My 9-Step Checklist for Buying Commercial BESS (Paid For With About $180,000 in Mistakes)

Who this checklist is for

I've been handling battery storage purchase orders for solar-plus-storage and standalone commercial projects for six years now. In that time I've made at least 14 significant mistakes, totaling roughly $180,000 in wasted budget — some of that was rework, some was liquidated damages from delays, and one order was a full container of equipment that only barely passed our own acceptance test. Now I maintain the checklist our team actually uses. It's not a procurement textbook. It's just what I do now so I stop repeating myself.

This works if your project is somewhere between 200 kWh and 20 MWh, you're getting quotes from battery energy storage system manufacturers or suppliers, and the person signing the purchase order is not the same person installing the thing. Nine steps, about 90 minutes if you already have the project data in front of you. More if you don't, and if you don't, that's the first problem.

The nine steps, in order

Step 1 — Write the requirement before you ask for a price

Most first-time buyers send an energy storage system manufacturer a single number, usually kWh, and wait. That number means almost nothing on its own. 500 kWh at 250 kW is a different product, a different cabinet count, and a different price than 500 kWh at 100 kW.

In my first year I sent out "500 kWh, commercial energy storage system, please quote." Three quotes came back: 500 kWh/250 kW, 480 kWh/500 kW, and 500 kWh/100 kW. I couldn't compare them because they weren't the same product, and I wasted about three weeks pretending I could.

Before you send anything, write down five things: the load profile or interval data for at least 30 days, what the system is actually for (peak shaving, backup, arbitrage, demand charge reduction), the C-rate you need, the usable state-of-charge window you're willing to live with, and the ambient temperature range at the site. That's your requirement. Now you can ask for prices.

Step 2 — Check the interconnection and permitting path before you compare pricing

This is the step almost everyone skips, including me, twice.

What I mean is: the authority having jurisdiction can require separation distances under NFPA 855 that change your footprint. The utility's interconnection study can change your inverter architecture. The fire code edition your jurisdiction actually enforces can be three cycles behind what you designed to. None of that shows up in a price sheet, all of it shows up in the schedule.

In September 2022 we designed a whole system to unit-level UL 9540A test data, then found out the AHJ wanted 3 feet of separation regardless. The design didn't fit. That was a two-month rework and roughly $40,000 of impact between engineering time and a delayed notice to proceed.

Call the AHJ and the utility first. Get the requirements in writing. Then price.

Step 3 — Read the test certificates, not the brochure

UL 9540 is the system-level listing. UL 9540A is the thermal runaway test — cell, module, unit, and installation level. They are not interchangeable, and "our cells are UL certified" tells you nothing about whether the integrated system is listed, or for which configuration.

Here's the thing: energy storage system manufacturers will all tell you they're certified. The question is certified for what, at which model number, with which cells, in which enclosure spacing. Ask for the certificate with the model number on it and match it against the quote. I have seen a quote reference a certificate for a different cabinet series. Twice. From two different suppliers.

Same logic applies to IEEE 1547-2018 for interconnection. Get the compliance statement against the current edition, not a 2015 document with a stamp on it.

Step 4 — Align the warranty language word by word

This is where I lost the most money, so I'll be specific.

Two warranty sheets can say almost the same thing and mean different things. "70% capacity retained after 10 years" and "degradation not to exceed 30% over 10 years" sound identical. They aren't — one anchors to calendar time, the other can anchor to throughput or cycle count depending on how the footnote reads.

Make a table. Columns: who the warranty obligor is (cell maker, PCS maker, or integrator — this matters enormously), guaranteed round-trip efficiency and at what condition, cycle count and the depth-of-discharge it assumes, capacity retention threshold, measurement method, who pays freight on a replacement, and the response-time SLA for a downed unit.

The freight clause alone has cost us $9,000 over three years. Nobody reads the freight clause.

Step 5 — Audit the factory, not the certificate wall

Every battery energy storage system supplier will offer a factory tour. Take it, but don't look at the lobby. Look at three things: the end-of-line test station and its actual raw data, the traceability records linking cell batches to finished serial numbers, and whether the line is running when you visit or was started up for you.

We've caught 47 potential issues using this pre-check in the past 18 months. Most were documentation gaps — a serial number recorded against the wrong cell lot — not equipment failures. But two were units that had been reworked and shipped without the rework record. That's a red flag, not a deal-breaker, but you want to know.

Step 6 — Put the factory acceptance test criteria in the contract, not the appendix

We didn't have a formal FAT acceptance process until 2024. Cost us when a unit showed up, passed a visual inspection, and then failed its first capacity test at site — at which point the vendor argued that site conditions weren't the agreed test conditions.

Write the pass/fail criteria into the main body of the contract: capacity test at the agreed C-rate, measured round-trip efficiency, point-by-point communications protocol verification, full alarm and fault code enumeration, and the exact test conditions (temperature, SOC start point, settling time). Then specify what happens if it fails — rework at whose cost, and whose schedule slips.

Step 7 — Map the long-lead items before you sign

Cells. PCS. Transformers. Fire suppression. Nobody quotes the transformer lead time unless you ask, and the transformer is the item most likely to slide.

In 2022 a medium-voltage transformer went from 16 weeks to 52 weeks between our budget quote and our purchase order. Nobody flagged it because nobody was tracking it. The project slipped into the next calendar year and the customer relationship took the hit, not the vendor's.

Ask each supplier for current lead times on every long-lead subcomponent, in writing, with a date on the email. Then ask again after the contract is signed, before you release the PO.

Step 8 — Define commissioning and site acceptance in the same document

Commissioning scope drifts because it's usually described in prose. Make it a numbered list with a witness for each item and a signature block. Who provides the load bank. Who provides temporary power. Who owns the site network during testing. What counts as a completed test versus a partially completed one.

This is boring and it's the step that saves the most arguing later.

Step 9 — Confirm the local service network, not the global one

A warranty issued by a company with a regional office four time zones away is a warranty with a response-time asterisk. Ask for the names of the field service partners within a 4-hour drive of the site. Ask how many certified technicians they have. Ask what the actual median response time was over the last 12 months, and ask who measures it.

If the answer is a call center, that's not a service network. That's a phone number.

Mistakes that still cost the most

Choosing the brand before writing the requirement. If you've already decided, you're not procuring, you're justifying. I've done this. It never goes well.

Treating cell certification as system certification. Covered in step 3, repeated here because it's the single most common misrepresentation I see from battery energy storage system manufacturers.

Skipping the AHJ conversation because "the last project was fine." Different jurisdiction, different edition, different answer. Every time.

Treating a commercial energy storage system like a big UPS. A UPS is a box with a battery. A BESS is a permitted, interconnected, thermally managed asset with a twenty-year maintenance obligation attached. The procurement process should look like the second thing.

One more, and this is the one that changed how I think about the whole job: the equipment is a brand asset whether you like it or not. When I switched from uncoated enclosures to properly finished ones on a mid-size project, the client walked the site, looked at the cabinets for maybe two seconds, and asked whether this was the final version. It was. The answer changed the tone of the entire handover. That difference was roughly $6,000 across the project. It was the cheapest brand investment we made that year, and I only found it because a client made me feel stupid about a cabinet door.

None of these nine steps are clever. They're just the ones I stopped skipping. Our procurement manual at EverCore Procurement has this list printed on the inside cover, mostly so I have to look at it before I approve anything.