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Why doesn’t watts = amps * volts?

Recently a post was made on a group I subscribe to touting the amazing usefulness of a webpage that calculates amps * volts = watts for you. All well and good (if a bit simple), but I commented that the basic premise, although good for dc, was flawed with ac. This caused great consternation in the group, as if I said the Pope wasn’t Catholic. I then explained that if you have a 1200w resistive load at 120vac, then yes, 1200 / 120 = 10a. But, and it’s a BIG but, if the load is a reactive load (motors, computers, fluorescent lights, etc.), a fourth entity comes into play, called Power Factor.

In this case, lets say a motor has a PF of .5

Now things look very different. We have our 1200 watt load / 120 vac = 10 amps. Now divide the 10a by the power factor .5, and we get 20 amps. That’s a much heavier load, requiring heavier wiring, and a bigger generator or inverter. This may help explain why some folks have problems with some loads and not others, that go beyond simple start up surges.

For more info, see


The Homeowner’s Handbook to Energy Efficiency: A Guide to Big and Small Improvements

The Homeowner’s Handbook to Energy Efficiency: A Guide to Big and Small Improvements

In The Homeowner’s Guide to Energy Efficiency, John Krigger and Chris Dorsi help homeowners set realistic personal goals for reducing their energy consumption. Their methods for making homes more energy efficient will also improve comfort, safety, durability, and resale value. They guide readers through the process of assessing current energy usage and predicting the benefits and estimating the costs of remodeling options. With projects ranging from simple fixes to large-scale renovations, this book offers solutions for the energy-conscious homeowner, regardless of budget, technical ability, or time.

A. T. Sterner says:

I am an energy auditor and I spend much time educating people about energy efficiency, conservation, products, building science, and more. This book is a life and time saver! Since it became available, I’ve been able to point folks to it and have saved myself lots of time. The book is very easy to understand, very thorough, with good graphics. Everyone who owns a home should have this book. And, everyone who is wanting to reduce their energy use and carbon footprint, AND fix their home should have this book.

About the Author
John Krigger is a highly respected authority in the field of energy conservation for buildings and the author of several books. He has been a passionate advocate of efficient construction technology for over thirty years. He is now working with other building scientists to develop the next generation of construction techniques for North America. Chris Dorsi has spent the last thirty years fine-tuning homes across North America. He has founded construction companies, developed real estate projects, and audited thousands of residential buildings. He is a widely acclaimed author and speaker who has motivated and enabled both homeowners and trade professionals to improve the efficiency and minimize the environmental footprint of their buildings.