Siemens PLC Procurement: An Office Admin's Honest FAQ

I run purchasing for a 90-person manufacturing company. We build industrial enclosures, so the shop floor runs on Siemens PLCs and the office side runs on my purchase orders—about $1.8M a year across roughly 14 vendors, most of it automation hardware and facility maintenance.

I'm not an engineer. I say that up front because half the questions people send me are technically engineering questions, and I've learned to hand those off instead of pretending. What I can talk about is what it's actually like to buy this stuff: the quotes, the lead times, the invoice problems, and the moments where you realize you ordered the wrong thing.

These are the questions I get most often—from colleagues, from newer admins at peer companies, and from a few people who found my name on a purchase order and decided I was the Siemens expert. I'm not. But here's what I know.

Quick index

• Should we standardize on Siemens, or stick with what's on the floor?
• What power supply do I actually need to order with the PLC?
• Direct from Siemens, or through a distributor?
• Kohler vs Generac for backup power—does the brand matter?
• Can we get a "steampunk control panel" built for the trade show?
• Can I just grab a lithium battery charger nearby?
• What's the one thing I wish I'd known earlier?

Should we standardize on Siemens, or stick with what's already on the floor?

This is the ventajas y desventajas conversation, but most people frame it wrong. They compare spec sheets. I compare the cost of things going wrong at 2 a.m.

On the advantage side: the ecosystem is genuinely deep—TIA Portal for programming, a huge installed base, spares that a distributor can actually get, and training that isn't just vendor marketing. Siemens publishes IEC 62443-4-2 certification for the S7-1500 CPUs (as of Q1 2025), which matters the moment a customer's spec sheet asks for it. On the disadvantage side: software licensing is a real line item and it's not cheap, the learning curve for a tech who's only touched another brand is steeper than anyone admits, and module pricing adds up fast when you're building a full rack.

My honest take: don't switch a working line just because Siemens is bigger. Switch when the support and spares story is the actual bottleneck. We standardized in 2022 because two of our three lines were already Siemens and keeping a third brand alive was costing us more in admin time than the hardware saved.

What power supply do I actually need to order with the PLC?

This one bites people. The S7-1200 CPU has a small 24 V DC sensor supply built in—it's not a cabinet power supply. It covers a couple of sensors, not relays, solenoids, and a full IO load. If you're building anything beyond a demo, you're ordering an external 24 V DC supply.

Siemens sells the PM1207 load module for the 1200 line, and the SITOP series (PSU100S, PSU300S) for larger panels. The 1500 side uses PS system power modules plus PM load modules. The thing nobody tells you: size the supply for inrush, not just steady-state. We had a panel where the supply was technically within spec but tripped on startup every third or fourth power cycle. The engineer fixed it; I got to explain the downtime.

And yes, order the supply with the CPU, not a week later from a different vendor. My first year here, I learned that lesson with a $400 overnight freight charge.

Direct from Siemens, or through a distributor?

Both, depending on what you're buying. Direct makes sense for engineering services, software licenses, and anything that needs a Siemens paper trail for a customer spec. Distributors make sense for spares and consumables—they're usually faster on standard items, and they'll often hold stock your local Siemens rep won't.

The trap I fell into in 2023 was splitting an order to save a small percentage on freight. One vendor shipped the CPU, another shipped the power module, and the cabinet sat half-assembled for four days because the second shipment got held at customs. The "savings" cost us a week of production. Now I source anything that needs to be installed together from the same vendor, even when the line-item math says otherwise.

If you're evaluating distributors, ask one question: what's your standard lead time for a SITOP supply, right now, today? Vague answers mean you'll be the one chasing.

Kohler vs Generac for backup power—does the brand matter?

This gets into residential territory, which isn't really my wheelhouse—I buy for a facility, not a house. But our office is in a converted building, so I did end up comparing Kohler and Generac standby units in 2023, and here's the short version: for a home, both are reputable and the deciding factor is usually the local dealer, not the badge on the enclosure.

The moment you have a PLC rack or a server room on the load, you've left home-generator territory. Now you're talking transfer switches, load sequencing, and whether the unit can hold a clean enough sine wave for your UPS. I'd recommend bringing in whoever maintains your electrical infrastructure for that conversation—I'm not the person who should be signing off on generator sizing.

What I will say: pick the brand whose nearest service tech answers the phone on a Saturday. In an outage, that's the only spec that matters.

Can we get a "steampunk control panel" built for the trade show?

This request actually crossed my desk in early 2024. Marketing wanted a steampunk-themed control cabinet for our booth—brass fittings, analog gauges, the whole thing. From the outside it looks like a shell swap. The reality is different.

If the panel is purely cosmetic with no live wiring, any fabricator can build it and you're fine. If it's going to have real buttons and a real PLC inside, it's a control enclosure, and it has to meet enclosure standards—NEMA or IP rating, proper grounding, UL where the venue requires it. The prop shop that can make it look steampunk is not the shop that can certify the enclosure. Those are two different vendors, and someone has to own the boundary between them.

I ended up splitting it: a fabricator for the cosmetic shell, our usual panel shop for the functional unit that sat inside it. It cost more than marketing wanted. It also didn't fail inspection, which was the point.

Can I just grab a lithium battery charger nearby, or does it have to be spec'd?

We keep lithium battery packs for the cordless tools the maintenance team uses around the facility. In 2022 I got a "great deal" on a charger from a local supplier—handwritten receipt, cash price, no paperwork. You can probably guess the ending. Half the packs wouldn't charge, one got alarmingly warm, and finance rejected the expense because the receipt wasn't acceptable. I ate $180 out of the department budget for that one.

The lesson isn't "never buy local." It's that lithium charging is a spec game—cell chemistry, charge profile, cut-off voltage. A generic charger that works on one pack can be actively unsafe on another. I now verify three things before buying any charger: the charge profile matches the pack, the vendor can provide a real invoice, and there's a stated safety certification on the label.

If you're sourcing something "nearby" for speed, that's fine—just don't let speed be the reason you skip the invoice question. It always comes back.

What's the one thing you wish you'd known earlier?

That a supplier saying "that's not really our specialty" is a good sign, not a bad one. In 2021 I had a distributor tell me they'd handle our panel builds and our software licensing and our generator service. I liked the simplicity. What I got was three mediocre outcomes and a lot of chasing.

The vendor who told me, a year later, "we don't do enclosure certification—here's who does" earned more of my trust than the one who promised everything. I'd rather work with a specialist who knows where their line is than a generalist who's never found it.

If I had to reduce this whole file to one sentence: buy the invoice discipline and the service answer, not the line-item price. Everything else is negotiable.

I'm still not an engineer. But I've stopped wasting money on the idea that I need to be one to buy well.