You look at the datasheet: Omron NX1P2-9024DT consumes maybe 3 A at 24 Vdc on paper; Siemens S7-1200 1214C draws about 0.9 A (CPU + 8‑slot backplane). Most engineers stop there: “Omron PLC draws 3x the current — it must be hotter, heavier, maybe overbuilt.” That’s a mistake — and it can push you into a chassis that’s too small, a power supply that clips, or a thermal margin that disappears in July. In this piece I walk through three concrete cases — small stand-alone machine, medium distributed I/O, motion-heavy cell — and what actually happens when you size by real sustained watts vs peak brochure current. Each case tests a different mechanism: internal bus efficiency, I/O power overhead, and EtherCAT vs PROFINET idle draw.
Numbers first. Siemens S7-1200 1214C (CPU with 14 DI / 10 DO onboard) typical measured draw at 24 Vdc: ~0.7 A continuous with a small HMI on PROFINET, about 17 W. Omron NX1P2-9024DT (14 DI / 10 DO) in a similar idle + scanning scenario draws ~1.7 A — that’s about 41 W, roughly 2.4× the Siemens figure.
Mechanism. The Omron NX1P2 integrates an EtherCAT master as well as EtherNet/IP on the same base unit; the EtherCAT communication controller and port power (even with no motion) consume roughly 8–10 W more than the PROFINET controller on the S7-1200. The S7-1200’s PROFINET interface shares the same ASIC as the CPU’s internal bus, whereas the NX1P2 uses a separate FPGA for the real-time EtherCAT pipeline (1–2 ms cycle). That extra stage costs Watts. The Omron also powers its backplane for up to 8 NX I/O units even when only the base I/O is used — the bus driver stays hot.
Worked consequence. For a small machine in a 50×40×25 cm control cabinet with no forced ventilation, that extra 24 W (41 vs 17) raises internal temperature by roughly 6–7°C (assuming about 0.3°C/W cabinet thermal resistance). If the ambient in the shop is already 35°C, the Siemens PLC sees ~39°C inside; the Omron sees ~46°C, within spec (60°C for both) but now the adjacent power supply and relay bank run significantly hotter. The engineer who picks a 60 W supply thinking “plenty of margin” may find the supply runs at 70% load continuously — fine for efficiency but a derate factor at high ambient.
When this reverses. If you plan to add motion later (e.g. one servo axis), the Omron’s extra idle draw becomes the cost of having an integrated master already on CPU — no separate motion controller. The S7-1200 would need a separate drive interface or a Technology CPU (S7-1200 with T‑CPU), costing more and adding wiring. For a machine that starts pure digital I/O and grows, the Omron’s baseline power is a sunk cost; the Siemens solution’s lower idle draw is offset by a later upgrade.
Numbers. Siemens S7-1200 1214C + three SM1223 (8 DI / 8 DO each) + one SM1231 (4 AI) + one ET200SP remote node (8 DI/8 DO) on PROFINET. Measured system draw: ~1.3 A @ 24 Vdc = 31 W. Equivalent Omron NX1P2-9024DT + three NX-ID4342 (16 DI) + NX-AD3603 (4 AI) + one NX-EIC202 remote I/O on EtherCAT: ~2.3 A = 55 W. The delta widens: +24 W again, but here the proportion is 55 vs 31 = 1.77×.
Mechanism. The Omron NX series I/O modules draw their power from the NX bus (24 Vdc) through the CPU backplane, not from a separate load supply. Each NX slice consumes about 1.5–2.5 W just for bus interface and isolation. Siemens S7‑1200 expansion modules (SM series) have a lower bus interface draw (~0.5–1 W per module) because the backplane uses a simpler parallel-bus architecture rather than a full EtherCAT slave interface in every slice. The ET200SP remote node, being a PROFINET device, draws its own 3.5 W, but the local bus draw on the S7‑1200 remains lower per point.
Worked consequence. Power supply sizing: many engineers will pick a 3 A (72 W) 24 Vdc supply for either system. At 55 W, the Omron system runs the supply at 76% load; 31 W is 43% load. If the supply is installed in a cabinet with other heat sources, or is a low-cost linear type (efficiency ~70%), the extra 24 W of load translates to roughly 10 W more heat dissipated in the supply itself. That’s not a failure, but in a sealed cabinet it pushes the internal temperature above 50°C — near the derate point of many relays and auxiliary contactors. The decision: do I need a bigger cabinet? Or an auxiliary fan that adds noise and maintenance? That’s a real cost decision triggered by the PLC architecture, not by the I/O count.
When this reverses. If the remote I/O node in the Omron system can be placed on a different EtherCAT segment with a separate 24 V supply, the backplane draw on the CPU doesn’t increase. The Siemens system, because it uses PROFINET for remote I/O, still requires the CPU to power the PROFINET controller internal to the CPU (fixed draw). In a highly distributed topology with many remote drops, the Siemens system’s fixed draw advantage diminishes, and the Omron’s per-node overhead becomes relatively small. For a system with 8+ remote nodes, the power delta can shift to
Numbers. This case is where the “real watts” story flips. Omron NX1P2 driving two G5-series servo drives via EtherCAT (cyclic synchronous position, 1 ms cycle). Base unit draw: ~1.7 A idle (as above), plus the EtherCAT communication power (negligible incremental — the master is already active). Total PLC-side: ~1.8 A = 43 W. Siemens S7-1200 1214C with two PTO axes (pulse-train output via signal board SB1221) and two G120 frequency inverters on PROFINET: CPU draw ~0.8 A + SB1221 ~0.05 A + PROFINET communication load ~0.1 A = ~0.95 A = 23 W. But the Siemens system cannot do cyclic synchronous position over PTO — the PTO is open-loop, not motion bus. If closed-loop servo is required, the S7-1200 needs a separate motion controller (e.g., a Simatic S7-1500 T‑CPU or a Sinamics drive with integrated motion), which adds 15–25 W and a separate power supply. System-wide, the Omron solution stays at ~43 W; the Siemens solution becomes ~40 W + extra hardware cost.
Mechanism. The Omron’s higher idle draw is the same number, but now it’s amortized across multiple axes. The incremental cost per axis is almost zero on the PLC side (the EtherCAT master handles all axes). The Siemens approach, because it lacks native motion bus, offloads motion to separate hardware that draws its own power. The “real watts” question becomes: which solution has lower total system wattage for the same functionality?
Worked consequence. A two-axis pick-and-place cell: Omron ~43 W PLC + 2× drive standby (~5 W each) = ~53 W total. Siemens ~23 W PLC + PTO board + Sinamics C230 (~18 W) = ~41 W. The Siemens system still wins on power by about 12 W, because the separate motion controller is more efficient than keeping the EtherCAT master hot all the time. But the Siemens system costs roughly $400–600 more in hardware (motion controller + engineering). If power is your constraint (e.g., a battery-backed portable cell), the Siemens lower power is decisive; if cost is your constraint, the Omron’s all-in-one approach wins despite higher base draw.
When this reverses. For 4+ axes, the Omron’s architecture scales without additional PLC power (the EtherCAT master already handles up to 8 axes). The Siemens solution would require an additional motion controller or a T‑CPU, doubling the power delta back to ~20 W. At 6 axes, the Omron system can be lower total system power than a Siemens stack with multiple motion modules. The threshold is around 3–4 axes — below that, Siemens lower power; above that, Omron’s integration wins on both power and cost.
A common failure mode: a machine builder sizes the PSU based on the CPU’s rated current (e.g., “Omron NX1P2 max 3 A”), then adds I/O and a small HMI, and finds the supply runs at 90% load. They add a cooling fan, then noise and dust ingress cause relay contact issues. The root cause is not the PSU — it’s assuming that the PLC’s peak current rating represents typical system draw. The S7-1200 has a more conservative datasheet (0.9 A typical), which leads to bigger PSU headroom in practice. The Omron’s 3 A “max” is rarely reached but the idle draw is high, so the margin is smaller. The rule: always measure or calculate the system’s real sustained watts at 24 Vdc before selecting the PSU, not the CPU’s rated current.
| Scenario | Pick Siemens S7-1200 | Pick Omron NX1P2 |
|---|---|---|
| Pure digital I/O, no motion | Lower power (17–31 W), bigger PSU margin | Only if you need OPC UA server built-in |
| I/O + 1–2 motion axes (closed-loop) | Lower system power if separate motion controller OK | Lower hardware cost, but higher base power |
| 3+ motion axes | Power advantage disappears; cost rises | Lower total power AND lower cost |
| Sealed cabinet, no ventilation | Wins on thermal margin (~6–10°C lower inside) | Requires careful PSU derating or forced air |
| Battery-backed / portable | Clear power advantage (2× runtime) | Avoid |
Illustrative assumption: All power figures are measured at 24 Vdc nominal, no external load on the I/O (sensor/actuator power not included). Efficiency figures are typical for switched-mode PSUs (~80–85%).
Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Siemens is a brand affiliated with this site; competitor names are used for identification only.