What Power Supply Do I Need? How to Calculate Your

Choosing the wrong power supply is one of the most overlooked decisions in a PC build. Too little wattage and you risk crashes, instability, and hardware damage during peak load. Too much and you’ve spent money on headroom you’ll never use. Getting it right isn’t complicated — you just need a reliable method. This guide walks through exactly how to calculate what you need, what efficiency ratings actually mean, and which PSUs are worth buying in the UK right now.

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The Two Components That Matter Most

Your CPU and GPU together account for roughly 80–90% of your total system power draw. Everything else — RAM, storage drives, case fans, USB peripherals — typically adds up to around 50–100W combined. That means the calculation always starts with two numbers: your CPU’s power draw and your GPU’s power draw.

The figure most manufacturers publish is TDP (Thermal Design Power). It’s not a precise measurement of actual power consumption — it’s an engineering reference figure used to size cooling solutions — but it’s close enough to be a reliable planning number. Under sustained load, some modern CPUs can significantly exceed their rated TDP, which is why headroom matters and why we’ll account for that in the formula below.

Common Component Power Draw Reference

The PSU Calculation Formula

Once you have the power draw figures for your CPU and GPU, the rest is straightforward. Here’s the method step by step:

1. Find your CPU’s TDP from the manufacturer’s specifications page.
2. Find your GPU’s TDP or typical board power figure — Nvidia and AMD both publish these.
3. Add 100W to cover everything else in the system: motherboard, RAM, storage, fans, and any connected USB devices.
4. Multiply the total by 1.2 to add 20% headroom. This accounts for power spikes under peak load and leaves room for future upgrades or component additions.

Formula: (CPU TDP + GPU TDP + 100) × 1.2 = Recommended PSU wattage

Once you have that number, round up to the nearest standard PSU size. Common sizes available in the UK are 550W, 650W, 750W, 850W, 1000W, and 1200W. Never round down — always go to the next tier up.

Real Build Examples

The formula makes more sense when you apply it to actual builds. Here are three worked examples covering a range of budgets.

Budget gaming build: Ryzen 5 7600X (105W) + RTX 4060 (115W) + 100W = 320W. Multiply by 1.2 = 384W. The next standard size up is 550W, which gives you comfortable headroom and room to add drives or upgrade peripherals later. A 550W PSU is the right choice here.

Mid-range gaming build: Core i7-13700K (125W) + RTX 4070 Ti Super (285W) + 100W = 510W. Multiply by 1.2 = 612W. Round up to 750W — a 650W would technically cover the calculation but leaves minimal headroom for a GPU of this class under sustained gaming loads. Go to 750W.

High-end workstation or gaming build: Core i9-13900K at peak load (253W) + RTX 4090 (450W) + 100W = 803W. Multiply by 1.2 = 964W. This build needs a 1000W PSU at minimum, and if you plan to push the CPU with workstation workloads alongside the GPU, a 1200W gives better sustained efficiency.

Understanding 80 PLUS Efficiency Ratings

A PSU doesn’t pass mains power straight to your components — it converts AC from the wall to DC at various voltages, and some energy is lost as heat in that process. The 80 PLUS certification was introduced to set a minimum efficiency standard: a certified PSU must convert at least 80% of the AC input to usable DC output at 20%, 50%, and 100% load.

There are five certification tiers, each requiring higher efficiency at all three load points:

• 80 PLUS (White): 80% efficient — budget tier, found on very cheap units. Avoid for anything beyond a basic office machine.
• 80 PLUS Bronze: 82–85% efficient — acceptable for entry-level gaming builds where budget is tight.
• 80 PLUS Gold: 87–90% efficient — this is the sweet spot. Gold-rated PSUs from reputable brands hit the best balance of efficiency, price, and reliability. Most builders should target Gold as a minimum.
• 80 PLUS Platinum: 90–92% efficient — diminishing returns in terms of electricity savings unless your PC runs 24 hours a day.
• 80 PLUS Titanium: 92%+ efficient — genuinely worthwhile only for servers, always-on workstations, or systems running under continuous heavy load.

For a gaming PC that runs a few hours a day, the difference in electricity cost between Gold and Platinum over a year is negligible. Spend that premium instead on a better-specced Gold unit from a reputable brand.

Recommended PSUs by Wattage Tier (UK)

These are well-regarded options available from Amazon UK, chosen for build quality, warranty length, low noise output, and consistent reliability in the UK enthusiast community.

550W — be quiet! Pure Power 12 M 550W (80 PLUS Gold): Quiet, fully modular despite its price point, and backed by be quiet!’s five-year warranty — an excellent choice for budget and mid-range builds alike.

650W — Corsair RM650x (80 PLUS Gold): The RM series has been a community favourite for years; the zero-RPM fan mode keeps it silent under light load, and the ten-year warranty reflects Corsair’s confidence in the platform.

750W — Seasonic Focus GX-750 (80 PLUS Gold): Seasonic manufacture OEM units for many other brands — buying direct means you get the same quality at a fair price, with a ten-year warranty and a reputation for outstanding voltage regulation.

850W — Corsair RM850x (80 PLUS Gold): The go-to recommendation for mid-to-high-end gaming rigs; fully modular, extremely quiet, and well-specced enough to handle a future GPU upgrade without touching the PSU again.

1000W — be quiet! Dark Power 13 1000W (80 PLUS Titanium): For high-end builds with flagship GPUs, the Dark Power 13 delivers exceptional efficiency and whisper-quiet operation — the overclocking connector and rock-solid build quality make it worth the premium at this tier.

Modular vs Semi-Modular vs Non-Modular

PSUs differ not just in wattage and efficiency but in how their cables are managed. It’s worth understanding the three types before you buy.

• Non-modular: All cables are permanently attached to the PSU. You can’t remove the ones you don’t need, which makes cable management more difficult. The trade-off is a lower price — acceptable in budget builds where aesthetics aren’t a priority.
• Semi-modular: The essential cables — the 24-pin ATX motherboard connector and the EPS CPU power cable — are fixed. Everything else (PCIe, SATA, Molex) is detachable. This is a solid middle ground that suits most builds.
• Fully modular: Every cable is detachable, including the ATX and EPS connectors. This gives you the cleanest possible cable routing, especially in a windowed case where the inside is on display. The cost premium is modest and worth it for any build where presentation matters.

Common Mistakes to Avoid

• Buying a no-brand PSU to save £20: The PSU is the component that powers and protects everything else in your build. A cheap unit with poor voltage regulation or no proper protection circuits can damage your CPU, GPU, and motherboard if it fails. It is the wrong place to cut corners.
• Undersizing for a GPU upgrade you’re already planning: If you’re buying an RTX 4070 now but have your eye on an RTX 4080 Super in eighteen months, calculate based on the GPU you intend to upgrade to, not the one you’re fitting today.
• Ignoring the ATX 3.0 and PCIe 5.0 connector: Nvidia’s RTX 4000 series and AMD’s RX 7000 series both benefit from a native 16-pin (12VHPWR or 12V-2×6) connector. Some older Gold-rated PSUs don’t include one natively and ship only with an adapter — check before you buy, especially if you’re running a 4090 or 4080 Super.
• Forgetting cable length in large cases: In a full tower or extended ATX case, cheaper PSUs with short cables may not reach the motherboard headers at the top of the case. Check cable length specifications if you’re building in anything larger than a mid tower.

The method is straightforward: add your CPU TDP and GPU TDP, add 100W for the rest of the system, then multiply by 1.2 to get your minimum recommended wattage. Round up to the nearest standard size, target at least Gold efficiency, and buy from a manufacturer with a proven track record and a meaningful warranty. Getting the PSU right once means you won’t have to revisit the decision until your next complete rebuild. If you’re still working out your case, it’s worth reading our guide to PC case types — your form factor affects both the PSU standard you need (ATX, SFX, or SFX-L) and the cable lengths required to reach every connector cleanly.