I've never really had issues with this and I"ve been running sometimes up to six or more "Retro-Pcs" all day long 24/7/365 in a networked setting since 2002. Most of these machines have power supplies that will never breech 300 Watts - and most modern systems, especially those made for gaming,; have 500 watts or more. Sure, it does add a LITTLE - and that will add up - but for most people, the price is pretty inconsequential compared to say, running your HVAC all day and all night.
Actually, funny story, in my single years in the 2000's, in my 1st apartment, I used to run my 8088-80486 machines in the winter time all day and all night to keep the apartment warm. That combined with leeching off the neighbors cooling really kept my power bill down. I've lived in the most expensive parts of the country (Seattle) and my power bill even with a 286, 386, 486, i1400 ThinkPad, ThinkPad 755CD, an old BLX2 Blue Lightning, a Power Mac 6400/180....and god only knows what else all running at one, some 24/7/362ish - and I never had a power bill over $115 bux, with an average of $75 or under even in the winter usually ($115 usually was when I started running power tools to build guitars or using the CRT TV a lot). I don't think I ever used my heater the entire time I lived there.