24VDC Power Supply Sizer

This free 24VDC power supply sizer adds up the DC loads in your control panel and tells you what size DIN-rail supply to buy. Enter your loads in six categories — industrial PC, PLC, HMI, network switch, I/O and sensors, and everything else — set your design headroom, and it sums the panel, applies the headroom, works out the required current, and rounds up to a standard DIN-rail wattage. It's for controls and automation engineers laying out a new panel, checking whether an added device overloads an existing supply, or replacing a failed unit with confidence instead of guesswork. No account, no spreadsheet — the method and its limits are shown below.

The formula

There is no exotic maths here — sizing a DC supply is a disciplined addition problem. The mistakes happen in what gets left out, which is why the method has three steps instead of one:

Connected load (six category fields, all in watts)
  P_load = P_IPC + P_PLC + P_HMI + P_switch + P_IO + P_other

Design load
  P_design = P_load × (1 + H ÷ 100)
  I_req    = P_design ÷ V          amps (V defaults to 24 VDC)

Recommendation
  P_supply = first standard size ≥ P_design, from
             30, 60, 75, 100, 120, 150, 240, 320, 480, 600, 960, 1200 W
             (above 1200 W: the next 240 W multiple)
  Loading  = P_load ÷ P_supply × 100%

Why watts first, and why the round-up: panel loads are documented in watts on most datasheets, so the categories are entered in watts, and the calculator derives the current from the design load at your bus voltage. The headroom step is not padding — datasheets quote a mix of typical and maximum draws, panels accumulate small loads nobody listed, and a supply that runs near full load runs hot and ages fast. The round-up step exists because DIN-rail supplies come in standard sizes: a 200 W design load doesn't buy a 200 W supply, it buys the next size up (240 W), and the difference between the two numbers is thermal margin and expansion room, not waste.

One side effect worth knowing: the supply turns a few percent of everything it delivers into heat inside your cabinet. At around 90% efficiency, delivering the 160 W in the worked example below means roughly 18 W of heat from the supply alone — feed that into the Enclosure Cooling calculator when you check cabinet temperature rise.

What each input means

Every field below is exactly as it appears in the live calculator (labels and defaults verified in source). All loads are entered as one watt figure per category — if a device is documented in amps, convert it first: watts = amps × bus voltage.

Worked example

Scenario: a mid-size machine control panel — an industrial PC, PLC, panel HMI, managed switch, the I/O and field devices, and the usual extras, all fed from a single 24VDC DIN-rail supply. The inputs below are the calculator's own starting values, so you can reproduce this example exactly by opening the calculator and pressing "Size power supply" without changing anything.

Inputs (the calculator's defaults):

• Industrial PC: 45 W
• PLC / controller: 20 W
• HMI / panel PC: 25 W
• Ethernet switch: 15 W
• I/O + sensors: 35 W
• Other 24V loads: 20 W
• Design headroom: 25%
• Bus voltage: 24 VDC

Step by step:

1. Connected load: 45 + 20 + 25 + 15 + 35 + 20 = 160 W
2. Apply headroom: 160 × 1.25 = 200 W design load
3. Required current: 200 ÷ 24 = ≈ 8.33 A
4. Round up the ladder (30, 60, 75, 100, 120, 150, 240, …): 200 W sits above the 150 W size, so the recommendation is the next standard size — 240 W (240 ÷ 24 = 10 A)
5. Loading on the recommended unit: 160 ÷ 240 = 66.7% of its rating in steady state

The calculator's verdict for these inputs reads: "Select at least a 240 W supply at 24 VDC, then verify inrush, redundancy, temperature derating, and future expansion."

What the example shows: headroom mostly decides which step of the ladder you land on. Same panel with 0% headroom: the design load is 160 W, still above 150 W, so the pick is still 240 W — but now the supply runs at 66.7% with nothing reserved for the next device. Take a lighter panel with a 100 W connected load and the same 25% headroom: the design load is 125 W, and the pick falls to the 150 W step. And at the small end, a 52 W panel load with 25% headroom is a 65 W design load, which lands on the 75 W size — not a 120 W unit. Each ladder step you land on decides how hard the supply works for its whole life: a unit cruising near two-thirds load in a warm cabinet is far more likely to stay inside its derating curve than one pinned near full rating (see Common Mistakes).

What the result tells you

The result panel shows four figures (labels as displayed by the live calculator), followed by a one-line verdict:

• Connected continuous load — the sum of the six category fields, in watts: your panel's steady-state draw before any margin. This is the number to compare against an existing supply's rating when you're checking whether an added device will overload it.
• Design load with headroom — the connected load with your headroom applied: the capacity you actually need to buy for. If this number sits close to a ladder boundary (say 148 W against the 150 W step), small load changes flip the recommendation — re-check it whenever the design changes.
• Required DC current — design load ÷ bus voltage, in amps. Use it to check the supply's current rating and your wire and protection sizing at the actual bus voltage.
• Recommended standard PSU — the first ladder size at or above the design load, shown in watts and amps (e.g. "240 W / 10 A"). The gap between this and your connected load is deliberate: thermal margin plus room to grow.
• The verdict line — restates the pick as a purchase instruction: "Select at least a {size} W supply at {voltage} VDC, then verify inrush, redundancy, temperature derating, and future expansion." Those four verifications are exactly what the result does not cover: whether the supply survives switch-on (inrush), a hot cabinet (derating), or a redundancy scheme sits outside a load sum, and the calculator's own assumptions add NEC/UL requirements and manufacturer curves to that list. Check them against the chosen unit's datasheet before you order.

Common mistakes

1. Sizing to the exact load. Set headroom to 0% and the design load is just today's connected load — the calculator will happily recommend a unit the panel already fills, in a sealed cabinet, forever. Supplies run at or near full rating run hot and fail early, and the first added sensor or relay has nowhere to go. Headroom is the cheapest reliability in the panel.
2. Forgetting inrush. The load sum is steady-state. At switch-on, an HMI, capacitive sensor loads, and a bank of inputs can draw several times their running current for a few milliseconds; a supply sized exactly to the sum can hiccup or trip on its overload characteristic at every power-up. The calculator deliberately excludes inrush — check the chosen unit's overload behaviour against your loads' startup draw.
3. Ignoring ambient derating. A supply's nameplate watts apply up to a stated ambient temperature; above it, the manufacturer derates output — in a 55–60 °C cabinet the same unit may only be good for a fraction of its label. Read the derating curve for your cabinet temperature, and note the interplay with Mistake 1: a supply running at two-thirds load has derating room, one pinned near full rating doesn't.
4. Leaving loads off the list. The forgotten ones are consistent: field sensors powered from the panel rail, the Ethernet switch, coil loads for contactors and solenoid valves, the "temporary" device from commissioning that never left. If it draws from this supply, it belongs in the sum — walk the panel and the field, don't size from the PLC configuration alone.
5. Treating watts as the whole job. A correctly sized supply can still deliver a bad installation: long, thin 24 VDC runs drop voltage, so the far end of the panel sees well under 24 V even though the supply is loafing. Sizing the supply and sizing the wire are separate checks (a voltage drop calculator is on this site's roadmap).

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