Commercial BESS Sizing: From Load Interval to Interconnection

Commercial BESS Sizing: From Load Interval to Interconnection

A storage system is sized from the interval data and the bill it is meant to fix - not from a catalogue. Here is the sequence that survives engineering review.

Commercial BESS Sizing: From Load Interval to Interconnection
By Marcus Rendon, PE · 2026-11-05 · 11 min read

Start from the interval data, not the load list

A commercial battery earns its money on demand charges, time-of-use arbitrage, or backup. Each of those is a different sizing problem, and each is answered from the utility's 15-minute interval data - not from a nameplate load list. Request 12 months of interval data before a single kW is specified.

If the site has no interval data, the first deliverable is a metering plan, not a quote.

Sizing driver by use case

Use case Primary metric Binding constraint
Demand-charge reduction Peak kW shaving Inverter kW and control response time
TOU arbitrage kWh shifted per day Usable kWh and round-trip efficiency
Backup / resilience kWh at critical load Continuous kW, transfer time, outage duration
Solar self-consumption Evening export offset DC/AC ratio and cycle limits

The sizing sequence that survives review

  1. 1

    Pull 12 months of interval data

    Identify the top 20 intervals and how often they recur. A demand charge set by 30 minutes a month is a different problem from one set daily.

  2. 2

    Set the control objective numerically

    e.g. 'hold site import below 350kW' or 'shift 800kWh per day'. Without a number, no size is wrong.

  3. 3

    Size the inverter first, the battery second

    Inverter kW sets how fast you can respond; battery kWh sets duration. Oversizing kWh to fix a kW problem is the most common error.

  4. 4

    Check the interconnection limit

    Export vs non-export, existing service capacity, and whether the utility requires relay protection. This frequently caps the design.

  5. 5

    Confirm NFPA 855 separation and permits

    Setback, fire suppression and separation distances decide where the enclosure can physically go - which can decide whether the project is viable at all.

Frequently Asked Questions

How do I size a battery for demand-charge reduction?
Find the peaks above the target threshold in interval data, compute the energy needed to shave them, then size inverter kW for response speed and kWh for the deepest recurring shave. Most projects land at 2-4 hours duration.
Does a BESS need to export to the grid to be worth it?
Not always. Non-export, behind-the-meter systems avoid much of the interconnection cost and still capture demand and TOU value. Restricting export often shortens the schedule more than it costs in revenue.
What certification will the permit package require?
UL 9540 for the system and UL 9540A cell-level test data are standard; NFPA 855 governs separation and fire protection. The authority having jurisdiction may add local amendments.

Send interval data for a sizing review

Get trade pricing, freight quotes, and portal calculators.

Register Now

For Registered Contractors -- Free Portal Toolkit

Register for free portal access ·>

Where to source it

Send a list to the quote desk for line-item trade pricing and freight.

Popular items in this category

Trade pricing and freight on any of these: quote desk.

Related guides

About the author

Marcus Rendon, PE - Commercial Systems Engineer. Expertise: C&I solar, commercial BESS, motor controls, industrial distribution. Written for contractors and integrators buying through PES Supply.

More by Marcus Rendon, PE: