5 MWh or 10 MWh BESS: Choosing the Right Containerised Storage Scale

Containerised battery energy storage has become the dominant format for grid-scale and large C&I storage projects. The standardised form factor — a standard shipping container footprint, pre-integrated at the factory, delivered as a commissioned unit — has driven down costs, compressed timelines, and made BESS bankable for smaller developers who cannot carry the risk of custom-engineered installations.

Two sizes cover most applications: the 20-ft container (typically 5 MWh usable) and the 40-ft high-cube container (typically 10 MWh usable). Choosing between them is not complicated — but several factors that are frequently overlooked determine the right answer.

What the Two Formats Deliver

20-ft container BESS (SIE-BESS5000):

• Usable capacity: 5 MWh at 0.5C
• Power: 2.5 MW continuous, 3 MW peak
• Footprint: ~30 m² (with clearance access)
• Weight: 24–26 tonnes loaded
• Transport: standard flatbed, no special permit required in most Indian states
• Stack: up to 4 units in parallel for 20 MWh without inverter replication

40-ft high-cube container BESS (SIE-BESS10000):

• Usable capacity: 10 MWh at 0.5C
• Power: 5 MW continuous, 6 MW peak
• Footprint: ~60 m² (with clearance access)
• Weight: 42–46 tonnes loaded
• Transport: heavy transport permit required in most Indian states; check route clearances
• Stack: up to 2 units in parallel for 20 MWh per inverter block

When to Choose 5 MWh

The site is access-constrained. A 20-ft container travels on a standard flatbed lorry. A 40-ft loaded BESS container requires a low-bed trailer and, depending on the route, an escort and transport permit. For projects in industrial estates, urban substations, or sites with narrow approach roads, the 20-ft format removes significant logistical risk.

The project capacity is 5–20 MWh. For projects in this range, multiple 20-ft units in parallel offer advantages: N+1 redundancy (one unit offline does not take the whole system down), staggered commissioning, and modular expandability. A 15 MWh project built from three 5 MWh units has better operational availability than a 10 MWh + 5 MWh combination.

Phase expansion is likely. If the project is phase 1 of a multi-phase development — a common structure in SECI-funded projects — the 20-ft format allows capacity additions without site redesign. Each new container is an independent power block with its own inverter interface and protection relay.

The application requires 4-hour or longer discharge. A 5 MWh / 2.5 MW system discharges at C/2 — comfortable for LFP cells and the optimal range for cycle life. For applications requiring 4-hour discharge (2.5 MWh at 625 kW from a 5 MWh container), the system operates at C/4 — even gentler on cells, extending cycle life further.

When to Choose 10 MWh

The project is 10 MWh or larger with a single-site concentration. For a 10 MWh standalone project, a single 40-ft container delivers the same capacity as two 20-ft units with a single inverter, single protection scheme, and a simpler control architecture. CAPEX per MWh is typically 8–12% lower for the 10 MWh format due to inverter and BMS sharing.

Inverter count must be minimised. Each 20-ft container in a parallel string requires its own bi-directional inverter (or a shared central inverter with switchgear). Inverter cost is significant — approximately 15–20% of total system cost. A project where minimising power conversion equipment count is a priority benefits from the 40-ft format's higher capacity per inverter.

The site is greenfield with unrestricted access. If you are building on a dedicated solar park or substation plot with a proper access road and crane clearance, 40-ft transport logistics are straightforward. In this scenario, the per-MWh CAPEX advantage of the larger unit should drive the selection.

The project is 20 MWh or above. Beyond 20 MWh, the optimal architecture is typically 40-ft units: two 10 MWh units (20 MWh), three (30 MWh), and so on. The per-unit inverter and control system overhead is lower, and the site footprint is more compact.

The Expandability Question

Both formats support modular expansion — but the 20-ft format is more flexible. Adding a 5 MWh block to an existing 5 MWh installation requires:

• Additional AC connection point and protection relay
• EMS configuration to add a new battery string
• Civil works for the new container pad (~30 m²)

Adding a 10 MWh block to an existing 10 MWh installation is identical in principle but involves a heavier transport event, more civil scope, and typically a larger EMS upgrade.

For projects where phase-expansion is in the project agreement (common for SECI projects with capacity obligation escalators), agree the expansion architecture with your BESS supplier before commissioning phase 1. The EMS, communications architecture, and grid connection sizing for the full project should be in place from day one.

A Sizing Decision Framework

Answer these questions in order:

1. What is the total project capacity? If ≤ 5 MWh: 20-ft is the only option. If 5–20 MWh: both are viable. If > 20 MWh: 40-ft is preferred.
2. What is the site access quality? Poor access (urban, industrial estate, narrow road): 20-ft. Unrestricted greenfield: either.
3. Is expansion planned within 5 years? Yes: 20-ft for modularity. No clear expansion plan: either.
4. Is N+1 availability required by the off-taker SLA? Yes: 20-ft in pairs (one unit down = 50% capacity available). No: either.
5. What is the CAPEX budget sensitivity? 8–12% lower per MWh on 40-ft: if budget is binding and access is good, 40-ft wins.

In most Indian project scenarios — SECI projects in the 10–50 MWh range, C&I behind-the-meter, and solar hybrid — the 20-ft unit is the right answer 60–65% of the time. The 40-ft wins in large greenfield utility projects where logistics are unconstrained.

For a project-specific sizing recommendation, contact our engineering team at sales@silicindiaenergies.com. We provide preliminary design and capacity sizing for free as part of pre-sales engagement.