Gas or Electric?

March 2024

Greetings.  Before I carry on, I want everyone to know that I am no expert on HVAC systems, but I have an engineering degree, so I think I am qualified to talk about energy efficiency.  We decided to upgrade the HVAC on our house.  It's a "gas pack" which uses a heat pump in the summer and burns natural gas in the winter.  Even though it was only 15 years old, every winter for the past three it failed one way or another to light up the gas burner reliably.  It is good to have an honest service tech that knows how to fix melted wires and replace controller boards.  Thanks, Juan!

For a replacement we considered the choice between a new gas pack, or a heat pump.  Initial cost and cost of operation are not the two most important things, I am more interested in reliability and energy efficiency.  If there is anything I can do to lower my carbon footprint I usually do it.  I am an avid recycler, saving all aluminum, glass and  PTFE containers, even scrap metal from remodeling, from the landfill. As a landlord it bugs the crap out of me when tenants don't sort out their trash, I have been known to correct their mistakes on occasion, which can be kind of nasty.

Federal tax break

A heat pump with sporty efficiency costs thousands more than a gas pack, but the federal government will help you out.  There is a federal tax credit to buy a "high efficiency" heat pump and take $2000 off of your income on your tax return.  To be eligible to claim the tax credit, an air-source heat pump (find out the cost of heat pump installation) must be purchased and installed between January 1, 2023, and December 31, 2032, (that is not a typo, it expires in 2032!) and meet the following criteria: SEER2 (Seasonal Energy Efficiency Ratio 2),rating greater than or equal to 16, or an EER2 (Energy Efficiency Ratio 2) rating greater than or equal to 12, or HSPF2 (Heating Seasonal Performance Factor 2) greater than or equal to 9. I'll refer just "SEER" here on this page for simplicity.

In our case the cost of a heat pack was around $10K and the cost of a high-end heat pump was around $12K, so with the tax break they are about equal.

What (or Watt?) does SEER mean?

SEER is seasonal energy efficiency ratio, a measure of "heat" power out of a system (cold air) to heat power into it (electricity). Arguably. this should be a unitless number.  You'd think "16 SEER" would mean you get 16 times the heat energy moved compared to utility energy consumed, but you would be wrong. SEE2 is BTUs divided by Watt-hours.  Let's explore the units.

What is a British Thermal Unit (BTU)? It is the energy needed to heat one pound of water one degree Fahrenheit, at temperature of 39F (where water has highest density).  The BTU dates back to the time of Beethoven and Napolean. As the linked article notes, the French and the Germans were not having this, and the metric system was born. Not much later, even the English abandoned their system of measurements, leaving only the hordes of ignorant Americans to enjoy talking about horsepower, miles per gallon,nautical miles, etc.

In the SI system there are only seven units

• meter (m)
• kilogram (kg)
• second (s)
• ampere (A)
• kelvin (K)
• candela (cd)
• mole (mol)

Derived units, such as watts for energy, can be broken down into Joules/sec (Joules is another derived unit) and further, into kg-m^2/sec^2.  Watts are kg-m^2/s^3.

The units of SEER are:  (Heat per pound of water/degrees F)*s^2*3600/(kg-m^2)

The 3600 term converts hours to seconds.   Why can't SEER be expressed without units, or even in percent?  I suppose it could be worse if meter units were in Potrezebes (1954, Mad Magazine), or Smoots (1958, developed at MIT).

Correcting SEER to "coefficient of performance"

Coefficient of performance  (COP) is often used as a generic measurement of energy out divided by energy in.  COP is almost always expressed unitlessly, so let's find the conversion between SEER and COP.

To convert BTUs to watt hours,  multiply by 0.293071.  

Thus, 1 SEER has unitless COP of 0.293, or 29.3% of energy is converted to heat. 16 SEER has energy ratio of 469%, which means you are moving 4.7X as much energy as you are consuming.  It goes without saying that anything that burns fuel to make heat, the COP is less than 1.  So it should be a no brainer to convert from burning gas to a high-efficiency heat pump and you are saving the environment, right?  It is not that simple.

Gas furnace efficiency

When I was borne, heating systems sent 50% of their heat into the atmosphere.  Now 90% efficiency is common. It is even possible to hit 95.5% but gas packs are not known for being in the that efficient.  Back in Massachusetts we had a house with high efficiency furnace. One outcome of high efficiency is that the flue gas is only slightly warm, and does not require an actual chimney.  In our case the flue was vented through the basement wall with PVC pipe. One thing the installer did not count on was that snow could cover that vent, it was maybe 3 feet off the ground.  I cursed him every time I had to shovel it out.  Thankfully there was some kind of sensor that would shut off the burner if the event was blocked, otherwise I would be dead from CO poisoning.

Let's assume that the efficiency of gas pack is 90% and the unit has 16 SEER.  In terms of COP, we see:

Gas pack: 90%
Heat pump: 469%

It's looking good for that heat pump!

Power plant efficiency and emissions

Arizona derives 46% from natural gas, 28% nuclear, 14% from coal, 5% from hydroelectric and 2% from wind according to chooseenergy.com.   We have the largest nuke plant in the country, and it consumes a lot of precious water.  Lately our grid is being expanded mostly solar, including the panels on my roof (see photo above). On the other end of the spectrum, West Virginia gets 88% of its electric from coal.  Interestingly, West Virginia has the highest percentage of cigarette smoking in the U.S.  You don't want to be downwind from that mess  In terms of solar, California and Massachusetts put the rest of the states to shame.

Let's assume that electricity for our heat pump is created 100% from natural gas. The efficiency of power plants is measure in "heat rate", which is expressed in the US as BTU/kW-hour (again, what a mess).  Apparently 6960 is pretty good, and lower is better (the reciprocal of COP).

To convert to our unitless COP,  multiply by 0.000293071 to convert BTUs to kW-hours, then take the reciprocal.  You will find that on a good day only 49% of natural gas energy is converted to electricity.

What about transmission line losses?  Turns out they are not so bad (Thanks, Nikola Tesla!)  Only about 5% is lost.  So about 47% of the energy content of the natural gas is delivered to homeowners.

Now, feed this into a 16 SEER heat pump unit, but take into account the conversion efficiency and losses of the public utility, 

16 SEER overall efficiency =467% x 47%=220%.

Compare that to 90% efficiency of the gas pack, and you will see it comes out way ahead by a factor of 2.44!

Cost of energy

In Tucson, electric rates in 2024 are  $0.19 per kW hour.

Cost of Gas is $21.33 per 1000 cubic feet.

There are 1050 BTUs per cubic foot, therefore gas costs $0.0203 per BTU. Converting to $/kW-hour, you get:

Cost of gas $0.069 per kW-hour

This should not be surprising, gas is always cheaper than electric for the same energy content.  But let's factor in the COP advantage:

Cost of electric heat/cost of gas heat=($0.19/$0.069)/2.44= 1.12 (112%)

Thus, the cost of heating your house with a heat pump if slightly more than burning natural gas. 

In conclusion, when you opt for the high-efficiency heat pump, you will help save the planet., even if your monthly utility bill will be slightly higher.   Throw some solar cells on your roof and you will save money with a heat pump. Speaking of solar cells, getting them installed is an IQ test.  If you let someone talk you into a free install, you fail.  Cough up the investment yourself and you will come out way ahead.

Further assumptions and variables

There are lots of other considerations if you  want an accurate cost analysis. SEER ratings are averaged for the country, we live in a place where winters are very mild.  in January our average temperature is 11.7 °C (53 °F), with a minimum of 4.7 °C (40.5 °F) and a maximum of 18.6 °C (65.5 °F). If you heat your house to 68 degrees the delta is only 15 degrees.  Heat pump efficiency gets better when the temperature difference is less. for more information look here.

In colder climates, historically heat pumps could not perform on the coldest days (below 32F for example), and heat strips were turned on.  Electric year is teh most expensive way to heat your home, with COP less then 50% compared to our 244% heat pump example.  .  When I first moved to Arizona (a different house than we live in now), we had a heat pump system that was old and feeble, so  I permanently disabled the heat strips.  So what if one the coldest day of the year your home temnperature drops a few degrees?  Suck it up!   Modern heat pumps can work down to 0F, so depending on where you live, heat strips will rarely turn on. Our heat pump doesn't even have heat strips!

SEER2 is an American thing.  In Europe it is ESEER.

Check out the Unknown Editor's amazing archives when you are looking for a way to screw off for a couple of hours or more!

Fan/hate mail can always be sent to UE@microwaves101.com