Hi Dominic,
I wanted to reply using the mailing list rather than reply directly as it can be of more use to others this way.
Regarding the error, you need to set the "Power Histogram Bin Width" variable to a number which divides the rated power (sorry for the typo in the error message!). Depending on the fidelity you want, an example that works would be 0.01. This value is recommended in the README_example_data.txt file provided with the database along with some others for this example.
The Array Rated Power refers to the required total output of the array and should be set in MW. For one device, you would therefore set it to 1.48.
I am not sure precisely what you mean by a power output graph? The hydrodynamics generates a set of probabilities for the power of the device given the environmental conditions, and these probabilities are also binned across the rated power of the device. If you are looking for turbine performance information, this is an input to DTOcean rather than an output.
I also thoroughly recommend you use the 1.0.3 version of the software, available through Anaconda Python, which has fixed some of the faults of the initial release. There is a post discussing that release on the message board.
Hope that helps,
Mat