Off Topic: Is a Gas or Electric Dryer Cheaper to Operate?

Our electric dryer went out this week. We bought it used 6 years ago when my wife and I got married, and I figured it would be a good time to buy a new set. Our family is growing, and we are doing more laundry than ever with two little ones in our house.

We have a gas hookup behind our dryer, in addition to a 240V outlet, so we have the option of buying either a gas or an electric dryer. I’ve never owned a gas dryer before. But my intuition is that all things considered, it may be the less costly choice. All the laundromats I’ve seen use gas dryers, and you typically heat your water and home with gas. I assume that’s because it costs less money than electric heating.

Is a gas dryer cheaper to operate than an electric one? How would changes in fuel or electricity costs affect the answer to that question? An electric dryer is about $100 less than a gas one as of this writing. Can the additional cost of a gas dryer be justified through lower fuel costs over time? How long would it take to recover that additional cost?

These are all questions that I had trouble finding answers to online. Most of the commentary on gas versus electric was about the quality of drying, not the cost. And when I did find answers to the cost question, there was little supporting analysis. This is my attempt to put numbers to those questions as I make this decision.

Natural Gas Cost

A good place to start with this analysis is the cost to power the dryer. I need to know what the next marginal kWh of electricity and Therm of natural gas will cost me.

To calculate these figures, I need to strip out the fixed costs in my electric and gas bills. These are costs I will pay regardless of my actual electricity or gas consumption. I can divide these non-fixed costs by my usage to calculate the cost of using one more unit of energy.

Let’s start with my gas bill. I want to figure out the cost of using one additional Therm.

My gas bill for the October and November.

To do this I need to identify the fixed costs in my bill. These appear to be the Customer Charge of $16.94 and the Gas System Reliability Surcharge (GSRS) of $3.19. Everything else appears to be an itemized charge based on usage.

Subtracting these fixed costs from my total bill gives the cost for the 57 Therms I used in this billing cycle:

The non-fixed natural gas cost.

Dividing this number by my usage is roughly what it would cost me to use an additional Therm of gas:

The cost of using one additional Therm of natural gas.

A Therm is 100,000 BTU. Google tells me that the energy in one Therm is equivalent to 29.3 kWh. The per kWh cost of natural gas is then:

The cost of natural gas in $/kWh.

This number represents the cost I will pay for one more kWh of energy from natural gas. It needs to be in units of kWh so I can compare it to my electricity cost, and to simplify subsequent analysis.

Electricity Cost

Below are the relevant charges from my electric bill:

My electric bill for roughly the same period as my gas bill.

The Basic Service Fee of $14.50 is a fixed cost. Everything else appears to be based on usage. I compared this bill to prior ones and every other charge varies with the amount of electricity used.

Subtracting the fixed costs from my total bill gives the cost for the 498.17 kWh I used in this billing cycle:

The non-fixed electricity cost.

And dividing by my total usage is roughly what it would cost me to use an additional kWh of electricity:

The cost of using one additional kWh of electricity.

If I stop here and compare these two numbers, it appears gas is the obvious winner. For the same amount of energy, electricity is more than 3 times the cost of gas.

Electric Versus Gas Dryer Efficiency

But there’s more to the story than just fuel and electricity costs. A gas burner doesn’t convert gas to heat with 100% efficiency, and electric dryers don’t operate with the heating coil on 100% of the time. Additionally, a gas dryer uses both gas and electricity in operation: they have a motor that turns the drum and an electronic control panel, same as an electric dryer. These details must be considered for a complete analysis.

Short of buying both units and measuring their actual gas and electricity consumption, there’s not a great way to answer these questions. Fortunately, most dryers marketed for sale in the US today have Energy Star ratings. The Energy Star website quotes a Combined Energy Factor, or CEF for certified dryers. It’s defined as:

The definition of the Energy Star Combined Energy Factor (CEF).

The CEF takes all energy used by the dryer into account. This formula can be used with the energy costs we calculated previously to determine whether a gas or electric dryer is cheaper to operate.

In this formula, C is the weight of a load of laundry. I’m guessing this is the weight of wet clothes that go in the dryer. C is 8.45lb when the Energy Star folks evaluate dryers for certification. Energy is in units of kWh, so the units for CEF are pounds per kWh.

For a dryer with a CEF of 3, you could dry 3lb of clothes using 1kWh of energy. Though the Energy Star website doesn’t state it explicitly, it stands to reason that energy used from both gas and electricity are measured and combined to calculate the CEF for a gas dryer.

For the rest of this analysis, I’m going to examine two GE appliance models that come in both a gas and electric version. The electric model number is GTD65EBSJWS, and the gas model number is GTD65GBSJWS.

The electric dryer has a CEF of 3.93. The gas dryer has a CEF of 3.48. This means the electric dryer can dry 3.93lb of clothes with 1kWh of energy, whereas the gas dryer can only dry 3.48lb of clothes with 1kWh of energy.

This makes sense, I think. If I remember my circuits and heat transfer classes from school right, converting electricity into heat with resistive heating is almost 100% efficient. Burning gas is pretty efficient, but only in the neighborhood of 80% from a brief web search, so we’d expect the electric dryer to be more efficient from an energy perspective. But what about the cost to operate them?

Cost to Operate an Electric Dryer

The only power source for the electric dryer is electricity, so the quoted CEF values and the cost of electricity I calculated previously can be used to estimate the cost to dry a given load of laundry. Drying a 10lb load of laundry in the electric dryer costs:

The cost to dry 10lb of clothes in the electric dryer at my current electricity rates.

Cost to Operate a Gas Dryer

Calculating the cost to dry the same 10lb load of laundry in the gas dryer is more complicated. A gas dryer sources power from both gas and electricity. The relative portions of gas energy and electric energy must be calculated so we can multiply both by their respective energy costs to get an accurate estimate of the total operation cost.

The owner’s manual for the gas dryer states it uses electricity at a rate of 6A at 120V, so the electric power the gas dryer uses is:

The electric power used by the gas dryer.

The owner’s manual also states that the gas dryer consumes gas at a rate of 22,000 BTU per hour. This comes out to:

The gas power used by the gas dryer.

The CEF values are for total energy consumption, both electric and gas. We can use the numbers computed above to breakout the portion of energy used by the gas dryer that is electricity and gas. The portion of power that is gas is:

The portion of power used by the gas dryer that comes from natural gas.

And the portion of power that is electric:

The portion of the power used by the gas dryer that comes from electricity.

The cost to operate the gas dryer per kWh of energy used, accounting for both the gas and electricity consumed, is:

The cost per kWh to operate the gas dryer at current natural gas and electricity prices.

Drying one 10lb load of laundry in the gas dryer costs:

The cost to dry 10lb of clothes in the gas dryer at my current natural gas and electricity rates.

Accounting for Differences in Machine Cost

The GTD65EBSJWS electric dryer costs $799. The GTD65GBSJWS gas dryer costs $899. How many 10lb loads of laundry would I need to do before the additional cost of the gas dryer is paid for?

The energy cost difference between the electric and gas dryer for one 10lb load of laundry is:

The savings from drying 10lb of clothes in a gas dryer instead of an electric dryer.

If I’m doing 7 loads of laundry per week, the cost savings per week is:

The cost savings of using a gas dryer per week if I’m doing seven 10lb loads of laundry.

The number of weeks to recover the $100 additional cost is then:

The time it would take to recover the additional cost of a gas dryer.

Alternatively, the annual cost savings of a gas dryer over an electric dryer is:

Annual gas cost savings at my current electricity and natural gas prices.

So not a bad deal. I need to verify with the Mrs. that we are indeed doing about that much laundry. But 7 loads sounds pretty reasonable. And as our family grows both in size and in quantity, it seems to reason that that number will only increase over time.

Variations in Natural Gas Prices

The price of natural gas seems to be rising. Let’s assume the price doubles. How does this affect the math?

To answer this question, the cost to operate the gas dryer needs to be recalculated using the new natural gas price. Recall that it is powered from both gas and electricity. Doubling the gas cost and leaving the electricity cost unchanged:

The cost to operate the gas dryer if natural gas prices double.

The cost to dry a 10lb load of laundry in the gas dryer under these circumstances is:

The cost to dry a 10lb load of laundry in the gas dryer if natural gas prices double.

In this scenario, the energy cost difference between the electric and gas dryer for one 10lb load of laundry would be:

The cost savings of the gas dryer over the electric dryer if natural gas prices double.

The gas dryer is still cheaper to operate than the electric dryer in this scenario. Assuming the same 7 loads of laundry per week, the cost savings per week is then:

The weekly cost savings for using the gas dryer if natural gas prices double.

The number of weeks to recover the $100 additional cost is then:

The time required to recover the extra gas dryer machine cost if natural gas prices double.

And alternatively, the annual cost savings of a gas dryer over an electric dryer is:

The annual cost savings of the gas dryer if natural gas prices double.

In this scenario, a gas versus electric dryer is closer to a wash, but still saves a little money. Over the life of the gas dryer, I will probably recover the extra cost.

Conclusion

Gas seems to be the winner in this analysis, even accounting for steeply higher natural gas prices. They would need to go up by a factor of 3.4 before the electric dryer is cheaper to operate.

The EPA says that 34% of electricity in the US is generated from natural gas. If natural gas prices increase, electricity prices will probably go up as well, because a portion of electricity is generated from natural gas. Alternatively, electricity costs would need to fall to about $0.03/kWh before the operating cost of an electric dryer becomes competitive with the gas dryer. I don’t anticipate that happening.

The more laundry you do, the better the gas dryer is. Our family isn’t going to be doing any less laundry over the next decade. If we did 10 loads of laundry that weighed 10lb every week, at current natural gas prices the additional cost to buy a gas dryer is recovered in 61 weeks, just over a year.

There are other, non-monetary benefits to having a gas dryer. Burning natural gas gives off carbon dioxide and water vapor. Most gas dryers I’ve looked at direct these exhaust gases into the drum and out the vent. The moisture in the drum can help reduce the static electricity in clothes after drying. We might save on dryer sheets if this is true.

You also get more heat when you’re drying. The gas dryer burner is capable of generating 6448W of heat when drying, whereas the electric dryer only outputs 5600W when operating at 240V. That might mean lower cycle times as the clothes dry faster, resulting in more laundry throughput, which I’m sure the Mrs. can appreciate.

One other thing that I find interesting: the CEF for the electric dryer is 3.93, while the CEF for the gas dryer is only 3.48. It’s true that the electric dryer uses the energy supplied to the machine more efficiently than the gas dryer. However, it’s not true that it costs less to operate, at least for my situation.

Furthermore, if the electric company is burning natural gas to make the electricity that powers an electric dryer, that’s actually a much worse scenario for the environment and your wallet than just burning natural gas in a gas dryer.

Every time you convert energy from one form into another you do so at less than 100% efficiency, so the extra steps of generating electricity from natural gas and transmitting it to your house involve losses that could be avoided by just burning that same natural gas in your dryer. This becomes less true if natural gas isn’t involved in electricity generation, but as stated above, that’s generally not the case. Some of that electricity comes from sources like coal, too.

I feel pretty confident in this analysis, but I’m posting it so I can have other folks verify its validity. Are these assumptions and calculations reasonable? Please let me know if you find any errors!

Comments from anyone who has owned a gas dryer are welcome, too. What was the experience like? Do you prefer gas or electric?

The Right Problem

When I mow my lawn, I spend five minutes putting on mowing clothes and walking out to the garage to grab the mower, and this experience has influenced the productivity solution I think the lawn care industry needs. The pain I experience involves pushing the mower. Lawn care companies, on the other hand, experience pain in a whole host of other areas. The two situations are worlds apart, and the comments on my last post made that crystal clear. Thank you to those who shared their thoughts!

As I think about these ideas, I’m starting to realize the problem I’m solving really isn’t the right one. The tag line on this blog is “a project devoted to an autonomous lawn mower.” But the real goal is actually increasing lawn care productivity. An autonomous mower is only a means to accomplish that goal.

Greenzie and Mowbotix haven’t succeeded yet because they haven’t built a system that increases lawn care productivity. While they have successfully removed the operator from the mower, they’re making a mistake by trying to sell it to lawn care companies. Integrating an autonomous lawn mower with the way lawn care companies operate today actually makes them less efficient.

Lawn care companies are just as much about logistics as they are about mowing grass. Successful lawn care companies are able to quickly ferry workers and mowers to multiple job sites throughout the day. But to do this they use expensive tools: a truck and a trailer. And beyond the cost of the truck and trailer, they also waste a lot of time travelling between job sites.

These are major costs that get baked into the price people pay for mowing services. Which is great: it means that there is a lot of waste in the way lawns are currently mowed, and that it’s possible that a new way of mowing lawns can exploit these inefficiencies for profit.

The Productivity Problem: A Really Long Editorial

I like searching the internet for other people who are making autonomous lawn mowers. You can learn a lot by seeing how others are approaching the problem. Over the years, I’ve found several folks who’ve made great progress. Yet everywhere I look I still see people on riding mowers cutting their grass the same old-fashioned way. What gives?

When I started the mower project, the problem I was solving seemed blindingly obvious. Mowing is unpleasant to do personally and expensive to hire out. Let’s build a machine that mows a lawn without a human. It will sell itself!

Both Greenzie and Mowbotix did just that. They built machines that can mow huge fields with great precision. Why haven’t they conquered the lawn care industry with their cutting-edge technology? The answer, in my humble opinion, has nothing to do with the maturity or sophistication of their technology. It has everything to do with productivity.

If you think back to economics 101, you’ll recall that productivity is the amount of output you get for a given input. For an autonomous lawn mower to be successful in the marketplace, it has to not only remove the operator from the machine but increase productivity while doing so.

Joe’s Mowing Company.

And therein lies the problem. To illustrate, imagine a fictitious Joe’s Lawn Care company, who is using standard lawn care technology available today. A typical day for Joe would go something like this:

  1. Joe drives to the job site he needs to mow.
  2. He unloads his mower, hops on, and starts cutting grass.
  3. When he’s finished, he loads the mower back on the trailer and drives to the next job site.
  4. He repeats steps 1 through 3 until he’s finished with the day’s work.

If Joe were to upgrade to an autonomous lawn mower, his day would look like this:

  1. Joe drives to the job site he needs to mow.
  2. He unloads his mower, opens his laptop, loads a mission, and starts cutting grass.
  3. When he’s finished, he loads the mower back on the trailer and drives to the next job site.
  4. He repeats steps 1 through 3 until he’s finished with the day’s work.

How much does an autonomous lawn mower improve Joe’s productivity? The answer: none. And that’s being generous.

Joe gets paid to be out there monitoring the autonomous lawn mower, even if he’s sitting in the truck sipping iced tea while it cuts the grass. He still needs to transport the mower to the job site, unload, and load it. In this light, an autonomous lawn mower doesn’t reduce Joe’s labor costs at all. In fact, it probably increases them because the setup time at each job site will be longer than the time it takes to hop on a riding mower.

And on top of that, an autonomous lawn mower will likely cost much more than a typical riding mower. To give you an idea of how much, I’ll direct you here and here. Essentially, Greenzie and Mowbotix are asking you to bring them your existing mower, $5,000, and they’ll retrofit it for autonomy.

The worst part? To use their solution, you need to pay a significant monthly fee. Wasn’t the whole point of this exercise to get rid of the monthly fee, i.e. the wages you pay the guy to run mower? Talk about back to square one. If that’s how we’re going to market the solution I understand why autonomous lawn mowers haven’t caught on yet.

Framing this information in productivity terms, the inputs for an autonomous mower solution:

  1. Cost thousands of dollars more than an ordinary riding mower.
  2. Still require a worker to setup, monitor, and load up when finished.
  3. Require a significant monthly fee to operate.

On the output side, you get to use your same riding mower at the same speed to cut the same amount of grass as before. And that assumes it doesn’t take longer to get the autonomous mower up and running once you’re at the job site.

I’m going to be honest, this has been a tough post to write because the solution I’m working on suffers from many of these same issues. I don’t intend to disparage Greenzie or Mowbotix: both of them have way cooler robots that are much more robustly autonomous than mine.

But as they exist today, these autonomous mowing solutions, mine included, cost more than traditional lawn mowing technology and result in about the same level of output. We’ve been solving the wrong problem, or a very small part of a much bigger problem.

Removing the operator from the machine is a step in the right direction, but to truly increase lawn care productivity it’s going to take more than a mower that can drive itself. I will be doing some pondering on that over the next few days.

I’ll leave you with a quote I wish I’d found back when Rod sold me the electric wheel chair many years ago:

It doesn’t matter how fast you move if it’s in a worthless direction. Picking the right thing to work on is the most important element of productivity and usually almost ignored. So think about it more!

Sam Altman

Please leave your thoughts below. I’d love to hear them!