Electric retrofit: anything but retro!

Explore • Pulse #6 • 4 min
By Adeline Tissier

Converting a diesel bus to electric drive rather than replacing the entire vehicle is an attractive idea for cities keen to speed up their ecological transition. Around the world, electric retrofit initiatives of diesel bus are taking shape. And while this technology is developing unevenly, it could be a real opportunity to reduce bus fleets’ carbon footprints.

Innovative enthusiasts

The concept of electric retrofit first emerged in California in the 1970s, when classic car enthusiasts began toying with it. What started as a niche trend soon spread. Today, all kinds of vehicles can be retrofitted — from private cars to trains, tractors and heavy machinery. More recently, it has attracted growing interest from cities wanting to run zero-emissions buses without the expense of buying new-build vehicles.

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What alternatives to conventional fuels?

2 min

The fine art of transformation

The idea is simple. Take a diesel bus that’s halfway through its service life — typically eight to ten years. Swap its combustion engine for a new drivetrain with electric motor. Add batteries and a charging port. Maybe make a few upgrades to the coachwork and internal fittings. And hey presto, you now have a ‘retrofitted’ electric bus, which is ecofriendly, super quiet and ready for another six to eight years of service!

Ticking all the low-carbon boxes

For transport authorities keen to embrace the circular economy model, it’s a great way forward. A retrofitted diesel bus reduces your carbon footprint on at least two levels: not only are exhaust pipe emissions eliminated, but so are all the industrial ones involved in making a new vehicle. Each existing bus converted to electric is one less new electric bus to be built!

Favourable regulations

The technology is moving forward apace, but it still needs a bit of stimulus. First, legislation could encourage or require cities to transition to clean buses — as in California, where it will be impossible to buy polluting vehicles after 2029. Specific guidelines also need to be set for converting existing vehicles. That’s what Germany and Italy have done, as well as France more recently with a government decree issued on 13 March 2020. Another effective lever is subsidies.

How much does it cost?

Electric retrofit also needs a viable business model. For now, the overall cost of retrofitting a diesel bus is about the same as the price of a new electric bus. The combined expense of a retrofit is still higher than the savings in maintenance and fuel over the ‘electric phase’ of a bus’s service life. A battery pack costs about €250,000. It then has to be installed, complete with charging ports. Plus there’s the purchase price of the original bus and maintenance and operation for the first eight to ten years, typically another €250,000. Going forward, the emergence of specialist companies to create an effective industry ecosystem will be vital for making this approach economically attractive.

Gas-powered buses can also be retrofitted to electric, but it’s easier to convert them to hydrogen.


The number of countries worldwide that allow retrofit.
Source: AIRE (electric retrofit industry association)


The number of charge/discharge cycles for a battery on a retrofitted bus over its six to seven years of service.

  Make old new again

After a successful trial in 2018, the city of Boulder, Colorado, is planning to retrofit nine diesel buses to electric drive. The decision was largely influenced by the much shorter delivery timeframe of six months, compared to two years for new-builds. A similar project is going ahead in Lane County, Oregon.

  A first in Oxford

For the first time, Oxford has retrofitted a diesel open-top sightseeing bus to electric (March 2020). Four others will be converted soon.

  European Commission support

A €107 million subsidy was granted to Germany in late 2018 to fund the conversion of 7,000 diesel buses to electric in almost 90 municipalities. A real boost!


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