Designing DC Block Augmentation Space Without Overpaying Today

Five years ago, a standard 20-foot BESS container held approximately 2 MWh. Current containers from major equipment manufacturers hold 6 to 7 MWh in the same footprint. By 2030 to 2033 — the window when most projects now in development will need their first augmentation — containers may hold 12 to 15 MWh. This trajectory creates a counterintuitive planning problem: a project designing augmentation space today for equipment that arrives in 2033 is reserving space for containers that will be 2 to 4 times denser than what it installs now. Restoration augmentation may need less physical space than the original equipment occupied to deliver the same MWh.

A project that installs twenty containers at 5 MWh each (100 MWh total) and loses 20% capacity over ten years needs to restore 20 MWh. If augmentation containers hold 12 MWh each, two containers restore the lost capacity. The project reserved space for twenty replacements but needs two. Reserve space based on today's energy density and the project over-reserves. Reserve based on projected density and the project risks under-reserving if the trajectory slows. The practical planning assumption: use 1.5 to 2 times current container energy density for the augmentation window, and design the layout so augmentation positions can flex if actual containers are larger or smaller than projected.

DC Augmentation vs AC-Coupled Augmentation

The type of augmentation a project will need determines the spatial and infrastructure requirements.

DC augmentation adds new DC blocks behind existing PCS units. The AC-side infrastructure remains unchanged. No grid code retesting is required because the MW export capacity stays the same. The site needs space for additional foundations near existing PCS positions and routes for new DC cabling.

AC-coupled augmentation adds new DC blocks along with new PCS units, new MV transformers, and new switchgear bays. This is required when the project needs to increase its MW export capacity, or when the existing PCS units are fully loaded. AC-coupled augmentation is a much larger spatial and infrastructure commitment. It often requires modifications to the grid connection agreement.

The grid connection agreement's MW cap frequently forces this decision. A project with a fixed MW allocation that only needs to restore degraded MWh can augment on the DC side. A project that needs to add both MW and MWh must go AC-coupled. The two paths have fundamentally different land and infrastructure requirements.

Projects that don't distinguish between DC and AC augmentation at the layout stage often over-build infrastructure they will never use, or under-build infrastructure they cannot retrofit.

What to Oversize From Day One

Not every piece of infrastructure is equally expensive to retrofit. The augmentation space decision is really a series of individual decisions about which components to oversize now and which to defer.

Oversize: Cable Corridors and Trenches

Widening a cable trench during initial construction costs EUR 30 to 60 per meter in additional civil works. The same work on an operational site — with live cables in adjacent trenches, operating equipment nearby, and outage coordination required — costs 5 to 10 times more and requires safety shutdowns that can take weeks to schedule. This is the clearest day-one investment in the entire layout. No other single line item has a comparable ratio of future-cost-avoided to present-cost-incurred.

Oversize: MV Switchgear Bays

Adding spare feeder bays to the MV switchgear at commissioning costs EUR 20,000 to 50,000 per bay. Adding bays mid-life requires outage planning across the entire MV bus, potential switchgear replacement if the existing panel cannot accept additional bays, and factory lead times that stretch to 12 months. A missed augmentation window because switchgear delivery pushes the schedule by a year is an avoidable failure. Spare bays are cheap insurance against it.

Oversize Conditionally: MV/HV Transformers

Transformer oversizing only matters if augmentation adds MW, not just MWh. A DC augmentation that restores degraded energy capacity behind existing PCS units does not increase the MW flow through the transformer. Only AC-coupled augmentation — which adds new PCS and increases MW export — requires transformer headroom. Oversizing a transformer for a project that will only ever do DC augmentation is wasted capital with no recovery path.

Do Not Oversize: DC Cables

With distributed PCS architectures, augmentation blocks get their own DC cables. There is no shared DC bus to oversize. The DC cables are installed when the augmentation blocks are installed, using the pre-built cable corridors.

Do Not Oversize: HV Cables

The MW capacity of the grid connection is usually fixed at permitting. HV cables are sized to the permitted MW. Unless the project has a realistic path to increasing its grid connection capacity, oversizing HV cables has no purpose.

Critical: Foundations and Crane Access

Pre-installing foundations or foundation-ready pads at augmentation positions is one of the highest-value day-one investments. Doing this work on an operating site means coordinating around live equipment, managing dust and vibration near operating battery containers, and accepting partial outages. A foundation that costs EUR 15,000 to 20,000 during bulk earthworks may cost EUR 60,000 to 80,000 as a standalone retrofit with all the associated access, safety, and reinstatement costs.

Crane access corridors to augmentation positions are equally critical. If a future container position cannot be reached by a crane because the access corridor was not preserved in the original layout, that position is unusable regardless of whether a foundation exists there.

The Commercial Frame

Augmentation space is not free. Pre-installed foundations sit unused for 7 to 10 years. A spread-out layout increases cable run lengths and day-one CAPEX. A larger footprint means more fencing, grading, drainage, and a bigger environmental permit.

These costs are real. But retrofitting foundations, trenches, and access routes on an operating site is consistently more expensive and more disruptive. Lenders and investors now require augmentation plans in their financial models. Augmentation is a modeled assumption with cost lines and revenue projections attached to it.

Recommendations

Most projects with a fixed MW grid connection should plan for DC augmentation. It is simpler, cheaper, and avoids grid code retesting. AC-coupled augmentation is the right path only when the business case explicitly includes MW growth with a confirmed grid connection upgrade pathway.

Pre-install cable corridors, switchgear bays, foundations, and crane access routes. Leave DC cables and transformers (for DC-only augmentation) for augmentation day. This split captures the items with the highest retrofit-to-install cost ratio while deferring items that add no value until augmentation procurement starts.

Use 1.5 to 2 times current container energy density as the planning assumption for space reservation. Design augmentation positions so they work with both the assumed density and a higher-density outcome — this means avoiding layouts that lock augmentation to a single container dimension. A flexible augmentation bay that can accept one large container or two smaller ones costs marginally more at the foundation stage and eliminates the risk of stranded space.