PC Power Supply Calculator
Pick your parts and get the wattage your build actually needs, sized against the spikes that trip undersized units rather than the average draw.
Your build
Add a second graphics card
Motherboard, chipset and the rest of the always-on load are counted at 50W.
The verdict
Pick a processor and a graphics card
Everything else is already set to a typical build, so two choices is usually all it takes. The numbers update as you go.
The link carries your whole build, so you can paste it into a forum thread and the answer travels with it.
These are estimates from published power figures, not measurements of your machine. Real draw moves with the game you are running, the ambient temperature and how your parts were binned. The recommendation is sized against the transient peak rather than the average, which is the number that actually decides whether a unit copes.
How PSU wattage actually works
The number on the box is a ceiling, not a target
A 750W power supply is rated to deliver 750W continuously. It is not a figure you are meant to reach. Sizing a build to sit right on that ceiling is how you end up with a machine that reboots halfway through a raid and passes every test you throw at it afterwards, because the fault only shows up under a combination of load you cannot easily reproduce on purpose.
The useful way to think about it is the proportion. A power supply is at its most efficient somewhere around half load, and that is also where its fan has least to do. Land a build near there and you get the quiet operation and the lower bill at the same time, with enough left over that a future graphics card does not mean a second purchase.
Nameplate TDP is not the power a part draws
This is the single most common way people undersize. A processor sold as a 65W part can hold well over twice that on an all-core workload, because the number in the marketing is a thermal design figure for the cooler, not a power budget for the supply. Intel publishes both, as base power and maximum turbo power, and AMD publishes TDP alongside PPT, which is the one that binds. This calculator uses the sustained figure and shows the nameplate next to it when the two differ, so you can see the correction rather than take it on trust.
Graphics cards have the same gap in a different direction. A card rated at 200W will sit near 200W all evening, then ask for 280W for a couple of milliseconds when a scene changes. That is a transient spike, and it is the reason PSU sizing guidance has moved up over the last few years even though average draw has not.
Why spikes matter more than averages
Power supplies protect themselves. When demand crosses what the unit can deliver, its over-current protection shuts everything down rather than letting voltage sag into the parts. That protection reacts in milliseconds, which is exactly the timescale a graphics card spike lives on. The result is a machine that cuts out under load while every monitoring tool insists the system was drawing 400W, because the tool was sampling once a second and the spike lasted for one thousandth of that.
This is why the recommendation here is sized against the peak rather than the steady figure. A build that averages 450W but peaks at 600W needs to be treated as a 600W build, and it is not being cautious to say so, it is being accurate.
What headroom is actually buying you
Three things, in order of how often they matter. First, it keeps the transient peaks inside the unit's capability, which is the difference between a stable machine and an intermittent one. Second, it puts the unit near its efficiency peak, which costs less to run and makes less heat for the case fans to move. Third, it means the next graphics card is a graphics card purchase rather than a graphics card and a power supply purchase.
There is a point where more stops helping. A 1000W unit powering a 300W build spends its life at 30 percent load, below the window where its efficiency curve is flattest, and you paid for capacity that never does anything. That is why the recommendations here have an upper bound as well as a lower one.
What efficiency certification is worth in pounds
The 80 PLUS tiers describe how much of the power drawn from the wall reaches your components. On UK mains the thresholds at half load run from 85 percent for Bronze to 92 percent for Gold and 94 percent for Platinum. Everything in that gap becomes heat.
The honest version of the Gold argument is that it depends on how much you use the machine. At UK price cap rates of 26.11p per kWh (1 July to 30 September 2026), a few hours of gaming a day on a mid-range build puts the Bronze-to-Gold difference in the region of a few pounds a year, which does not pay back a large price gap quickly. Push the hours up, or the wattage up, and it starts to. The comparison in the tool runs the numbers on your build rather than on a hypothetical one, so you can see which side of that line you are on instead of taking a rule of thumb.
Efficiency is not the only reason to buy a better unit, and it is usually not the best one. Build quality, the warranty, the quality of the protection circuitry and whether the fan stops at idle all matter more to how the thing feels to live with. Treat the running-cost figure as one input, not as the argument.
Common mistakes
- Sizing off the graphics card alone. Card makers publish a recommended system wattage, which assumes a processor they cannot know about. It is a starting point, not an answer.
- Trusting a wall meter. A plug-in meter samples far too slowly to see a transient, and it reads what comes out of the socket rather than what reaches the parts. It is useful for running cost and useless for sizing.
- Reusing an old unit at its old rating. Capacitors age. A ten-year-old 650W unit is not a 650W unit any more, and the tolerances that matter most are the ones that went first.
- Counting the cooler twice. An AIO pump and its fans are worth around 25W together, not the number on the radiator box.
- Assuming daisy-chained PCIe cables are fine. On a card pulling over 300W they are not. The wattage can be right and the cabling still wrong.
Where these numbers come from
Component power figures are published manufacturer specifications, corrected to sustained power where the nameplate figure is a thermal rating. Transient estimates follow the same ratio across the table rather than being set per card, so no single card is quietly favoured. The efficiency figures are the 80 PLUS thresholds at half load on 230V mains. The electricity rate is the Ofgem price cap unit rate, dated on the page and editable if you are on a different tariff.
The units suggested underneath your result come from our own catalogue, filtered to what is in stock at the size your build needs. We earn a commission if you buy through those links, which does not change which units appear: the ordering is wattage, certification and price, and the same filter runs whatever the commission looks like.
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