Technical Advisory & Assurance
Technical study · Engineering & Design
Electrifying a 120-Bus Depot Without a Network Upgrade
Managed charging, an on-site battery and rooftop solar fit a 120-bus electric fleet inside a depot's existing supply, cheaper and sooner than the network upgrade on offer.
What the client needed
A contracted bus operator in South East Queensland had five years to move a 120-bus depot to battery-electric buses. The network provider's first offer was a new feeder and depot substation, at around $7.5 million and with a two to three year lead time.
The question they needed answered: Can the depot be electrified on schedule, and is there a cheaper way than waiting for a full network upgrade?
- 11 to 3.6 MWpeak grid demand, unmanaged versus managed charging with a battery
- ~$1.9Mcheaper than the network upgrade
- 120 to 60chargers needed, by sequencing dual-outlet units
What we did
- Worked through timetables and blocks to find the energy each bus needs overnight and how long it is actually parked.
- Modelled depot demand for unmanaged charging, managed charging, and managed charging with an on-site battery and rooftop solar.
- Compared the network upgrade with the battery-based option on cost, timing and risk.
- Developed a staged charger layout and electrical concept matched to fleet delivery dates.
What we found
The fleet needs about 36 MWh a night and is parked for nine hours. If every bus plugs in on arrival, demand spikes past 11 MW. Spread across the night, it's about 5 MW.
With managed charging, a 3 MW / 12 MWh battery and 1 MWp of rooftop solar, the full fleet fits within the existing 3.6 MVA supply, for about $5.6 million and 12 to 15 months to energise.
Sequenced dual-outlet 150 kW chargers serve two buses each, halving the charger count. The first 40 buses need nothing beyond chargers.
If the battery is out of service, the existing supply with managed charging still delivers about 70% of the overnight energy. A modular battery and charging prioritised by each bus's next-morning block keep the timetable running.
Deferring the $7.5 million upgrade by seven years is worth about $2.8 million in today's money, even if it is eventually needed.
What we recommended
- Stage 1 (40 buses): install chargers and a charging management system within the existing supply.
- Stage 2 (80 to 120 buses): add the battery and rooftop solar, and defer the network upgrade until growth beyond about 150 buses needs it.
- Specify chargers and the management system on open protocols (OCPP) to avoid vendor lock-in.
- Size switchboards and cable routes for the eventual upgrade now, so it can be added without rework.
Key assumptions
250 km per bus per day at 1.2 kWh/km, nine-hour overnight dwell. Battery around $380/kWh installed, rooftop solar around $1.0/W. Network upgrade cost and timing typical of SEQ depot connections.
Related work
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