We may want to estimate the profit of renting storage vs electricity cost.
Say that you have a PC with Celeron CPU and 2x 6TB internal hard drives and you leave it running 24/7 just for SIA.
How much is it going to consume ?
Assuming that you rent all your 6 TB at 12-15 $ / month ($2/TB/month) , i am not sure you can cover the energy costs, because it depends on the disk reads/writes operations done in that month, as the disk too consumes power based on I/O activity.
you leave it running 24/7 just for SIA
You said it all, that won't be worth it. I've heard that a well configured machine with low rpm hdd can consume only around 100-150w, I haven't done the math, but could be a beginning.
The idea is more to have a machine running 24/7, with its own purpose, and as you have free space on it, you rent it.
It will consume much less than that. A Celeron CPU is rated usually at 35W, RAM will be 2W more, low-power disks (5400rpm) 2-3W each one. With a bit of overhead and loss of efficiency of PSU, the final consume is going to be an average of 50W or less (those values I mentioned are max values). A Minebox consumes less than 50W, for instance.