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NREL analysis of Proterra electric bus demo finds average fuel economy nearly 4x that of CNG baseline buses

Proterra battery-electric buses in service in a 12- vehicle demonstration by Foothill Transit in California offered significant fuel savings compared to similar conventional vehicles, according to a recently published analysis of the results of the demonstration by the National Renewable Energy Laboratory (NREL).

The NREL team found, inter alia, that the battery-electric buses (BEBs)had an overall average efficiency of 2.15 kWh per mile on the service route, which equated to 17.48 miles per diesel gallon equivalent (DGE). The baseline CNG buses had an average fuel economy of 4.04 miles per gasoline gallon equivalent (GGE), which equates to 4.51 miles per DGE. These results indicated that the BEBs have an average fuel economy that is nearly 4 times higher than that of the CNG buses.

The NREL team also found that reliability, measured as miles between roadcalls (MBRC), was high during the data period (April 2014 through July 2015). The overall bus MBRC for the BEB fleet was more than 9,000. During the data period, there were only three roadcalls that were attributed to issues with the battery system, the primary power system for the buses.

This resulted in an ESS-related MBRC of more than 133,000. The NREL team called this “exceptional for an advanced technology bus in the early stage of commercialization.” The CNG baseline buses achieved an overall bus MBRC that was more than 45,000, which is expected of a new, fully commercial product.

Other findings included:

  • The average energy efficiency was 2.15 kWh per mile over 399,663 miles of use.

  • The average battery pack SOC is 75.4%, indicating a possibility for a usage window shift.

  • The average runtime per day is 13.2 hours with an average of 13 charges per day. Each charge averages 20 kWh energy delivered.

  • Accessory loads contribute to the overall range capability, as more than 50% of “system on” time is spent at a speed of 0 mph where lighting and HVAC loads are still required.

Source: “Foothill Transit Battery Electric Bus Demonstration Results”

Advanced technology demonstrations typically experience challenges and issues that need to be resolved. The challenges and lessons learned from the demonstration included bus-related problems as well as programmatic issues. One major challenge is addressing demand charges and time of use charges that affect electricity cost. This will be a major challenge for any fleet looking to deploy electric buses that charge during peak times.

The industry needs to work on a permanent solution for all BEB adopters to keep costs reasonable in the future. Another challenge is training operators and maintenance staff in the differences between BEBs and conventional buses. Foothill reports that there is still a lot of learning as the agency ramps up to a larger BEB fleet. The team needs to understand how service can transition to a higher number of buses.

—“Foothill Transit Battery Electric Bus Demonstration Results”

Background. Foothill Transit began a demonstration of three Proterra battery electric buses (BEBs) in October 2010. That first demonstration went well, and Foothill moved forward with an order of 12 next-generation BEBs.

Foothill’s project goal for these buses was to fully electrify one route in its service area and to investigate the feasibility of the technology for other routes. The buses are charged on route at a charging station built at a transit center mid-way along the route. In March 2014, Foothill Transit began operating the new fleet in its service area.

Source: “Foothill Transit Battery Electric Bus Demonstration Results”

Foothill Transit is collaborating with the California Air Resources Board (CARB) and the US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) to evaluate the buses in revenue service. CARB staff has been gathering data on zero-emission buses (ZBuses) to assess the status of the technology; the majority of ZBus data collected and reported to date are for fuel cell electric buses. CARB would like to have similar analysis and reporting for the other primary ZBus technology being adopted in the state of California: BEBs. CARB enlisted NREL to conduct a third-party evaluation of the Foothill Transit fleet.

Foothill does not operate conventional buses that are similar in size, weight, and year to the BEBs; for the primary baseline buses, NREL selected eight new NABI compressed natural gas (CNG) buses of the same model year as the BEBs.

Futures. Foothill Transit will continue operating the 12 BEBs on the same line and is evaluating other routes that might be well suited for electric buses. The agency has orders for Proterra’s new Catalyst 40-foot BEB. The first two will be fast-charge buses, followed by 13 of the Catalyst XR extended range model BEBs.

NREL plans to continue the in-service performance evaluation for another year and expects to publish a second report in late 2016.

NREL is also very interested in performing chassis dynamometer testing of the BEBs and the CNG buses and hopes to precisely quantify both energy and emissions savings in a controlled environment on a variety of drive cycles.

Foothill Transit serves a 327-square-mile area covering the San Gabriel and Pomona Valley region of Los Angeles County.


  • Leslie Eudy, Robert Prohaska, Kenneth Kelly, and Matthew Post (2016) “Foothill Transit Battery Electric Bus Demonstration Results” (NREL/TP-5400-65274)



Thanks Captain obvious.

The real issue is cost. It seems that if cities collaborated to purchase high volumes of electric buses then they could put in high enough volume orders to support the building of large plants. Let's say 50 large cities collaborated to buy EV buses in quantities that would overturn their fleets in ten years. Such a long term purchase would supply a stable market for the builders and drive down cost and yet each individual city would not be spending extreme amounts of money. It's funny that the DOE clean cities groups don't influence collaborative purchasing to support the growth of the industries they supposedly support. I suppose there is some ideological view that says government must be benign or inept, otherwise it is picking winners. Although, why government would want to pick losers is beyond me. I think we as a group (humans) have beliefs in ideologies and religion that will prevent us from making rational plans and decisions.


At 13 charges per day I wonder how long the batteries will last? Also wonder how much replacement batteries would cost? Can't imagine being more than $100,000 for 88 KWH of high power battery. In spite of these questions it appears that the economics of electric would be close at high oil prices.

I agree with Brokenny. If the world really is serious about reducing CO2 emissions, public transit buses appear to be some of the lowest hanging fruit.

My personal circumstances would benefit most if a clever scientist would come up with a study that completely refutes the correlation between CO2 and global warming but I've given up hope of that happening any time soon.


Yes B4. Large orders get lower purchase prices. In China, many cities ordered 1000+ e-buses each. BYD is mass producing many of those (extended range) e-buses at a much lower local price.

North America is just a few years behind. Orders of 1000+ e-buses per major NA cities will not happen till 2018/2022.

Volvo Canada has built 2 or 3 units as a pilot project (operating on one single route equipped with high capacity quick chargers) in Montreal City but regular mass produced units will not be available until 2018/2020.


By the way, our local politicians have decided to spend up to CAN$5B to extend the Montreal Subway instead of spending CAN$2B to replace 2,000 diesel city buses.

The same Subway is getting 453 new cars (built by Bombardier for CAN$2.8B) to replace older 1963/1973 units. Another 50+ cars may have to be added when the subway is extended.

Both projects are required but e-buses should be given the priority.


They are about twice as efficient, when you take into account the purchase price and battery replacement costs they are about even. Make the CNG hybrids and the numbers change again. EVs buses are cleaner with less noise, this is good enough.


The Washington State DOT awarded BYD last year a contract for up to 800 buses. The buses should have an initial cost almost equal to the CNG buses (around $625k) and of course lower operating costs. Washington State has a very low cost of electricity and is also predominately hydro.


In use the ebus is shown to be ~ 4X the CNG efficiency as claimed.17.5DGE as compared to 4.5DGE that's close enough the claim.

The average speed of the EV at 10.6 as opposed to 17.6 suggests the ebus could be on an inner city route (sensibly) which would easily explain the high DGE in service number.
The Ebus has a lower continuous H.P. but higher peak. As it is unlikely that either run at max power I would have to think that the ebus could perform at least as well as the CNG.

Without knowing the weight / gearing / route etc it is a mystery as to why the speed difference. Possibly the ebus is operating a more conservative economy mode strategy?

No fuel wasted idling or during inefficient rpm ranges as well as the likely high rate of regenerative braking.

I think the 4x claim should stand.

CGary I3 charge stops delivering only 20kw into the small @ 88KWH battery would have very little affect on the battery life as we know from various studies and manufactures lifecycle ratings across a range of Li chemistries.

We also know that fast charge to ~ 80/90% capacity can be achieved at the high or rapid charge rate.
My guess would be that the wireless charging stops will take the battery to that 80/90 fill in a short 5-15 minutes.
"Less than 10 minutes"is claimed.


The company will no doubt ramp up production as reliable ongoing orders can be found.
It is like Tesla in that the venture capital risk disallows over optimistic forecasts.

The uncertainty in American and world economy must be overcome to enable more aggressive rational expansion.
Then there will still be the difficulty of competing against Chinese cost structures.

Comparing Proterra private capital local manufacturer with BYD With its large resources of the Chinese state economy misses the mark. Very different opportunity/ reality / scalability and therefore cost structure.


It is the way they calculate MPGe, they pretend it does not take any energy to make electricity, so NO it does not stand.

Nick Lyons

The comparison needs to be well-to-wheels. On a natural gas-powered grid, the electric bus is going to be burning gas, just remotely. 4 to 1 might come down to 2 to 1 or lower. On a nuclear- or hydro-powered grid, the e-bus looks like more of a winner, at least from an emissions point of view. A full life cycle costs/emissions comparison would be interesting.

Brian Petersen

Battery size seems small for this size of vehicle (88 kWh). I was under the impression that the same company had developed a bus with sufficient capacity to run a whole typical shift. Then it could be recharged overnight, which would make the economics better, and would eliminate charging delays during peak operating times.

The incremental electricity to charge during the daytime is probably coming from a natural gas fired plant, even if the base power is mostly nuclear (which is the case where I am). Even so, a central combined-cycle generating station ought to be a fair bit more efficient than a mobile CNG combustion engine ever could be.



I would concede the CNG bus example carries significant penalties.
It is .9 Meters longer wheelbase and 2.3 M or 20% longer in the body.
The CNG also weighs 22% more but as there is capacity for 5 extra passengers on the smaller ebus, the GVWR or max rated load is only 13% heavier for the CNG example.

Somehow the 35 foot Proterra carries 5 more passengers than the 42 foot cng example.

Regarding diesel gallon equivalent:

This is about the energy on board which is used at ~ 40% efficiency for ice as opposed to 90 + % for electric machine.

Another advantage for the electric comes from the fact that it does not waste energy at stops idling and not being able to stay at its most efficient speed.

An efficiency dividend also comes from braking energy recuperation.

There is nowhere here reference to greenhouse gas equivalence or well to wheel as that is entirely dependent on the source.

To say that it takes energy to make electricity is not the point it takes energy to deliver petrol or diesel currently around X 2
is a good starting point for estimating the more recent global average.
That number is going north at a high rate as deep sea, arctic drilling and tar sands not to mention potential world wars are becoming more common.

The days of oil flowing from the ground under it's own pressure are long gone.

First principles 101.


Tried to show that Washington State DOT has already approved a large order for eBuses.
Proterra is also one of the approved vendors for this contract. Did not have specific pricing for the Proterra eBus on this contract, probably is close to BYD pricing.


Nick Lyons,
Foothill Transit has Solar panels at the Pomona and Arcadia facilities generating 421 MWh of electricity. They also purchase Solar and Wind Energy credits from SCE. This eBus looks like a winner.



Tks for the info on Washington State large order for e-buses. It is probably the first large order in NA. That's the way to go.

The BYD unit was successfully tested in our City a few months ago but our politicians prefer to wait another 3 to 4 years for the Volvo Canada units. (at twice the price?)

The 88 kWh battery pack on the Proterra Units is way too small and will require very frequent refills, specially on our very cold winter days (we have -25C today). A gas heater would be required. The BYD Units can be ordered with 230 to 324 kWh battery pack.

Brian Petersen

New Flyer's electric bus has 200 kWh storage standard (up to 300 kWh available), Winnipeg has been testing one for the last year, and as far as I can tell, they're available today. There's no need to send the manufacturing to China ...


Ditto, no need to go to China. And what cost structures, Arnold, to contend with? PPP, or purchase power parity rules the day. As energy, water, material costs of infrastructure, and certain capital overhang at McDonald's and other corporations become increasingly important in modernizing economies, it becomes increasingly harder for China to sustain itself with basket weavers or cheaper and cheaper food imports to keep the price of labor low (You'll hear crickets when the subject of Chinese agricultural modernization comes up.)

E-buses are one great way to kick them in the pants. A range of sizes for different passenger/mile requirements will really do well: It is actually the jitney to 30 passenger classes that will most fit in the 13 recharges per day, leaving 90 passenger versions for the rush hour.

An intermodal rail-road vehicle would do just great. Many older branch rail lines are suffering for lack of feed into the trunks, and lack of investment justified in either diesel or third rails for relatively few passengers. You've heard of Dial-a-Bus, how about Dial-a-Train? The UK was onto somethign like this 3 decades ago, to cut expenses to its unwieldy National Rail.


People who have heard a bus roar away in a cloud of diesel smoke will appreciate an EV bus.


One main reason to buy BYDs (or other e-buses from China) is much lower initial cost, specially the units with very large battery packs (300+ kWh).

The other reason is availability or quicker delivery.

However, if the CAN $ keeps dropping (to 50 cents or so), a Canadian built e-bus may become more competitive.

Ad Van der Meer

Why would a 88 kWh battery cost $100,000 when you can buy a Tesla Model S 90D for $88,000?
I am shocked by the price tag of the electric version and I can't think of a reason why.
Of course the CNG bus can use most the parts of a diesel bus which are made in greater number, but still $330,000 extra?

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