The Cost of a Watt

Thought:

As a[n easy] rule of thumb;
Every watt a device consumes, costs me ≈10¢ per month.

So, if I have a 2watt LED porch light, that I leave running 24/7, it will cost me ≈20¢/mo to operate.

Actual Math: (0.002kw * 0.1125/kw * 24hrs * 31days) = 16.74¢
Granted, this is a rule of thumb, as a general guideline, for easy on-the-fly estimates, with room for the cost of electricity to go up a bit too.