ManagingEnergy provides a convenient way to check if your tariff definition is working. First get an actual invoice. Then click the Test Tariff tab and fill in values from the invoice, including the Start Date and End Date for each Reading Group, and all Meter Readings (highlighted in yellow). Constant values are not highlighted, but they can also be changed.
The Start and End Date are important if any of your components have formulas that reference them, such as, Elec1.Period.Days. If Elec1 is a Reading Group and the Start Date was Feb 17, 2010 and the End Date was Mar 16, 2010 then the value returned would be 27 Days.
Once all values are filled in, press Run. If everything is correct, calculated values should fill in on the right and the Final Total value should be the same as the total on the invoice. Hover over any flags in the Errors column to help diagnose syntax errors.