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Please help
we need to define the block tariffs so when we calculate the account that the system can identify the difference between start and end reading and then break the cost up to 0-500 kwh , 501- 1000 kwh 1001 to 2000 kwh , 2001 to 3000 kwh and 3001 and above

This is for consumption. Therefore, under Tariff Line item, specify Consumption.
Each block would be a separate tariff line item.
Select the checkbox to enable "Enable Banding on Consumption:"
Two fields will then become visible: 'Band Lower Limit' and 'Band Upper Limit'. For the bottom, the lower limit would be 0, and the upper limit would be 500. Then for the second one, the lower limit would be 500 (not 501); and the upper limit would be 1000, etc. The last item would have a lower limit of 3000 and an upper limit of something very big, e.g. 9999999999.