NREL Study Compares Costs Of Operating CNG Buses Versus Electric Buses


If you operate a public transportation fleet, cost is a critical factor. Over millions of miles, a difference of a penny or two per mile can add up to serious money. All electric buses tend to cost more to buy but are expected to cost less to operate over their anticipated 12 year useful life. Is that assumption accurate?

Proterra electric buses

A new study by the National Renewable Energy Lab tabulated data collected by Proterra electric buses operated by Foothill Transit in southern California and other buses in the Foothill fleet that were powered by compressed natural gas. The results show the Proterra buses were up to 8 time more efficient than their CNG powered cousins.

When operated on the same route, average fuel economy for the CNG buses came to 2.1 miles per diesel gallon equivalent. By contrast, the Proterra electric buses had an observed MPDGe of 17.35 — more than 8 time higher. Cost of fuel per mile worked out to be 41 cents for the electric buses and 50 cents for the CNG powered vehicles.

There are other costs associated with operating a transportation fleet that include such items as how often repair crews have to be dispatched to deal with breakdowns away from the repair facilities located at the terminal. Here, the Proterra fared less well against the competition, averaging 16,405 miles between road calls versus 56,710 for the CNG buses. Still, overall maintenance costs came out slightly in favor of the electric buses — 21 cents per mile versus 22 cents per mile.

Cost per mile for unscheduled maintenance again showed an advantage for the CNG buses, $0.14 versus $0.10 — but Foothill Transportation attributes half that difference to replacing damaged tires more frequently on the Proterra buses. The operate exclusively on city streets with potholes and broken curbs as opposed to the CNG buses in its fleet, which spend most of their time on smooth, well maintained highways.

Foothill Transportation found virtually no degradation in battery power during the 6 month observation period, suggesting that the batteries should not need replacing during the 12 year projected useful lives of the electric buses. It also found the charging equipment used by Foothill Transportation to keep the buses charged while away from the terminal was almost entirely trouble free during the test period.

On balance, the Proterra electric buses are operating well in commercial service and equal or exceed the expectations of the transportation authority, which plans to transition to 100% electric buses by 2030. Another entry in the good news column is that the cost of the Proterra buses has declined by more than one third since Foothill Transportation bought its first Proterra buses in 2009. They cost $1,200,000 a piece back then versus $789,000 each today.

Source: NREL


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I have been a car nut since the days when Rob Walker and Henry N. Manney, III graced the pages of Road & Track. Today, I use my trusty Miata for TSD rallies and occasional track days at Lime Rock and Watkins Glen. If it moves on wheels, I'm interested in it. Please follow me on Google + and Twitter.
  • kevin mccune

    How come no one mentioned , there is no oil change on an electric buss.?

    • bioburner

      Yup. Also there would be a big savings replacing brake parts on the Electric bus. No mention of “scheduled” service costs for either bus.

      • Cosmo Kramer

        Oil changes, brakes and all other maintenance items are included in the NREL study and in this article… “Still, overall maintenance costs came out slightly in favor of the electric buses — 21 cents per mile versus 22 cents per mile.” The unscheduled maintenance is discussed in the next sentence with scheduled maintenance obviously being the difference of the two. You are right, brakes, oil, even spark plugs and air filters are either better or non-existent costs for the EVs, but EVs have costs that CNG doesn’t have. I encourage everyone to read the source NREL study (the article author included a link at the bottom of the article). It’s pretty interesting and an easy read.

  • Cosmo Kramer

    Forgive me Mr. Hanley, as I’m going to offer some complaints on this article that are a result of the many articles I’ve seen on this NREL report. Everybody is reporting on the NREL report with what I have to believe is a bias towards EV=good, fossil fuel=bad. I want to start by saying that I believe EVs are the future of transportation and cannot wait for it to happen. That said, I’m not ready to paint EVs as Mother Teresa quite yet.

    You report that EVs are 8 times(!) more fuel efficient than CNG buses. That’s impressive. Of course, when most electricity is produced by coal or natural gas at the plant, the production of electricity is pretty inefficient. The net efficiency is not 8 times more efficient, but it’s still better than the CNG bus. The number is meant to mislead so you don’t think about what really matters. A car can get 100 miles per gallon of gold, but gold is expensive so what really matters is cost per mile. The NREL study found CNG was $0.25/mile for fuel and electricity was $0.41/mile. Because that number didn’t fit the story of EV=good, even NREL “estimated” the cost of CNG buses in the same exact service as the EV. Why did you report the estimate and not the actual? How did they even arrive at the estimate? Why did Foothill Transit and NREL do 2 years of comparison study and not put the vehicles in comparable service? The answer is the EVs can’t be put in the service of the CNG buses. EVs still have limited applicability for the services that bus riders require. Notice the EVs do half the miles per month and at half the speed. There’s a reason. What NREL doesn’t properly report is that even the $0.41/mile cost of electricity is with a special PUC exemption of “normal” electricity costs. They do mention that their test case was during a period where these charges were eliminated, but they don’t help the uninitiated understand what that truly means. In other words the electricity is subsidized for this special, short-term, test fleet case. The utilities are artificially lowering the price of electricity in this case to get EVs into the market in a much bigger way to increase their revenues. When 100’s of EVs are running for years and years in these cities, they will charge full demand charges. NREL could easily of “estimated” those actual costs back in. It is a much more accurate “estimate” than the very arbitrary estimate they made about CNG on the EV route. CNG costs can also be subsidized. The NREL hints at this, but doesn’t “estimate” it. They don’t even have to estimate it. Most SoCal transit fleets already use renewable natural gas and through cap-and-trade carbon pricing CNG costs/mile are close to zero. They have this data.

    Increased tire maintenance on the EVs is not because of the roads/pot holes. Los Angeles is one bid pot hole. The increased tire maintenance is because regenerative braking is hard on tires. You are essentially braking as often as possible with hard applications of the brakes each time. If you operate this way with any fuel, you will experience excessive tire wear. The benefit, is all that energy is stored for future power applications instead of lost to heat, thus the great mpg.

    MBRC (miles between roadcalls). Nobody seems to think this is noteworthy to report on. It may be that uninitiated don’t really know what it is. This is a standard transit metric for how many miles a vehicle can go before it breaks down while in service and requires a mechanic to go to the vehicle (in service) to fix it OR tow the vehicle back to base. All of the passengers on the bus get delayed and transfer to another bus that is sitting available back at the base, which requires buying extra buses to sit and wait for a roadcall. The EVs had 5 times as many roadcalls per mile traveled; even when only considering the motive force (ignoring broken tail lights, etc…). If your car breaks down every 45,000 miles on road or 9,000 miles on road, what would you do?

    The NREL report removes accident and warranty related maintenance expenses from their overall cost of maintenance. That sounds reasonable, except that EV buses are so few and far between still that most OEMs include extended warranties on them, especially the battery/motor technologies. So all the actual maintenance costs on those systems are not included in the cost of maintenance. Now that is fine if it shows up in the cost of the bus purchase price where extended warranties normally would show up, but in the case of ultra-low volume buses the warranty costs are a mix of “cost of building a new product line” for the OEMs and subsidized by enormous government grants to get these technologies out there. Eventually, those costs will actually pass down to the transit agency and ultimately the rider.

    Finally, battery degradation is normally low and linear until you reach a certain limit on number of recharges. 6 months is not indicative of 12 years. The federal government requires transit agencies to keep their buses for 12 years if they are receiving any federal funds for those buses. They are receiving ~90% federal funds for those buses. Look up the Nissan Leaf warranty for their batteries. They expect to lose 33% of the battery capacity in 8 years. This assumes recharging once or twice per day, at most. These Proterra buses used in the NREL study can only go about 20 miles before recharging and they have fast chargers installed along the bus route to keep them charged throughout the day (another cost not properly accounted for in the study. it is a minimum of a half million dollars to install a fast charger on a public street and you need one every 10 miles or so on all the major roads/routes you service in a city). The batteries get cycled 10-15 times per day. Luckily on the buses, they are covered by the extended warranty that is offered on a few test buses. What happens when a city has 100’s of buses?

    My very negative response is again not meant to be directed at Mr. Hanley so much as the broader conversation that remains out in the public space about EVs. I believe they will take over the world and we need them, along with solar/wind power production replacing gas/coal. But there will be a cost to do the right thing for our environment and inconveniences. Spreading inaccurate versions of the facts so as to fool people to do what somebody wants them to do will backfire. These reports give facts, but try to move facts into propaganda. I’d prefer to rip the band-aid off knowing it will hurt, because when it surprises people that it hurts they will stop.

  • deh

    What was the total life cycle cost for each that includes bus and vehicle charging / gas compression?