The units I'd use are:
Lenovo ThinkCentre M57e 6305
System info:
CPU Intel Pentium E2200 / 2.2 GHz
Number of Cores Dual-Core
Data Bus Speed 800 MHz
Chipset Type Intel G31
Sata SSD
4x 1GB RAM DDR2
No Monitors
Cost of Hydro in my city:
OFF peak$ 0.065 / kWh
MID peak$ 0.095 / kWh
ON peak$ 0.132 / kWh
At Lenovo's website, look up the power draw of that particular model. It's under 'specs'. Then do some multiplying… And figure out how many hours a day are off, mid, and on peak… Including adjusting for weekends.
Hint: if this relatively simple computational task is beyond you, I guarantee that your "render farm" is not going to work out for you.