Hi and thanks for these questions. 1) You may input into DER-VET negative values of day ahead energy prices. However, if you are asking DER-VET to perform size optimization using the GUI, negative energy prices will not be allowed. In the GUI, an error will occur that prevents the case from being run. The GUI's Summary page will display red text describing the error. This validation is meant to prevent concurrently performing size optimization and using negative energy prices (this causes concurrent charge and discharge for storage systems, see [DER-VET User Guide | Mixed Integer Optimization](https://storagewiki.epri.com/index.php/DER_VET_User_Guide/Model_Details/Mixed_Integer_Optimization) for more details). The work-around here is to run the case through DER-VET's command-line. With DER-VET's command-line operation, a warning occurs (written to the log file) in this situation, but the case will run. Example work-around: You have set up a case with the GUI, and are getting an error indicating Day Ahead Pricing values must all be positive in value. Save a copy of yo ur file that has the negative energy prices. Create a new file of energy prices, all set to zero, and load this into the GUI. Run the case in the GUI (click Green play button), but stop the case right away (click Red stop button). These actions will create the input files on your local computer needed to run the case with DER-VET's command-line. The last step is to modify the input timeseries.csv with the negative energy prices that you saved, and then run the case via the command-line.