Your PC is probably one of the most power-hungry electronics in your home, and you're probably aware of how much power it is capable of consuming under load. However, modern PCs can consume a fair bit of power even when you're doing absolutely nothing, and there are a multitude of reasons for that.
A low-efficiency PSU
Not only a concern under load
Power supplies aren't perfect; they draw extra power from the socket to account for losses that occur during AC-to-DC conversion. So, if your system needs 50W at idle, it could draw 60–65W instead. The most efficient power supplies reach up to 96% efficiency, which is mighty impressive, but that's for expensive 80+ Platinum units and at 50% load. When you are above or below 50%, efficiency decreases, and cheaper PSUs compromise on efficiency in general. While there are cheaper steps to take to reduce idle power consumption, a PSU upgrade is still worth considering if you want to minimize the bills your PC is generating as much as possible.
CPU power management isn't in check
Don't let it consume more power than it needs
Though the default power management modes set for your CPU always tend to favor the "Balance" configuration, you should double-check. If your CPU is set to "High Performance" in Windows, it will consume more power at idle. The CPU will run at boost frequency at all times, which doesn't drastically increase power draw, but does make a tiny difference that can add up. CPU C-states are set by the motherboard, and these also tend to be set to "Auto," but the settings can be more conservative on some boards than others. If your board allows, you should set your C-states to allow C6, which minimizes idle power consumption from the CPU as much as possible. In very rare cases, this can possibly cause latency spikes, but it is generally only a concern for ultra-low-latency workloads like real-time simulation.
Background tasks are intervening
They can interrupt C-states
Some background apps can utilize resources at idle and either prevent (or wake the CPU/GPU from reaching) deep C-states; this keeps idle power consumption higher than it needs to be. Cloud service apps, game launchers (like Steam), RGB sync software, and communication apps like Discord can be responsible for this, among others. Even idle websites on the browser can keep your CPU busy through dynamic content like refreshing ads. Microsoft has a guide on detecting apps if you want to pinpoint specific apps that might be interrupting your system. Some of these apps (plus Windows overlay like Instant Replay and live wallpapers) can also keep your GPU from idling properly, so fixing them could help significantly reduce idle power draw.
For timer-based wake-ups, allowing important wake timers under the advanced power management settings should help reduce interruptions from background apps. For polling, some apps (like HWiNFO and Slack) allow increasing the polling time, which you should increase to 20 seconds or higher to maintain C6 residency.
USB peripherals
They're always around
Devices like RGB keyboards and mice, DACs, USB microphones, and webcams may keep the chipset running (since it handles almost all USB connections) and prevent it from entering deeper low-power states. Polling peripherals (e.g., gaming mice, keyboards, external drives, and capture cards) can ping your PC consistently and prevent the CPU from going into deep C-states.
To counter unnecessary consumption from USB devices, make sure USB Selective Suspend in Windows power management settings is enabled. This only works if the device/drivers support it, though.
Multi-monitor setups add a decent chunk of power draw
The cost adds up
Monitors can consume as much power at idle as during normal activity, depending on the image on display. Multiply that for multi-monitor setups, especially those that include high refresh rate and/or high-resolution screens, and that's a decent amount of power. Plus, outside their own power use, monitors in these configurations force the GPU to run at higher idle clock speeds and draw more electricity.
The simplest way to get around high-monitor power consumption is to switch them off when not in use, or configure them to switch off after X minutes of inactivity. Many monitors have power-saving or ECO modes that are worth looking into for secondary displays. Also, make sure any high refresh rate secondary monitors are running at 60 Hz unless you need the higher rate.
RGB devices
They demand their own tax
Modern gaming PCs include a handful of light sources, though most are down to personal preference and choice. RGB from the motherboard, RAM, AIO pump block lighting, and ARGB strips are some of the common lights, one or a combination of which are in every other gaming PC. Depending on the combination, you may have anywhere from 40 to 60 (or even more) LEDs running, which together can consume 5–15W or more. Brighter colors consume more, with full white using the highest amount of current.
If you're worried about idle power consumption by these devices, you can use an RGB controller to switch them off (or lower their brightness). Some RGB software also supports switching off the LEDs automatically with your display.
High idle power consumption can be bothersome, but there are solutions
Modern gaming PCs use software, components, and peripherals that keep consuming power even when you're not using the PC. The strategy to reduce idle power draw is recognizing the sources and then dealing with them. Though putting your PC to sleep is the best step if you're stepping away for more than just a few minutes. Other than that, you can tinker with power settings, reduce background apps to a minimum, and disconnect/switch off any peripherals that continue eating power when you're AFK.