The Constraint Stack: What Actually Limits the Battery You Can Build

Battery sizing works better when the feasible operating envelope is established before the revenue model chooses a configuration. A 100 MW / 400 MWh design can be a reasonable starting case; it should not be treated as a default that every site can or should support.

The reverse order is better, and it is not a stylistic preference. Constraints in a storage project bind on different variables, which means finding the binding one first tells you which of the remaining decisions matter at all.

Interconnection can limit export MW, import MW, charging behavior or combinations of them; site layout and adopted safety rules constrain the physical energy configuration; and a capital budget constrains combinations of power and energy. Those constraints can bind different variables at the same time.

This post opens the sizing and dispatch series with the stack itself: what limits a battery, which variable each item binds, and how to find the one that actually decides your project.

ℹ️ Note: Figures are labelled assumptions except where attributed. Fire code and siting requirements are jurisdiction-specific — NFPA 855 is widely adopted but the authority having jurisdiction governs.

The constraint stack in one picture:

flowchart TD
  A[Candidate site] --> B{POI / interconnection MW}
  B -->|Power cap| C[Maximum charge & discharge MW]
  C --> D{Land + fire-code footprint}
  D -->|Energy cap| E[Maximum usable MWh]
  E --> F{Capital budget}
  F --> G[Feasible MW / MWh configuration]
  G --> H{Offtake + market value}
  H --> I[Buildable and economic battery]

What Is the Constraint Stack?

The ordered set of physical, regulatory, commercial and financial limits on the battery you can actually build at a given site. Each binds a specific variable, and the project is determined by whichever binds first — not by the configuration you wanted.

The Seven Constraints

Constraint	Primary variable affected	Evidence to inspect
Interconnection service	export MW, import MW and operating assumptions	queue request, study assumptions, interconnection agreement
Site / layout	equipment count, spacing, usable MWh	civil layout, parcel constraints, vendor design
Safety / fire code	layout, spacing, protection systems	adopted code, AHJ requirements, UL 9540A / vendor test evidence
Capital budget	combinations of MW and MWh	sources and uses, vendor pricing, contingency
Technical capability	real/reactive power, ramping, telemetry, SOC limits	equipment data and interconnection schedules
Offtake / market rules	commercially valuable MW, duration and throughput	toll / PPA, capacity and ancillary-service rules
Supply chain / schedule	feasible COD and configuration timing	procurement schedule, transformer / PCS / container delivery

The column that matters is the middle one. A developer that treats all seven as a single "feasibility" question misses that relieving a power constraint does nothing for an energy constraint, and vice versa.

Interconnection: The Power and Charging Envelope

Interconnection can be the binding constraint because it defines the import / export rights and operating assumptions the project is allowed to use. Whether it eliminates more candidate sites than acreage, permitting or other siting constraints is portfolio-specific and should not be asserted from a generic benchmark.

Two features make interconnection distinctive as a constraint.

Do not reduce the interconnection question to one export-MW number. FERC Order 2023 specifically requires transmission providers, at the customer’s request, to use operating assumptions in interconnection studies that reflect the proposed charging behavior of storage. Import capability, export capability and the proposed charge/discharge envelope can therefore all matter.

Adding energy behind an unchanged MW rating may be a non-material change in one project and a restudy issue in another, particularly if charging behavior, equipment characteristics or modeled system impacts change. The interconnection documents decide.

It can resolve late and outside the project's control. As the interconnection queue post and the network-upgrades post explain, queue studies, network upgrades and restudies can change cost and timing after meaningful development spend has already been committed.

FERC’s pro forma LGIA includes concepts such as Limited Operation and Provisional Interconnection Service, but those are specific interconnection mechanisms rather than a generic “battery can take a cheaper conditional connection” option. Treat any limited-service arrangement as a document-specific operating constraint in the revenue model.

Land: Model the Actual Layout

Land does not convert to MWh through one stable industry ratio.

A DOE environmental assessment for a New York storage project noted that existing upland BESS facilities ranged widely in site size and described a rough MW-based planning range of about one acre per 30–40 MW, depending on arrangement and interconnection location. That is useful evidence of variability, not a universal design rule.

For a real project, start from the vendor / civil layout:

Container or enclosure count
× equipment footprint
+ required separation
+ PCS / transformer / MV equipment
+ fire access lanes
+ drainage / stormwater
+ setbacks and easements
+ substation / gen-tie facilities
= usable project footprint

The binding site constraint can therefore be energy, power equipment, fire access or substation geometry depending on the design. Converting parcel acreage directly into MWh before that layout exists creates false precision.

Fire Code: Start With the Adopted Edition and the AHJ

NFPA 855 is the Standard for the Installation of Stationary Energy Storage Systems. NFPA lists the 2026 edition as the current edition and describes it as providing minimum requirements for mitigating ESS hazards.

That does not mean every project is governed by the 2026 edition. The legally relevant package is the edition adopted by the jurisdiction, local amendments, the fire code, permit conditions and the authority having jurisdiction.

The diligence file should therefore identify:

• which NFPA 855 edition is adopted;
• applicable local fire-code amendments;
• equipment listing / certification requirements;
• UL 9540A or other large-scale fire-test evidence relied upon by the design;
• separation, fire-access and hazard-mitigation requirements actually imposed by the AHJ;
• whether the selected enclosure / container arrangement remains valid after those requirements are applied.

Fire-test evidence can create layout flexibility where the adopted rules and AHJ permit it, but do not convert a vendor test result into a universal spacing entitlement.

Why the Binding Constraint Has to Be Found First

Because it determines which optimisation is worth doing.

Consider the same project under two different binding constraints.

Power-constrained (a 50 MW interconnection envelope on a large parcel). The worked case treats the MW as fixed. Every remaining decision is about what to do with those 50 MW — how many hours behind them, which products to sell and how hard to cycle — unless the project is willing and able to modify the interconnection rights.

Energy-constrained (a 100 MW interconnection on a small urban parcel). The MWh is fixed by land and fire code. Duration analysis is nearly pointless — you cannot add hours. The decisions that matter are about power rating and product mix: with limited energy, high-power short-duration products like frequency response become relatively more attractive.

Those are different projects requiring different analyses, and the answer to "how many hours should it be" is only interesting in the first case. A developer that runs the duration study before establishing which constraint binds may have done a careful piece of work on a question its site does not ask.

Grid Code: The Constraint Written Into the Agreement

Less visible than the first three because it arrives as a schedule to the interconnection agreement rather than as a physical limit, and it can bind either variable.

The requirements that matter for a battery are mostly about behaviour rather than size: fault ride-through, reactive power capability across a defined range, frequency response characteristics, ramp rate limits, and the control and telemetry the operator requires.

Three of them have configuration consequences.

Reactive-power capability is an apparent-power constraint. Real and reactive power share the inverter / PCS MVA envelope: S² = P² + Q². Whether a reactive requirement actually derates real-power capability depends on the PCS MVA rating and the required operating point. An oversized PCS can preserve the intended MW; an undersized one cannot.

Ramp rate and telemetry requirements can constrain market participation. Storage is physically fast, but the market will only credit the capability the resource is certified, telemetered and permitted to offer.

Import / export operating limits can create dead bands or asymmetric operating ranges if they appear in the interconnection or equipment capability. Those limits should be taken from the actual schedules rather than assumed.

The practical instruction is to read the technical schedules to the interconnection agreement as commercial documents, because their effect lands entirely on the revenue side. A project that models 100 MW of energy-market participation against an agreement requiring reactive support has modelled a machine it has not agreed to build.

Offtake: The Constraint on What Is Worth Building

Distinct from the others because it does not limit what you can build. It limits what is worth building, which is the same thing commercially and a different thing physically.

A merchant battery can be any configuration its site allows. A contracted one is shaped by the contract.

A tolling agreement can specify the asset. If the contract defines required MW, usable MWh, duration, availability, round-trip efficiency or throughput, those become contractual design constraints. Some tolls leave more operating flexibility than others, so the agreement — not the label “tolling” — decides what is fixed.

Capacity-market rules can change the marginal value of duration. PJM, for example, publishes separate ELCC class ratings for storage durations such as 4, 6, 8 and 10 hours. That is a revenue input to sizing, not a universal duration floor across US markets.

A hybrid or co-located arrangement imports the host's constraints. Sharing a point of interconnection with a solar array means the battery's available export capacity is whatever the array is not using, which is the subject of a later post in this series.

The order of operations matters here. Signing a toll before establishing the site's physical constraints risks committing to a configuration the site cannot deliver. Establishing the constraints first and then contracting to them is slower and avoids the worst outcome in development, which is a signed obligation against an unbuildable asset.

Supply Chain: The Constraint That Binds on the Calendar

The odd one out, because it rarely limits size and frequently limits timing.

Cell availability, container delivery slots, PCS availability and transformer lead times can determine when a project can be built rather than how large it can be. That timing can interact with financing milestones, offtake COD obligations, interconnection milestones and tax assumptions.

Do not import wind-and-solar statutory deadlines into a standalone storage model by analogy. Storage tax treatment has its own eligibility and placed-in-service analysis. The broader modelling point is simpler: procurement dates should be mapped against the actual financing, offtake, interconnection and tax milestones that apply to the project.

That makes procurement timing a financial constraint wearing an operational costume. Model the delivery schedule against the actual date ladder and quantify the value of earlier equipment against the specific COD, financing, tax and contract consequences it protects.

The Constraint You Can Buy, and the Ones You Cannot

Worth separating, because it changes the negotiation.

Some constraints can be relaxed with money or redesign; others require a different permit, interconnection position, contract amendment or site. The useful classification is therefore relaxation cost and lead time, not “buyable” versus “not buyable.”

A project should report what it would take to relax each binding constraint and whether that relaxation is actually available within the project schedule.

How Do You Model the Constraint Stack in Excel?

As a table of limits with the binding one identified and the shadow price of each reported.

The constraint table

                          Limit on power   Limit on energy
Interconnection              100 MW              —
Land (20 acres usable)          —            870 MWh
Fire code spacing               —            (in the land figure)
Capital budget $110m          varies          varies
Grid code                    100 MW              —

Binding on power:     Interconnection at 100 MW
Binding on energy:    Capital budget

The budget constraint, which is a line not a point

The capital budget does not limit power or energy on its own — it limits a combination, and the trade-off is set by the cost structure developed across the power-sizing post and the cost-curve post:

Budget = Power_MW × BOS_$/kW × 1,000 + Energy_MWh × Pack_$/kWh × 1,000

$110,000,000 = MW × 380,000 + MWh × 180,000

At 100 MW that leaves (110,000,000 − 38,000,000) / 180,000 = 400 MWh, or exactly four hours. At 80 MW it leaves 442 MWh — 5.5 hours on a smaller power rating.

That line is the worked budget frontier. Plotting it alongside the physical limits makes clear which configurations are feasible before revenue optimization begins.

The shadow price of each constraint

Shadow price = value of relaxing the constraint by one unit

Interconnection:   value of one more MW of POI
Land:              value of one more acre
Budget:            value of one more dollar of capex

A constraint with a shadow price of zero is not binding and can be ignored. A constraint with a large shadow price is where effort should go — and if that constraint is the interconnection, the honest conclusion may be that the effort belongs on a different site.

The screening test that comes before all of this

For a developer looking at a pipeline of candidate sites rather than a single project, the useful output is not a shadow price at all. It is a rejection rule.

For each candidate site:
   Distance to usable substation         → reject beyond threshold
   Available interconnection capacity    → reject below minimum viable MW
   Usable acreage after setbacks         → compute max MWh
   Implied duration at max MW            = max MWh / interconnection MW

Flag unusually short or long implied duration for commercial review.
Do not hard-code 2-hour / 8-hour rejection thresholds unless the market, offtake or economics justify them.

That ratio is a useful screening diagnostic because it shows whether the site geometry and interconnection envelope are naturally aligned with the duration range the commercial case is considering. It is not, by itself, a reject rule.

ℹ️ Note: Report which constraint binds as a model output, not an assumption. A model told that the project is 100 MW / 400 MWh cannot tell you whether that configuration was chosen or merely inherited.

To build the constraint table, the budget frontier and the shadow prices, prompt Dezzmond with your site, interconnection and budget parameters.

What Do Developers Actually Check?

• What are the separate import and export limits, and what charging assumptions were studied?
• What is the usable site layout after the adopted code, AHJ conditions, access and electrical equipment are applied?
• Which NFPA / fire-code edition is legally relevant to this site?
• What real/reactive capability does the PCS have at the POI?
• Which contract or market rule creates a minimum valuable duration or throughput requirement?
• What would it take — cost and time — to relax each binding constraint?
• Which constraint has the highest shadow value in the model?

Frequently Asked Questions

What limits the size of a battery project?

Different things limit different dimensions. Interconnection capacity limits power; land and fire code limit energy; the capital budget limits a combination of the two. Finding which binds first determines which analysis is worth doing.

Why is interconnection such a central constraint?

Because it defines the grid-facing operating envelope and can create long lead-time, study and upgrade obligations. But whether it is more important than land, permitting or fire-code layout is site-specific.

How much land does a battery need?

There is no durable acres-per-MWh rule. Use the vendor and civil layout plus adopted safety requirements, access, drainage, transformers, substation and setbacks. DOE project documents show utility-scale sites varying materially with equipment arrangement and interconnection geometry.

What does the 2026 NFPA 855 edition mean for a project?

NFPA lists 2026 as the current edition, but the project must follow the edition and amendments adopted by its jurisdiction. The practical diligence task is to identify the adopted code, AHJ conditions and fire-test evidence that support the actual layout.

Does a tolling agreement constrain the configuration?

It can. If the contract fixes MW, usable MWh, duration, availability, efficiency or throughput, those terms become design constraints. Other tolls leave more operating flexibility.

Why does supply chain matter if it does not limit size?

Because it limits the feasible COD and can interact with financing, offtake, interconnection and tax milestones. Model those dates explicitly rather than treating procurement as a generic schedule buffer.

Does adding duration require a new interconnection study?

Not automatically, and not never. If export MW is unchanged the change may be non-material, but a different charging profile, equipment model or system impact can still matter. Check the interconnection procedures and transmission provider determination.

Closing: Find the Edge Before You Optimise Inside It

The instinct in a development team is to start with a configuration, because a configuration is something you can model. The constraint stack is unglamorous by comparison — a list of limits, mostly discovered by asking other people.

But the configuration is downstream of the limits, and the limits are not interchangeable. An additional hour can be relatively attractive on a power-constrained site because some power-related infrastructure is already fixed, while a land- or safety-constrained site may have no physical room for it. An extra MW can be economically valuable and still unavailable inside the studied interconnection envelope. The same analysis produces opposite answers depending on an input that most models do not contain at all.

The discipline is one line of output: which constraint binds, on which variable, and what would it cost to relax. With that, the rest of this series is a set of tractable questions. Without it, the duration study, the dispatch model and the revenue forecast are all being run on a configuration nobody established was available.

the next post takes the first of those variables on its own terms: what the megawatts are actually for, and how to size the power rating against the revenues that pay for it.

Sources: FERC — Order No. 2023 explainer, including storage charging assumptions in interconnection studies · FERC — Pro Forma LGIA, including Limited Operation / Provisional Interconnection Service provisions · NFPA — NFPA 855 standard development page; current edition 2026 · DOE — Floating Energy Storage System environmental assessment, siting examples · Dezzmond analysis; worked budget-frontier assumptions are illustrative.