I was considering this but then I stumbled across some ~7 year old micro towers. Those little thin client boxes that were popular for a short time as a thing that can connect a keyboard, mouse and monitor to a corporate network.

Refurbished with a Windows license (if I ever want to use it) was less than a surplus laptop. Power usage seems acceptable but I'll compare with a Kill-A-Watt. I have one on the way and if it works out I'll grab a few more.

I do like the idea of a surplus laptop cluster, mostly because of the built in UPS factor but there's a lot of wasted hardware there, such as the monitor and keyboard. But I suppose that's kind of a feature too.

I have shower thoughts of some kind of custom bladecenter made of old laptops but that's a lot of work for probably zero benefit.