I have been following BYD Auto for a while; their electric buses are in use, successful, and being sold to more and more places quite quickly.
The economics of switching to electricity for a city bus are *massive*. The duty cycle is entirely start-stop-start-stop, so you want high acceleration and deceleration. The bus is enormous and very heavy, requiring even more acceleration, and causing low fuel efficiency for ICEs. But since they're stopped all the time, with ICEs, you spend a huge percentage of the time idling, with even lower fuel efficiency. People hate localized diesel emissions, which are pretty nasty -- it's bad enough that a lot of cities have switched to compressed natural gas buses, but those are complicated, expensive to refuel (due to the complicated fuelling stations), cost $30K more than diesel buses, and break quickly. Electric buses last more years than diesel or CNG buses, as well (we know this from trolleybuses), and the charging stations are relatively cheap one-time expenses.
BYD has got a bus which can easily do an 250-kilometer (155 mile) duty cycle (which is common for a city bus) on one charge. The battery is *324 kwh* (it's a bus, there's a lot of room to put it on the base of the bus).
BYD and New York Metropolitan Transportation Authority Complete Successful Pilot Test of BYD Electric Bus | Inside EVs
There are a few major competitors with very similar products (such as Proterra).
The payback time for public transit agencies from switching to electric is very fast. The typical lifespan for diesel buses is only 12 years, while electric buses last a bit longer (about 15 years). The savings is from $47K - $62K per year depending on what estimate you read. I've seen BYD price estimates ranging from $320K-$480K. A typical diesel bus costs $300K-$600K, so the BYD buses *cost about the same amount*. An agency may worry that they will have to spend another $100K after about 7.5 years for battery replacement, but even with this cost, the savings is massive and immediate; even using the worst-case estimates, the BYD bus will pay for itself in fuel savings vs. a diesel or CNG bus, in less than 6 years.
Expect to see BYD electric buses (and other electric buses, too) sweeping the world within the next 10 years as the old diesel/CNG fleet wears out. Fuel buses will be relegated to poor areas which can only afford secondhand buses, or maybe kept for express "commuter" routes. This will happen *very fast* because transit agencies are always looking at the bottom line, and with the very high mileage which gets put on city buses, the economics are overwhelmingly in favor of battery buses.
(Side note: people have suggested putting a fuel heater in electric cars. This is actually being done in electric buses, where the hit to range from heating could be disastrous for the schedule.)
