Calculating charges is something best done with a spreadsheet program like Microsoft Excel. Label the columns Date, Reading, Used, Rate and Amount. Enter the date of the reading and the reading in KWH. In every row after the first one, enter a formula to subtract the reading from the month before from the amount you just entered. Enter the rate per KWH in the Rate column and enter a formula in the Amount column that multiplies the value in Used by the value in Rate. The amount will be the amount to charge the tenant for the current period.