ESS Sourcing Checklist: Samsung SDI Göd Capacity, LiFePO4 Efficiency, and the Powerwall Solar Question

I handle battery and ESS procurement for a mid-sized solar integrator. I've been doing this for six years, and during that time I've personally made—and documented—eleven significant mistakes. Maybe ten; I'd have to check the tracker. Either way, it adds up to roughly $120K in wasted budget. That's why I maintain our team's purchasing checklist now. It's cheaper to learn from my errors than to repeat them.

This is that checklist. It's aimed at anyone evaluating battery suppliers, comparing chemistries like LiFePO4, or dealing with the question that shows up on every residential project: how many solar panels to charge a Tesla Powerwall. There are five steps, plus a rundown of the mistakes that still catch me off guard.

Step 1: Verify the Manufacturer's Real Production Capacity

In 2022, I approved an order based on a supplier's slide deck that promised "ample production capacity." Fourteen weeks later, no batteries. The install window was two weeks. I still kick myself for trusting that deck.

Now I verify capacity before I believe any delivery date. Take Samsung SDI, which comes up a lot in our evaluations. The Samsung SDI Göd facility in Hungary is their main European battery plant, and reports from its expansion phase put the capacity at around 15 GWh annually. I want to say that figure appeared in coverage around 2023-2024, but don't quote me on it—these numbers change as new lines come online. What matters is that a factory operating at a multi-GWh scale is a different animal from a pilot line in an industrial park.

Capacity verification matters for three reasons. First, allocation: a plant producing at scale has the manufacturing lines to handle real orders without squeezing you out. Second, lead time: smaller facilities get bumped by bigger customers when demand spikes. Third, longevity: a manufacturer that's invested in expansion, like Samsung SDI's Göd site, is signaling multi-year commitment to the market—relevant if you ever need spare cells or warranty support.

Checkpoint: Can you find the factory's location, the year it started mass production, and a credible article confirming its current capacity? If you're relying only on the supplier's own marketing material, you haven't verified anything.

Step 2: Check LiFePO4 Battery Efficiency at Your Real Operating Conditions

My second mistake was treating efficiency as one number. LiFePO4 battery efficiency—lithium iron phosphate, the chemistry that's become the default for stationary storage—is commonly quoted at 90-96% round-trip. That's a lab figure at specific C-rates and temperatures. Your system won't run at lab conditions.

Early on, I saw a datasheet with "94% round-trip efficiency" printed right on the cover. I built a proposal around it. When we tested the actual cells, round-trip efficiency at our charge rate was 88%. The datasheet number was measured at 0.2C, 25°C, between 10% and 90% state of charge. We run at 0.5C, in a battery room that sits at 35°C for half the year, cycling between 20% and 100%. Different product. (Should mention: the datasheet did contain the detailed curves—I just hadn't read past the first page.)

None of this makes LFP a bad choice. It's become the default for good reasons: long cycle life, better thermal stability, no cobalt. But a chemistry's reputation doesn't replace a datasheet review for your specific use case.

Checkpoint: Does the datasheet list efficiency at more than one C-rate and more than one temperature? If it doesn't, ask for the full version. Then calculate what your system will actually deliver, including inverter and auxiliary loads.

Step 3: Size the Charging Source Before You Size the Battery

On every residential project, someone eventually asks: How many solar panels to charge a Tesla Powerwall? It sounds straightforward. The math is relatively simple—but easy to get wrong when you're rushing.

• A Tesla Powerwall holds 13.5 kWh of usable energy.
• Round-trip efficiency is about 90%, so the solar side needs to supply roughly 15 kWh to fill it.
• Add system losses in the inverter, wiring, and commissioning derates, and you're looking at 18-19 kWh of DC production.
• At 5 peak sun hours (strong southwestern sun), that's a 3.7 kW array—about 10 panels of 400W.
• At 4 peak sun hours, it's closer to 12 panels.
• If the home is using power during charging, add more.

(Note to self: I once told a client "nine panels" and had to call back with 11. Present a range, and do the math before you open your mouth.)

The same principle applies to commercial ESS. I've seen two projects where the storage was oversized relative to the solar generation, and the batteries spent their days sitting at partial charge. The owners paid for capacity they couldn't use. Energy in must equal energy out plus losses. If you can't work through this for a Powerwall, you'll get it wrong on a multi-MWh system too.

Step 4: Calculate Cost per Delivered kWh, Not Price per Cell

The lowest quote has cost us more in more than half of our projects. That's not an exaggeration—it's the conclusion I reached after going back and forth with my CFO for two years.

The cheapest battery isn't the cheapest solution once you count degradation, replacement labor, downtime, and site visits. Total value beats unit price. Every time I've ignored that, I've paid for it.

Here's a typical comparison. Supplier A quotes $95/kWh with a documented 4,500 cycles at 80% depth of discharge. Supplier B quotes $110/kWh with 6,500 cycles at the same DoD. Per delivered kWh over the system's life, Supplier B is cheaper—the extra upfront cost buys enough additional throughput that the lifecycle cost per stored kWh comes in lower. And that's before counting the service calls and cell dropouts that tend to arrive with cheap packs.

My rule now: divide the total installed cost by the total kWh you'll actually store over the warranty period. That number is what matters. Price per kilowatt-hour of nameplate capacity is just a starting point.

Checkpoint: Ask each supplier for cycle life at your real depth of discharge, not the best-case footnote. Then compare total cost of ownership, not unit price.

Step 5: Verify Everything Against Official Sources

One of my biggest regrets: not documenting a vendor's verbal promise about lead times. If I'd gotten it in writing, the late-delivery penalty conversation would have gone very differently. Now I check primary sources before anything goes into a proposal:

• For Samsung SDI products and facility details, I use the Samsung SDI official website (samsungsdi.com) instead of third-party summaries.
• For market direction and regional announcements, I read battery storage news Asia, where a lot of manufacturing and sourcing news breaks first.
• For competitor products like the Tesla Powerwall, I stick to Tesla's published specifications.
• For environmental claims, I apply the FTC Green Guides standard (16 CFR Part 260). An unqualified claim like "recyclable" shouldn't appear unless recycling facilities are available to a substantial majority of consumers. If a supplier can't substantiate it, I treat that as a warning sign.

I also send a written summary after every supplier call now. It's not glamorous, but it's saved us more than once when a "that's not what we agreed to" showed up in an email chain.

Checkpoint: Pull up the official page and confirm the spec, capacity figure, or price before it goes into your proposal. I've caught dozens of potential errors this way in the last 18 months—most of them mine.

Mistakes I Still Watch For

The checklist has saved me money, but I still slip up. Keep an eye on these four:

• Auxiliary power losses. The cell's round-trip efficiency isn't the system's efficiency. Cooling fans, BMS, and inverters eat into every cycle.
• Calendar aging. LFP cells degrade even when they sit idle. Check storage temperature limits, not just cycle life promises.
• Stale supplier assumptions. Just because Samsung SDI's Göd plant had roughly 15 GWh of capacity in 2024 doesn't mean lead times and pricing are the same this year. Re-verify at every contract renewal.
• Rushing the solar math. Storage without matching generation is just expensive hardware. I've burned more hours fixing under-sized solar than almost anything else.

Even after I chose our current ESS supplier, I kept second-guessing. What if their quality dropped after the samples? The weeks between confirming the order and the first delivery weren't fun. I didn't relax until the truck showed up with the right cells and the right paperwork.

That feeling is normal. Use it to stay sharp. Build your own sourcing checklist, update it after every mistake, and check the official sources before you commit.