Electric bus use is growing, with nearlyhalf of municipal buses on the road globally expected to be electric by 2025 and the size of the U.S. electric bus marketprojected to reach $1.95 billion by 2024. With all this change in the (cleaner) air — amid growing environmental concerns, legislation mandating electric vehicle adoption, low maintenance costs, and funding and tax incentives — transit companies that aren’t already using electric buses would do well to consider planning how to integrate them into their fleet.

Though the infrastructure for electric vehicles needs to be taken into consideration, including issues such as which batteries to use and where charging stations should be located, integrating electric vehicles into a fleet is about much more than hardware. The underlying challenge is that electric buses have different needs than diesel ones — perhaps most significantly, they need recharging more often than diesel buses require refueling — and that makes efficient scheduling both more complicated and more necessary than ever.

Here are three important factors that transit agencies, operators and schedulers should consider when thinking about incorporating electric buses into their planning and scheduling:

Charge times

Introducing electric buses into transit systems requires operators to consider how charge times affect the planning and scheduling of bus routes. It’s essential for operators to figure out each vehicle’s range limits and the minimum charging requirements for each route, which dictate the level of energy needed for a given trip and the amount of spare energy required. Charging time for each battery type must also be considered.

Since electric buses need more recharging than diesel buses, some drivers may face range anxiety, the fear that the vehicle won’t have enough juice to make it to the end of the route. The goal here is to schedule buses in a way that alleviates range anxiety while also getting the most value out of the vehicles by keeping them on the road while incorporating charge times into the schedule at the right junctures.

To do that, it’s important to build a charging strategy that answers questions such as:

• What’s the range and minimum battery capacity for a given trip? For example, a bus with 70% remaining charge could have a remaining range of 45 miles and remaining time of 4 hours and 20 minutes. Once you know the baseline, the schedule can take that into account.

• What’s the maximum widespread? The duration of a break at the depot is more significant for electric buses than diesel ones, since it dictates the potential for increasing the vehicle’s charge.

• How many recharging events do you want to have? For instance, if you have multiple short charging events instead of long charges, you may be able to keep a vehicle at a lower remaining charge if you know it will be recharged shortly. But in some circumstances it may be better to charge just once at night.

• What kind of batteries and chargers are used for each vehicle? Important variables include the type, capacity and discharge rate of the batteries as well as the battery types supported by the chargers and their charging rate.

Cost

Charging electric buses comes with a new set of cost considerations for transit operators. There’s a lot of interplay between cost and charging times, since bus schedules must take into account both the amount of time electric buses need to charge and the time of day they do so.

The cost of electricity changes over the course of a 24-hour period, and also varies based on the day of the week and the season. This means that transit companies can exert some control over operational costs for electric buses by timing their charging events accordingly, but must be careful to take into account the demand charges imposed by electric utilities at peak times, which can have a significant impact on cost.

Cost can also be affected by the type of recharging event, whether for longer periods during off-peak hours or shorter bursts throughout the day.

The lifetime cost of electric buses is lower than diesel buses, despite the demand charges and the higher initial cost of electric buses, a Columbia University electric busanalysis for New York City Transit found. The savings are due to lower operational costs, including maintenance and charging. In addition, battery costs are expected to decline in the coming years, further lowering the overall cost of ownership.

Fleet size

Just as charge time scheduling can affect cost, it can also affect peak vehicle requirements and overall fleet size, since there need to be enough vehicles to allow for quick rotations as one bus gets charged and another moves into active use.

Analyzing multiple scheduling scenarios makes it easier to experiment in order to find the one that best meets needs like reducing PVR.

For example, we used two different charging strategies to see how the length of the charge time would affect the peak vehicle requirement for a hybrid fleet with six diesel buses. We found that when two full, long charges were scheduled per day, the peak vehicle requirement came to nine buses. But it dropped to eight buses with partial recharges that averaged out to four short bursts of energy a day.

Charge times, cost and fleet size are important factors to take into account when planning how best to integrate electric vehicles into a bus fleet. Once these factors are expressed as preferences and constraints, sophisticated scheduling platforms can optimize electric miles by creating efficient schedules that eke the most value out of each bus.

Amos Haggiag is CEO/cofounder of Optibus, a Tel Aviv-based company that provides technology to make mass transit operations more efficient.