This is an interesting discussion.
" • on a 32-bit machine with 4GB of memory and with 2.5GB reserved for SIP server,
the server could support 43 000 simultaneous TLS connections – consumed energy 209W"
http://www.kamailio.org/w/2011/05/green-voip-energy-efficiency-and-performa…
That paper by long-time contributor Jan Janak is a good one to return to now and then.
/O