First experiments with Modbus RTU successful! Got an electricity meter that supports modbus, got a USB RS485 adaptor, connected a 13 amp plug to the meter, connected a spare length of twisted pair taken from a cat5e cable from the meter's RS485 port to the adaptor, plugged that into my server and ran mbpoll against it. A bit of finding the meter's protocol manual and tweaking and I have readings! (It ought to have terminating resistors at each end but it worked well enough without to prove the thing works.) With that knowledge I should be able to configure collectd to monitor meters and CGP to generate graphs of it for me, so hurrah. So, the plan to properly meter my customers' energy use at the datacentre (which will probably have to be implemented as part of a rack move which I really ought to do sometime this year - the DC want back the space my original, first rack has been in since 2006), can go ahead! This also means I should be able to get data from my solar/battery inverter at home at some point, as it also uses Modbus RTU over RS485. But that will be Another Project™.