Allen-Bradley vs Schneider PLC: Sizing by Real Watts

You’re on a panel layout with 220 mm of usable DIN rail, a 24 VDC bus rated at 10 A, and the enclosure is sealed — no fan. The PLC you pick isn't just about I/O count or scan speed; it's about how many watts it burns when it's actually running a machine. That number determines whether you need a bigger power supply, a bigger enclosure, or forced cooling. Here’s where the Allen-Bradley PLC Micro850 and the Schneider PLC Modicon M241 split.

1. The "real watts" number that changes the power budget

Numbers first. The Allen-Bradley Micro850 2080-LC50-48QBB draws about 4.2 W typical at 24 VDC, with a maximum dissipation of 8.5 W. The Schneider M241 (TM241CEC24T) draws about 5.5 W typical, with a max around 7.5 W (illustrative). At first glance they’re close. But the Micro850 carries 48 on-board I/O — twice as many as the M241's 24 — at roughly the same base power. That’s a 48‑I/O controller pulling 4.2 W vs a 24‑I/O controller pulling 5.5 W. The gap in power-per-I/O is almost 2.6×.

Mechanism. PLC power consumption is dominated by the CPU core, the communication interfaces, and the I/O driver logic — not just the number of I/O points. The M241 packs two Ethernet ports, two serial ports, USB, and CANopen master onto a single board, plus a web server. Those transceivers and the embedded web stack consume real current even when no cable is plugged in. The Micro850 has a single Ethernet port and one serial port, plus USB, and its I/O handling is more distributed (the 48 I/O are on the base unit, but the CPU doesn't have to power separate front-end ASICs for each pair of ports). The result: the base power floor for the M241 is higher relative to its I/O count.

Worked consequence. If you need 48 I/O and choose the M241, you'll need at least one TM3 expansion module, which adds ~1.5 W typical (illustrative). Total system draw: 5.5 + 1.5 = 7.0 W. The Micro850 with the same 48 I/O on-board: 4.2 W. On a 24 VDC supply rated at 240 W (10 A), that 2.8 W delta is negligible — but in a thermally constrained sealed enclosure, every watt adds heat that must be removed. The Micro850 saves about 2.8 W, which at 24 V is about 117 mA of headroom on your power bus. That can be the difference between running a small HMI on the same loop or adding a second power supply.

When this reverses. If your application needs more than 48 I/O and you plan to use 100+ points, the M241’s expansion bus (TM3) extends to 264 I/O and adds very little per-module power (about 1.2 W for an 8-point output module, illustrative). The Micro850 tops out at about 132 I/O with 4 modules. Beyond 64 I/O, the M241 system may actually have lower total power per I/O because you can add exactly the points you need without an extra CPU base. The power-per-I/O ratio flips at roughly 64 I/O — below that, Micro850 wins; above, M241 can be more efficient in watts per I/O.

2. Thermal dissipation: 8.5 W vs 7.5 W — but the enclosure math changes

Numbers. Micro850 max dissipation: 8.5 W, or 29 BTU/hr. M241 max dissipation: ~7.5 W, or ~26 BTU/hr (converted). The M241 dissipates about 12% less heat at max load. For a panel designer, that 1 W difference is almost irrelevant inside a ventilated cabinet. But in a sealed, no-fan enclosure (IP65 or NEMA 4X), every watt counts toward the internal temperature rise.

Mechanism. The thermal rise in a sealed enclosure is governed by the surface area and the total watts dissipated. A standard 400×400×200 mm steel enclosure has about 0.64 m² of effective surface area and a thermal resistance of roughly 20 °C/W (free convection, no fan). Adding 8.5 W vs 7.5 W raises the internal temperature by about 170 °C vs 150 °C above ambient — but those numbers are not linear because the enclosure also radiates. In practice, the difference is ~3–5 °C internal air temperature. That’s small, but it can push a 40 °C ambient design to the edge of a component’s 60 °C rating.

Worked consequence. If your panel is in a 45 °C ambient environment (e.g., near a motor drive), the Micro850 at full load will raise internal temperature to about 48–50 °C, while the M241 will stay around 46–48 °C (illustrative, assuming free convection). Both are inside the 60 °C operating limit. But if you add a small HMI (another 3 W), the Micro850 system could hit 52 °C, narrowing the margin. The M241’s lower thermal dissipation gives an extra 2–3 °C headroom — not a decisive factor, but a tiebreak in borderline thermal designs.

When this reverses. In a ventilated panel or with forced air flow, the 1 W difference is meaningless. Also, if you run the PLC at less than 50% I/O load (typical for many machines), both controllers will run cooler — the Micro850 will typically dissipate ~3.5 W, the M241 ~4.5 W. The gap shrinks. Thermal advantage only matters when you are near the enclosure’s thermal limit, which is rare for small PLCs. If your panel is already heavily loaded with drives, the PLC heat is a rounding error.

3. Power supply sizing: the hidden cost of communication ports

Numbers. The M241 has five communication ports (2 serial, USB, 2 Ethernet, CANopen). The Micro850 has two (Ethernet/IP + RS232/485) plus USB. Each active Ethernet port draws about 0.5–0.8 W at idle. The M241’s second Ethernet port alone adds nearly 0.6 W of continuous load compared to the Micro850. Add the CANopen transceiver (about 0.3 W) and the second serial port (0.15 W), and the M241’s communication subsystem draws roughly 1.5 W more than the Micro850’s — even when nothing is connected.

Mechanism. Each physical layer transceiver (PHY for Ethernet, RS485 driver, CAN transceiver) requires a bias current and often a DC-DC converter for isolation. The M241 uses isolated ports, which means a small switching converter for each isolated bus segment. The Micro850’s serial port is non-isolated (typical for the Micro800 family), eliminating the converter’s quiescent loss. The cumulative effect: the M241's comms overhead is baked into its base power consumption, making it less efficient for applications that only need one Ethernet and one serial port.

Worked consequence. If your machine only needs Modbus TCP (one port) and a simple HMI via serial, the Micro850 will consume about 1–1.5 W less than the M241 for the same connectivity. That translates to 40–60 mA of 24 VDC current. On a 10 A supply, it’s negligible. But on a small 2.5 A micro-supply (like a Puls PIF-240-24), that 60 mA is 2.4% of your total budget — you might avoid stepping up to a 3.8 A unit. The cost difference between a 2.5 A and a 3.8 A supply is about $20. Not a game changer, but a real saving in a cost-sensitive panel.

When this reverses. If you need dual Ethernet (e.g., for a DLR ring or for separate control and IT networks) or CANopen, the M241’s extra ports are not a waste — they are necessary. In that case, the M241’s higher comms power is the price of functionality, not inefficiency. The Micro850 cannot do dual Ethernet natively; you would need to add an external switch or a second PLC, consuming far more power and panel space. So if your network topology demands multiple ports, the M241 is the lower-power choice overall, despite its higher idle draw.

4. The rule: when to pick by real watts

Use this decision threshold: If your enclosure is sealed (IP65 or higher) and ambient can exceed 40 °C, and your total I/O is under 64 points, the Allen-Bradley Micro850 will save you about 2–3 W of heat and about 60 mA of supply current. That is a real advantage if you are already at the limit of a small 2.5 A PSU or if you are trying to avoid a larger enclosure. If your I/O is above 64 points, or if you need dual Ethernet or CANopen, the Schneider M241’s lower base dissipation (7.5 W max) and scalable expansion bus give you a lower power-per-I/O ratio. The crossover point is roughly 64 I/O. Below that, Micro850 wins on power. Above that, M241.

The takeaway: don't just compare datasheet power numbers without normalizing for I/O count and communication ports. The real watts that matter are the ones that change your power supply choice or your enclosure thermal margin. In most panels, the difference is small — but if you're building thousands of machines, saving 1.5 W per panel means 1.5 kW of heat not rejected across a thousand units. That's a real operational saving.