65W TDP? Let's say we want to run a PC so we're switching to a newer low-end Ryzen with a 35W TDP and that that's a 30W difference for the whole system. Let's say we're running the system 24/7 and the CPU is pulling its full TDP constantly. Average US residential electricity price is $0.18/kWh.

0.03 kW * 24 h * 365 d * $0.18 = $47.30/year

Watts in TDP are not the same as watts in electricity, although they're both measures of energy.

TDP is a thermal measurement, it's how much heat energy your heatsink and fan need to be able to dissipate to keep the unit within operational temperatures. It does not directly correlate to the amount of electricity consumed in operation.

PC power draw at the wall is different than TDP. Idle power goes to a lot of components.

Even CPU TDP is not an accurate measure. My latest AMD CPU will pull more than it’s rated “TDP” under certain loads.

In the UK, residential electricity tariffs are currently capped by the regulator at 27.69p per kWh, resulting in a total yearly cost of £72.77. Much higher than in the US, but still much cheaper than a new PC.

So $50/yr for 4 years gives you ~$150 with $50 extra for shipping or whatever, which gets you a decent Lenovo M700 Tiny with much better performance in both power and power consumption.