The Power

Chapter 9 — Power Supply Unit (PSU)

The PSU converts mains AC (230V in the UK) to the clean, stable DC voltages that every component in your machine depends on. A quality PSU from a known manufacturer running well within its rated capacity will likely outlast everything else in this build. Your Corsair TX550M is that PSU. This chapter confirms it handles the full component stack comfortably, documents every cable it needs to supply, and gives you a clear picture of when a higher-wattage unit would become necessary.

Verdict up front: The Corsair TX550M 550W is the right PSU for this build. At worst-case peak load (i7-13700K at full PL2 burst + RTX 3050 LP + all storage), the system draws approximately 340–380W — roughly 65–70% of the TX550M's capacity. This is the textbook ideal operating range for efficiency and longevity. No PSU purchase needed.

What a PSU Does

Your mains socket delivers 230V AC (alternating current) at 50Hz. Every component in the PC runs on DC (direct current) at far lower voltages. The PSU performs three functions:

  1. Rectification — converts AC to raw DC using a transformer and diode bridge
  2. Regulation — smooths the raw DC into stable +12V, +5V, and +3.3V rails using capacitors and feedback circuits. A good PSU holds these within ±5% of target under any load condition.
  3. Protection — shuts down (rather than frying your components) if it detects overvoltage, overcurrent, overtemperature, or a short circuit

The power number on the label (550W) is the maximum continuous DC output the PSU can deliver. It tells you nothing about efficiency. The efficiency rating (80+, Gold, etc.) tells you how much of the AC power it draws from the wall actually reaches your components vs. how much is lost as heat.

Your PSU — Corsair TX550M 550W

Corsair TX550M 550W 80+ Gold Semi-Modular ✓ Keep — excellent headroom for this build
Rated output550W continuous
Efficiency rating80+ Gold
Peak efficiency~92% at 50% load (~275W draw)
ModularitySemi-modular
Fan size140mm Hydraulic Bearing (HDB)
Fan start modeZero RPM / semi-passive at low load
+12V railSingle +12V, 45.8A (549.6W)
+5V rail25A (125W)
+3.3V rail25A (82.5W)
ProtectionsOVP, UVP, OCP, OPP, SCP, OTP
Warranty5 years
Form factorATX (150mm depth)
Single +12V Rail
45.8A
All power-hungry components (CPU, GPU, most drives) run from the +12V rail. A single high-current +12V rail is the modern standard — no current sharing issues between virtual rails.
Semi-Modular Design
Fixed + Detachable
24-pin ATX and EPS CPU cables are permanently attached. PCIe, SATA, and Molex cables are modular — attach only what you need. Unused cables stay in the box, keeping the case cleaner.
Protection Suite
OVP / OCP / SCP / OTP
OVP = over-voltage, OCP = over-current, SCP = short circuit, OTP = over-temperature. If any rail goes out of spec, the PSU shuts down before damaging connected hardware.
Hydraulic Bearing Fan
140mm HDB
HDB fans last longer and run quieter than sleeve bearings. The TX550M's fan stops completely at low load (semi-passive mode) — the PSU is completely silent during typical dev workloads.
80+ Gold Efficiency
87–92% efficient
At 50% load (~275W draw), the TX550M is ~92% efficient — only ~22W wasted as heat. Lower efficiency units waste 20–30% of the power you pay for, generating more heat in the process.
Voltage Regulation
±1–2% typical
Gold-rated units hold rails tighter than the ATX spec's ±5% requirement. Stable voltage means stable clocks — CPUs and GPUs boost more consistently when voltage doesn't droop under load.

Understanding 80+ Efficiency Ratings

80+ EFFICIENCY CHART — WHAT THE RATINGS MEAN Rating 20% load 50% load 100% load Heat at 50% load (275W out) ───────────────────────────────────────────────────────────────────────────── 80+ White 80% 80% 80% 69W wasted 80+ Bronze 82% 85% 82% 49W wasted 80+ Silver 85% 88% 85% 37W wasted 80+ Gold ★ 87% 90% 87% 31W wasted ← TX550M 80+ Platinum 90% 92% 89% 24W wasted 80+ Titanium 92% 94% 90% 17W wasted Example: TX550M delivering 275W (50% load) ┌──────────────────────────────────────────┐ │ Wall power in: 275W ÷ 0.90 = ~306W │ │ DC power out: 275W (to components) │ │ Lost as heat: ~31W │ │ Efficiency: ~90% ✓ GOLD │ └──────────────────────────────────────────┘ Sweet spot: 40–60% of rated load maximises efficiency. For TX550M: 220–330W draw = peak efficiency zone. This build runs ~230–300W under typical heavy load — in the sweet spot.

Power Budget — This Build

Three scenarios matter: idle (productivity, browsing), sustained dev (compilation, Docker builds), and worst-case peak (CPU at full burst + GPU under simultaneous load). The TX550M must handle worst-case without exceeding 80% of rated capacity — a safety margin recommended to preserve longevity.

Scenario 1 — Idle / Light Productivity ~80–110W total
i7-13700K (idle ~8–12W)~10W
RTX 3050 LP (idle / fan-stop ~5W)~5W
Motherboard + RAM + fans + storage~55W
0W~70W total (13% of 550W)550W
PSU fan completely stopped at this load level — silent
Scenario 2 — Heavy Dev (Sustained Compile + Docker) ~230–280W total
i7-13700K at PL1 (125W sustained TDP)~125–150W
RTX 3050 LP (video decode, display)~25–35W
Motherboard + DDR4 + NVMe + SATA ×2 + fans~65–80W
0W~255W total (46% of 550W)550W
Well within the 90% efficiency sweet spot — TX550M barely working
Scenario 3 — Worst Case Peak (CPU PL2 burst + GPU load) ~360–400W total
i7-13700K at PL2 (253W short-term boost)~253W
RTX 3050 LP under GPU load (~80W max)~80W
Motherboard + DDR4 + storage + fans~65–75W
0W~400W total (73% of 550W)550W
Within safe headroom. CPU PL2 burst only lasts seconds before throttling to PL1 (~125W)
PL2 is a short burst, not sustained: The i7-13700K's 253W PL2 figure is a brief power spike lasting 28–56 seconds (called Tau) that occurs at the start of a load spike — like when a build kicks off. The CPU then settles to its PL1 (125W). In practice, even a 10-hour compilation run spends the vast majority of time at PL1, not PL2. The 73% peak load figure above is a worst-case snapshot, not a steady-state condition.

Semi-Modular Cable Guide — What This Build Needs

The TX550M separates its cables into two groups: fixed (permanently attached, always needed) and modular (plug in only what the build requires). Only attach the modular cables you need — unused cables contribute to clutter and reduce airflow.

24-Pin ATX (Motherboard Main) Fixed — always connect
Connector24-pin ATX
Plugs intoMotherboard main power header
Purpose+12V, +5V, +3.3V, standby, sensing
The largest connector. Permanently attached to the TX550M. Route it behind the motherboard tray first before connecting — it's thick and difficult to hide once the board is seated.
4+4 Pin EPS12V (CPU Power) Fixed — always connect
Connector4+4 pin (clips to form 8-pin)
Plugs intoEPS12V header, top-left of motherboard
PurposeDedicated +12V power for CPU VRM
Permanently attached. Connect both halves of the 4+4 clip together for the full 8-pin needed by the i7-13700K's VRM. Most B760 boards have one 8-pin EPS header; some have a second 4-pin booster (not needed here — TX550M handles it on one cable).
6+2 Pin PCIe (GPU Power) Modular — may not be needed
Connector6+2 pin (configures as 6-pin or 8-pin)
Plugs intoGPU power connector (if present)
TX550M provides2× 6+2 pin PCIe cables
Required only if your RTX 3050 LP variant has a 6-pin or 8-pin connector on the card's edge. If the card has no external power connector, skip this cable entirely — the PCIe slot provides 75W directly from the board.
SATA Power Chain Modular — connect for drives
ConnectorSATA power (L-shaped, 15-pin)
Devices poweredSamsung 870 EVO + WD Blue SA510
TX550M providesTwo SATA chains (3–4 connectors each)
Connect one SATA chain for both 2.5" drives — the daisy-chain cable can reach both bays. NVMe drives receive power through the M.2 slot and do NOT use SATA power. Do not confuse SATA power (L-shaped 15-pin) with SATA data (flat 7-pin cable from motherboard).
Molex (4-Pin Legacy) Leave in box
Used forLegacy fans, older devices
In this buildNothing requires Molex
All case fans connect to motherboard headers. No Molex devices in this build. Leave the Molex cables in the Corsair cable bag — attaching them unnecessarily just adds bulk inside the case.
Second PCIe Cable Leave in box
TX550M provides2× 6+2 pin PCIe total
In this buildOnly 1 GPU (RTX 3050 LP)
The second PCIe cable is for dual-GPU or high-power single GPU setups. The RTX 3050 LP needs at most one 6-pin connector. Leave the second cable in the bag.
Never mix modular cables between PSU brands: The modular connector on the TX550M uses a proprietary pin layout. A Seasonic or be quiet! modular cable physically plugs into the TX550M port but wires different pins differently — this can cause a short-circuit or fire. Use only the cables that shipped with the TX550M. Never borrow cables from another PSU.

Installing the PSU

1
Identify PSU orientation (fan direction)
The TX550M's 140mm fan faces one direction — it must face down toward the bottom vent of the case when installed in the Fractal Define 7. The PSU mounts in the bottom-rear chamber with the fan pulling fresh air in from outside the case through the floor vent. Check the Fractal Define 7's PSU shroud has its bottom vent unobstructed — there is a removable dust filter on the case underside.
The label side of the PSU (with wattage and brand) typically faces down when the fan faces down correctly. Confirm visually before sliding the unit in.
2
Route fixed cables before inserting the PSU
The 24-pin ATX and 4+4 EPS cables are permanently attached. Feed them through the appropriate cable routing holes in the motherboard tray before sliding the PSU into the chassis. In the Fractal Define 7, the 24-pin routes through a large grommet on the right side of the main chamber; the EPS cable routes through a smaller hole near the top. Pre-routing avoids struggling to thread stiff cables through tight gaps later.
The EPS cable may be too short to reach the top-left CPU power header if you don't route it through the correct hole. If it's just barely reaching, check whether there's a routing cutout near the top of the tray specifically for this cable — most mid-tower cases include one.
3
Slide the PSU into the chassis and secure
Slide the TX550M into the PSU bay from the inside of the case (some cases load from the back panel). Align the four mounting holes on the PSU's rear face with the case's PSU bracket, then secure with the four screws provided with the case. Corsair TX-series PSUs fit all standard ATX PSU mounts.
Finger-tighten all four screws before fully tightening any of them — this helps ensure the PSU sits flush before the screws are locked down.
4
Attach modular cables — SATA chain and PCIe (if needed)
Connect modular cables to the TX550M's output ports first, then route them to the components. The modular ports on the TX550M are labelled — match the cable plug type to the correct port. Do not force connectors; they only fit one way. Attach: one SATA chain (for Samsung 870 EVO + WD Blue), and one 6+2 PCIe cable if your RTX 3050 LP has a power connector.
5
Connect all power headers
Connect in this order: (1) 24-pin ATX to motherboard — press firmly until the latch clicks; (2) 4+4 EPS to the CPU power header (top-left of board) — both halves clipped together for a full 8-pin; (3) SATA power to each 2.5" drive; (4) 6+2 PCIe to GPU if required. Every connector has a keyed latch or notch so it can only insert in the correct orientation.
The 24-pin ATX requires firm, deliberate pressure to seat fully. A half-seated 24-pin connector is one of the most common causes of "no POST" on a first boot. You should hear/feel a definite click from the plastic latch.
6
Cable management — bundle and secure unused length
Use the Fractal Define 7's cable tie-down points and velcro straps (included with the case) to bundle cables behind the motherboard tray. The 24-pin cable is the thickest and least flexible — coil excess length behind the tray rather than letting it bunch in front. Unused modular cables stay in the Corsair cable bag, not stuffed inside the case.
Good cable management improves airflow past the GPU and storage devices, and makes future component swaps dramatically easier. Take 15 extra minutes on this now.

The Paperclip Test — Verifying the PSU Standalone

If you want to test the TX550M before or after the build — without risking connected components — the paperclip test lets you power on the PSU standalone to confirm the fan spins and voltages are present.

24-PIN ATX CONNECTOR — PAPERCLIP TEST Pin 16 (PS_ON — green) shorts to any Ground pin (black) This tricks the PSU into thinking the motherboard has signalled "power on" ┌─────────────────────────────────────┐ │ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Row 1 │ │ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Row 2 │ │ ↑ ↑ │ │ Pin 16 (GRN) Pin 17 (BLK) │ │ PS_ON Ground │ │ │ │ Short these two with a paperclip │ │ or a short piece of wire │ └─────────────────────────────────────┘ With the paperclip bridge in place: ✓ Switch PSU on (rear toggle) ✓ Fan should spin up → PSU is alive ✓ Connect a SATA device or multimeter to check +12V / +5V / +3.3V ⚠ Do this with mains power OFF until the paperclip is in place ⚠ Do not touch the mains plug terminals A PSU tester (£5–10 on Amazon) is safer and shows all rail voltages on an LCD
When to use the paperclip test: Mainly useful if you're troubleshooting a system that won't POST and you're not sure whether the PSU is at fault. For a new build with a known-working PSU like the TX550M, you can skip directly to the full system test. If the paperclip test shows the fan doesn't spin at all, the PSU is dead — return/replace it before connecting it to any components.

Testing the PSU In-System

POST confirms all power rails present: If the system reaches the BIOS/UEFI after first boot, all required power rails (+12V, +5V, +3.3V, +5VSB) are working. A system that powers on but doesn't POST could indicate a PSU issue, but is more often a RAM or CPU installation error — check those first.
HWiNFO64 voltage monitoring — Sensors view: Download HWiNFO64 (free). Open → Run Sensors. Expand the "Nuvoton NCT6798D" or equivalent motherboard sensor section. Look for +12V, +5V, and +3.3V readings. Acceptable ranges: +12V between 11.4–12.6V; +5V between 4.75–5.25V; +3.3V between 3.14–3.46V. Any rail more than 5% outside its target during load is worth investigating.
HWiNFO64 under Prime95 load — voltage droop: Start a Prime95 "Large FFTs" run (maximum power consumption mode) and watch the +12V rail in HWiNFO. A quality Gold PSU like the TX550M should show no more than 0.1–0.2V droop under load. Significant voltage droop (0.5V+) under load indicates a PSU struggling at capacity — not expected with the TX550M at this build's power draw.
PSU fan behaviour: At idle (Windows desktop, no load), the TX550M fan should be completely stopped in semi-passive mode — the PSU is silent. Under sustained load (Prime95 running), the fan should spin up smoothly. A fan that rattles, grinds, or pulses irregularly indicates a bearing issue — this PSU should be silent during spin-up given its hydraulic bearing design.
No unexpected shutdowns under load: Run Prime95 "Small FFTs" (CPU stress) for 30 minutes alongside FurMark (GPU stress). A clean PSU with sufficient headroom will not trigger its OCP or OPP protections under this combined ~350W load on the TX550M. If the system shuts down cleanly during stress (no BSOD, just powers off), the PSU's overcurrent protection may have tripped — check that no SATA cable has a loose connection creating a short, and verify cable seating before suspecting the PSU itself.
Wall power draw (optional — smart plug or Kill-A-Watt meter): If you have a smart plug with power monitoring (e.g. Tapo P110, ~£12), plug the system into it and check actual wall draw. At idle you should see ~80–120W at the wall. Under full Prime95 load, expect ~280–380W at the wall. These figures reflect wall power (AC in), not DC out — the difference is the PSU's conversion losses. Higher than expected wall draw at a given load indicates the PSU is less efficient than rated.

When the TX550M Would Need Replacing

The TX550M is the right PSU for this build. However, future GPU upgrades change the power equation significantly. Here's when 550W becomes insufficient:

✓ Fine — Keep TX550M
RTX 4060 upgrade
RTX 4060 TDP: 115W. Total system peak: ~115W GPU + ~150W CPU + ~75W sys = ~340W. Well within 550W. TX550M handles this with no changes.
~340W peak — 62% of 550W ✓
⚠ Marginal — Monitor carefully
RTX 4060 Ti upgrade
RTX 4060 Ti TDP: 165W. Total system peak: ~165W GPU + ~253W CPU PL2 + ~75W sys = ~493W. At 90% of the TX550M's capacity during CPU+GPU peak — technically within spec but leaves little headroom. Consider an 850W PSU if this is the GPU direction.
~493W peak — 90% of 550W ⚠
✗ Replace PSU
RTX 4070 or above
RTX 4070 TDP: 200W. Combined with i7-13700K PL2 and system: ~530W+ peak. This exceeds 550W rated capacity. A 750W or 850W Gold/Platinum unit is required before installing any GPU above the 4060 Ti class.
~530W+ peak — exceeds 550W ✗
PSU rule of thumb: System peak load should not exceed 80% of rated PSU capacity. For an RTX 4070 build with an i7-13700K, a 750W 80+ Gold unit is the minimum. The Corsair RM750x (modular, 80+ Gold, 10-year warranty, ~£110–130) is the natural upgrade if you later need it.
Next: Type PC10 to generate Chapter 10 — Case & Cooling: mounting the components in the Fractal Define 7, fan placement for your build's airflow, cable routing strategy, and the final assembly checklist before first power-on.