I have a spare LGA1150 motherboard with a G1850 in it - I want to repurpose this as a new desktop for a bit until I can build something more modern, from scratch, in the future.
I am going to be using the desktop for working from home, plus some occassional video editing and music recording - nothing hugely taxing.
My current destkop is based upon an i3-2105, which does lack a bit of oomph at times - e.g. if I am working but also need to run Netflix in the background.
So I was really looking to swap out the G1850 CPU for something with more grunt than an i3-2105.
That being said, the work computer is going to be idling for most of the time whilst I am just doing work on it so I really need to be aware of basic energy use (especially with electricy prices at the moment).
Ultimately, I was looking at the best compromise between energy use and performance but I am struggling a bit trying to work that out.
If you're referring to Intel systems, then since Sandy Bridge the biggest influence on idle power (assuming no HDDs) is usually the graphics card, the second is the motherboard, CPU comes third. In many systems, a high-end graphics card at idle uses more than the motherboard, memory and CPU combined.
I'm not sure why, maybe because they're often just cut-down larger CPUs (with all the gubbings still present), but the difference between low-end and high-end Intel CPUs tends to be very small. I can't speak for 11th and 12th gen, but certainly for anything since Haswell, we're talking a few watts
at most.
The motherboard can make a huge difference to idle power and with high-end CPUs it can have a big influence on load power consumption. Generally speaking for idle power: ITX motherboards are the best format, ASRock are the best mainstream brand and the worst chipset is Z boards because they have so many additional features (many of which require more components). The difference between H and B chipst is usually minimal unless it's a high-end board.
If you're using an Intel system since Sandy Bridge, without an IGP and minimal hardware, then 20-30 watt (whole system) is normal for a mainstream motherboard, anything ITX or ASRock will be more like 15-20 watt. That's regardless of the CPU used, whether it's a Celeron or an i7.
When you get down to 30 watts, bronze and gold-rated ATX PSUs (well, all of them, to be honest) have very poor efficiency at this level of load, so for the ultimate idle (anything sub 15 watt) you usually need to use pico psus and power bricks.
For a long time, 8th and 9th gen CPUs and low-end ASRock (or better: Fujitsu motherboards) offered the best combination of idle power and performance in average desktop use, but Ryzen APUs are pretty competitive with the right motherboard.
Graphics cards: from what I've seen AMD's RDNA has great idle power, especially the lower-end and the Sapphire models and nvidia's Pascal is very good too.
Ryzen is consistently not great for idle power, I'm not sure why, if it's the CPU or the motherboard, but it appears to use significantly more (1st-3rd gen) if the memory is clocked beyond 2666 and needs more effort to maintain (for example: some applications like Steam, seem to prevent the system from idling properly). I don't know if these issues have been resolved since I last looked into it, or if the latest CPUs are any better. APUs are different to the other desktop CPUs, I guess since they're meant for laptops and are very competitive with Intel at idle, even with middling motherboards.
The most efficient CPUs under load is a different story, when multi-threaded it's usually AMD, but lightly-threaded and gaming Intel is often better (except for 11th gen, which is usually crap). Limiting a CPU to low clocks (e.g. a T or S model, or disable turbo) keeps it within the architectures highest efficiency bands which is relevant for continuous use like gaming, but like you said, it's often less efficient than high clocks when the task is finite.
If you're only upgrading your current CPU for now, then my suggestion would be an E3-1245 V3 or E3-1246 V3 for approx £45. They'll be efficient at idle and clock high when needed, with hyperthreading for productivity stuffs if supported.
My most efficient 9th gen system uses under 15 watts idle (using a standard ATX psu and no graphics card), but compared to my gaming system (30 watts idle) we're only talking about a tenner for the 15 watts (for 8 hours use per day @ 25p kwh).
