kWh x 30 days x 0.10 dollars per kilowatt hour equals $43.20. So running a 1,200-watt space heater that’s on 50% of the time could add around $43 per month to your electric bill.
Here’s the math in a simple string. Multiple the kWh of the appliance times the percentage of time it’s on per day, times the hours in a day, times the days in the month, times the kWh rate charge. Of course, since there’s 720 hours in a 30-day month (24 x 30 = 720), you can simplify this as 1.2 (kilowatts of power usage) x .50 (50% duty cycle) x 720 (hours in the month) x 0.10 (dollars per kWh), which is 1.2 x .50 x 720 x 0.10 = $43.20
So a single 1,200-watt electric space heater in your RV or basement can easily add $43 (each) to your monthly electric bill. And that adds up quick.
While this appears to be a lot of math, mathematics is really a wonderful tool to help us figure out our daily lives, not just some abstract science to determine how fast the universe is expanding (although it’s really great for that kind of thing as well). And I admit to hating math in grade school until I started taking high school physics and collage engineering classes. Then I began to see everything around me as a bunch of equations to be solved.
Pencils down, as that’s enough math for today. But I hope this has helped you determine when it’s cost effective to run a space heater, or even a bunch of light bulbs. And doing basic arithmetic is great for your brain, much as doing crossword puzzles or playing Sudoku is good for you. Math is indeed power.
Okay, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.