For some real numbers, I picked up a 2018 generic office PC (Core i5-7500) used for $50 as a backup machine to run linux on when my laptop was in for repairs, and it idles at 14 W, vs the OpenWrt One which appears to run at 5.5 W.

So that's 10 kWh/mo for the PC vs 4 kWh/mo for the OpenWrt One.

Yep, and that's anywhere from about $1.30 to $3 something in the really expensive states for electric rates in the US. Half that if you only count the delta between that and a low power device.

Spending hundreds on new hw to save $20 a year in power cost.