Fleet electrification creates one of the most compelling commercial cases for on-site solar and battery storage. A depot charging 50 electric trucks overnight can easily add 500–1,000 kW of new electricity demand — a load that, without mitigation, triggers expensive utility demand charge upgrades and capacity fees. A well-designed solar + BESS system can slash this demand penalty, provide energy cost savings and meaningfully reduce the fleet's carbon footprint.
The EV fleet depot charging challenge
Fleet depot charging creates specific energy challenges that differ from typical commercial solar:
- Concentrated overnight charging: most fleets charge 22:00–06:00, when solar generation is zero — storage or grid power is required
- High peak demand: simultaneous charging of multiple vehicles creates sharp demand peaks; a fleet of 20 × 150 kW chargers theoretically draws 3 MW if all charge simultaneously
- Demand charge exposure: commercial electricity tariffs often include a demand charge of $10–30/kW/month; a 3 MW demand spike adds $30,000–90,000/month in charges
- Grid capacity constraint: utilities may require expensive grid upgrades ($500K–5M+) to supply high-demand fleet depots; on-site generation and storage can defer or avoid this cost
Smart charging as the first line of defence
Before sizing solar or BESS, implement smart charging to reduce the raw demand spike:
- Managed charging: a fleet charging management system (FMS) staggers charging start times to avoid simultaneous peaks — 20 vehicles charging at 75 kW average vs. 3 MW simultaneous peak
- Depot energy management system (EMS): coordinates chargers with on-site solar and BESS to minimise grid peak demand
- V2G (vehicle-to-grid): where available, EV batteries can discharge back to the building during peak demand periods — each truck with 300 kWh usable becomes a temporary BESS
Smart charging alone can reduce peak demand by 40–60%. BESS then handles the residual demand peak; solar reduces the total energy imported from the grid.
Solar sizing for fleet depot
Fleet depot solar should be sized to offset daytime grid imports, not to power overnight charging directly:
- Daytime building loads (office, warehouse, lighting, AC): size solar to cover 80–100% of these loads
- BESS charges from solar during the day; BESS discharges during the evening charging peak
- Solar does not directly power overnight charging — that comes from BESS discharge + grid
Typical depot solar sizing: 20–50% of the total annual charging energy demand. Example: 50-truck depot consuming 3,000 MWh/year → 600–1,500 MWh/year from solar → 400–1,000 kWp solar system depending on location.
BESS sizing for demand management
The BESS must be sized to flatten the evening charging demand peak:
| Fleet size | Charging demand (managed) | Demand target (grid) | BESS required |
|---|---|---|---|
| 10 EV trucks | 300 kW peak, 4 hrs | 150 kW | 600 kWh / 150 kW |
| 25 EV trucks | 750 kW peak, 5 hrs | 300 kW | 2,250 kWh / 450 kW |
| 50 EV trucks | 1,500 kW peak, 6 hrs | 500 kW | 6,000 kWh / 1,000 kW |
| 100 EV trucks | 3,000 kW peak, 6 hrs | 800 kW | 13,200 kWh / 2,200 kW |
Sungrow's ST255CS-2H liquid-cooled BESS (255 kWh per unit) scales readily for fleet depot applications, with up to 5 MW/10 MWh per containerised installation. The integrated EMS coordinates with the depot's charging management system via Modbus/BACnet.
Financial model: a 50-truck depot example
- Fleet: 50 electric trucks, 200 kWh usable per truck, 80% utilisation = 8,000 kWh/day charging demand
- Without solar/BESS: utility grid peak demand 2 MW → demand charges ~$40,000/month
- With 800 kWp solar + 3 MWh BESS: grid peak reduced to 500 kW → demand charges ~$10,000/month
- Energy cost savings: 2,400 kWh/day from solar at $0.12/kWh = $104,000/year
- Demand charge savings: $30,000/month × 12 = $360,000/year
- Total annual savings: ~$460,000/year
- System cost (800 kWp solar + 3 MWh BESS installed): ~$2.5M
- Simple payback: ~5.4 years; IRR ~18%