Yeah ... expanding on what mike_forum said, the rates in CA are tiered. Here in SF the rate ranges from 13 cents to 35 cents per kwh depending on your overall usage. This also varies seasonally (and, I think, due to other mysterious factors). The rates are calculated based on your usage within each tier that you reach.
So in reality, adding a device like the OBi can make a measurable difference in one's monthly bill, as you'd need to figure it at the rate of the highest tier you reach.