Picking a power supply that’s too small can cause instability or shutdowns under load, while one that’s needlessly oversized just wastes money. This calculator adds up the estimated power draw of your components and applies a safety margin to recommend a PSU wattage, so you can shop with a target number instead of guessing. It’s useful whether you’re planning a new build, checking if your current PSU can handle a GPU upgrade, or troubleshooting random reboots under load.
What the Result Means
The calculator gives you a recommended minimum PSU wattage, not the exact number your components will draw at every moment. The recommendation is deliberately higher than your components’ combined typical draw, because power supplies run most efficiently and safely when they aren’t operating right at their rated limit, and because real-world power draw spikes briefly above average, especially under a demanding GPU load. If the result says, for example, “650W recommended,” that’s the wattage rating you should look for when shopping, not the wattage your PC will constantly consume — actual draw from the wall will usually be lower than that number during normal use.
How This Calculator Works
The calculator sums the estimated wattage of your listed components — CPU, GPU, storage, RAM, motherboard, fans, and any other listed parts — using their rated or typical TDP (thermal design power) figures as the base for CPU and GPU draw, since those two components dominate total system power. It then applies a headroom margin on top of that sum, typically in the range of 20-30%, before rounding to a common PSU wattage tier (450W, 550W, 650W, 750W, 850W, and so on) that manufacturers actually sell.
That headroom margin covers three real things: PSU efficiency loss (a power supply pulls somewhat more from the wall than it delivers to your components, with the difference lost as heat), the fact that components rarely all hit peak draw at exactly the same instant but occasionally get close, and — increasingly important on modern high-end GPUs — transient power spikes. Modern GPUs, particularly higher-end ones, can briefly spike well above their rated TDP for fractions of a second during load changes, and a PSU with no headroom can trip its protection circuits during one of these spikes even though the average draw is well within spec.
Worked example: Take a build with an AMD Ryzen 7 7700X (105W TDP) and an RTX 4070 Super (220W TDP), plus roughly 60-80W for the motherboard, RAM, an NVMe SSD, and a couple of fans. That’s a component sum of roughly 385-405W. Applying a 25% headroom margin brings the target to about 480-505W, which the calculator would round up to a 550W PSU recommendation — the nearest commonly sold tier that comfortably clears the padded estimate.
Important Limitations
This is a planning estimate, not a lab measurement of your specific PSU or components. Actual power draw varies by silicon quality (even two units of the “same” chip can differ slightly), overclocking or undervolting, ambient temperature, the specific game or workload, and how many drives, fans, and peripherals you have connected. The calculator also can’t know your exact PSU’s real efficiency curve, since that depends on the specific unit and its efficiency certification (80 Plus Bronze, Gold, Platinum, and so on) as well as load percentage.
Because of the transient spike behavior of modern GPUs, we’d rather round up than down — if your result sits right between two common wattage tiers, choosing the higher one is the safer call, especially if you plan to add drives, overclock, or upgrade the GPU later without replacing the PSU. This calculator is not a substitute for checking your GPU manufacturer’s own recommended PSU wattage, which is also listed for each GPU in our spec tables.
Frequently Asked Questions
Is it bad to oversize my PSU by a lot?
Not dangerously, but it’s a waste of money and PSUs are usually least efficient at very low load percentages. A PSU sized for roughly 50-70% load under typical gaming use is a reasonable sweet spot rather than one so oversized it rarely runs above 20-30% load.
Why does the calculator recommend more wattage than my parts' TDP adds up to?
Because TDP figures are typical or sustained ratings, not worst-case peaks, and because PSUs lose some power to heat rather than delivering 100% of what they draw from the wall. The headroom margin covers both of those plus brief power spikes from the GPU.
Does PSU efficiency rating (80 Plus Bronze vs Gold vs Platinum) matter for sizing?
It affects running cost and heat output more than the wattage you need, since the rating measures how much of the wall power becomes usable DC power rather than how much total capacity you need. A higher-rated PSU of the same wattage will typically run a bit cooler and cost a bit less to operate over time.
Do transient spikes really matter for PSU sizing?
Yes, especially with higher-end GPUs. These are very brief power draws above the GPU’s rated TDP that can occur during sudden load changes, and a PSU with little to no headroom can shut down protectively during one even if the GPU’s average power draw is normal.
Can I reuse my old PSU for a new GPU?
It depends on both its wattage and its connector availability (some newer high-end GPUs need specific 12VHPWR or multiple PCIe power connectors). Run your new component list through this calculator and compare the result against your current PSU’s rated wattage and connectors before deciding.