I’ve been making a statement for a long time that sometimes gets called out as inaccurate. The story began in the second month of this blog back in April 2010 when I wrote an article titled When Is 100% Efficient Not Good Enough? I’ve been claiming that when you turn electricity into heat (think toaster), the energy conversion is 100 percent efficient. But is it really?

Efficiency vs. COP

Sometimes this critique comes up when I contrast getting heat from electric resistance (aka strip heat) with heat from a heat pump. I discussed this a bit in my article on the problems with electric resistance auxiliary heat in a heat pump. And I acknowledged that it drives some people crazy:

The reason talking about the efficiency of strip heat and heat pumps drives some people crazy is that it’s the wrong quantity. We really need to talk about coefficient of performance (COP), which is defined as the ratio of the amount of useful heating provided to the amount of energy used to provide that heat. So technically, I should say electric resistance has a COP of 1 and a heat pump has a COP of about 3.

The real problem here, though, is with heat pumps, not electric resistance heat. And yeah, that conversion does happen at pretty close to 100 percent. In an article I wrote about what happens to “used electricity” from all the electrical devices in a home, I said,

Some of the electrical energy gets converted to heat immediately. The rest gets converted to whatever form of energy is needed for the purpose of the device or appliance. It’s mostly motion, light, or sound for the things that aren’t meant just for providing heat. Eventually … those intermediate forms of energy used in your home become heat, too.

We can refer to that energy conversion process in terms of COP or even annual fuel utilization efficiency (AFUE), which is how it’s given on electric furnaces. But I contend that there’s nothing in the terminology to prevent us from saying electric resistance heat is 100 percent efficient.

Site energy vs. source energy

For most people, talking about energy efficiency means talking about what happens after the energy arrives at the house.  But the efficiency picture is bigger than that. I covered this topic in some detail in my book. Here I’ll give a brief overview, beginning with how I define the terms in the book:

Site energy: This is the energy used within the boundary of the site. For electricity, it’s the kilowatt-hours you get billed for. For gas, it’s the amount that flows through your gas meter. For propane, it’s the amount used from your tank. Each fuel delivered and used at the site counts toward the total site energy …

Source energy: This is the energy used on-site plus the energy that went into getting that energy onto the site.  For electricity, it’s the kilowatt-hours you get billed for plus the kilowatt-hours of energy that are “consumed in the extraction, processing, and transport of primary fuels such as coal, oil, and natural gas; energy losses in thermal combustion in power generation plants; and energy losses in transmission and distribution to the building site.”^†

I think you probably see the answer now.

So is it 100 percent efficient?

As with most things in building science, the answer is … it depends! If we’re talking about site energy, the answer is yes.  There’s little to no loss in the conversion of electricity to heat in your home.

But it’s different with source energy. There are significant losses of the energy that went into making the electricity delivered to your home via the electrical power lines. And that means the answer is usually no when you consider the big picture of source energy.

Now there’s good news here too. Electricity is getting cleaner and more efficient every year.  The less we rely on fossil fuel power plants, the more efficient our source electricity becomes. If you’re generating your own electricity from photovoltaics and using it for electric resistance heat, you can say that your strip heat is 100 percent efficient in terms of both site and source energy.

And that takes you to the edge of the next rabbit hole: embodied energy.

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Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.

 

^†A Common Definition for Zero Energy Buildings, U.S. Department of Energy, 2015