Watts, kWh, TDP and PSU ratings

Your power supply says 750 W on the label, your CPU box says 65 W, and your electricity bill counts kWh. Only one of those three numbers ends up costing you money.

The short answer

Watts tell you how fast a device uses energy right now. Kilowatt-hours tell you how much energy it used over time, and that’s what you pay for. TDP and the wattage on a power supply are design figures, not readings of what your computer actually pulls from the wall.

How it works

1. Power vs. energy: a watt (W) is one joule per second, so it describes a rate. Energy is power multiplied by time. 100 W running for ten hours is 1,000 watt-hours, or 1 kWh.
2. Cost: your bill multiplies kWh by a price per kWh. Watts alone can’t give you a cost until you know how long the device runs.
3. TDP: thermal design power is a target for designing the cooling and power delivery around a processor. Intel defines it as the power under maximum theoretical load and has called it Processor Base Power since its 12th-generation chips. Under lighter load the processor uses less. With turbo or certain heavy workloads it can briefly go above it.
4. PSU rating: the wattage on a power supply unit (PSU) is the maximum DC output power the manufacturer specifies. It’s a ceiling, not a consumption figure. A 750 W unit powering a light load delivers far less than 750 W.
5. Efficiency: a PSU turns AC from the wall into DC for the components and loses some energy doing it. Efficiency is DC output divided by AC input. That’s why the wall reading is always higher than what the components receive.

In practice

A device that draws a steady 50 W at the wall:

50 W × 24 h      = 1,200 Wh = 1.2 kWh per day
1.2 kWh × €0.40  = €0.48 per day
50 W × 8,760 h   = 438 kWh per year  →  €175.20 per year

€0.40/kWh is a rounded example price based on the German regulator’s average for household customers (40.05 ct/kWh on 1 April 2025). Use the price per kWh on your own bill instead. Efficiency works the same way: if the components need 200 W of DC and the PSU runs at 90 % efficiency, it draws about 222 W from the wall (200 ÷ 0.9).

Volt-amperes are not watts

Some meters and every UPS label show VA (volt-amperes) next to or instead of W. VA is apparent power: voltage times current, without asking whether the two are in step. W is real power, the part that actually does work and that your meter bills. The ratio between them is the power factor (PF), a number between 0 and 1. A PSU with a power factor of 0.95 drawing 200 W real power pulls about 211 VA from the socket (200 ÷ 0.95). That matters in two places: a UPS is sized in VA and in W, and you must stay under both; and cheap plug-in meters sometimes display VA while calling it watts. If a reading looks too high compared with another meter, check which of the two it is showing.

What 80 PLUS actually guarantees

The 80 PLUS badge on a PSU is a certification of efficiency at a few fixed load points, measured at 20 %, 50 % and 100 % of the rated output (Titanium adds 10 %). For the 230 V EU internal class, Gold means at least 90 % at 20 %, 92 % at 50 % and 89 % at 100 % load, and a power factor of at least 0.90 at half load; Titanium requires 90 % already at 10 % load, 93 % at 20 %, 95 % at 50 % and 91 % at full load. Two things follow for a homelab. First, efficiency peaks around half load and drops at both ends, so a 750 W Gold unit feeding a 60 W idle server is running at 8 % load, below any point the certification covers. Second, the badge says nothing about standby power, fan noise or how the unit behaves at 30 W. For an always-on machine, a smaller PSU running nearer 30–50 % load is usually both cheaper and more efficient than an oversized one.

What an oversized PSU costs in idle, in numbers

Assume a server that needs 50 W of DC in idle. A PSU running at 80 % efficiency at that point draws 62.5 W from the wall; one at 90 % draws 55.6 W. The difference, 6.9 W, is 60 kWh a year, about €24 at the example price. Not dramatic for one machine, but it is the same money every year, and the gap grows with every extra always-on box. That is the whole case for matching the PSU to the real load instead of to the biggest number on the parts list.

Sizing a PSU without guessing

1. Add up the measured or datasheet DC draw of CPU, GPU, drives and fans under the heaviest load you actually run, not the TDP sum.
2. Add 20 % headroom for turbo peaks and ageing.
3. Pick the smallest rating above that number, and check that the 12 V rail (where CPU and GPU draw from) is specified for most of it.
4. Then measure at the wall. If idle sits below 20 % of the rating, you have bought headroom you pay for every hour.

Checklist

• Write down every power value with its unit and where it was measured: at the wall, from internal sensors or from a datasheet.
• Note how long and under which load a value applies before you turn it into kWh.
• Keep TDP, sensor readings and wall measurements in separate columns instead of mixing them.
• Calculate costs only with a price per kWh you’ve stated, and say where it comes from.
• Check whether a meter shows W or VA before you compare it with another one.