The actual calculator is below. The image above is to show the input areas. There are 5 levels on the sliding scale.

You can only buy as much as the allocated units in the left column are, per level, per month. So say, you are buying R1220 worth of electricity and your slip indicates that you got 555.8 units.

You now start filling in the levels, until the units bought equals the amount you got. 555.8 in this case. So, it is 50 in the first level, 300 in the next level. That equals 350 units.  555.8 – 350 is 205.8. Enter 205.8 in the next level. Units bought now stand on 555.8.

To the right under Total (top right), you will see the amount for that is R1145.94.  But you paid R1220. The difference is what has been deducted as ‘dept’, or indicating you are in arrears and they deduct an amount. In the bottom right corner, you fill in the R1220 you actually paid. Now the fee row to the left (bottom) of that updates and shows you you paid 6.46% and an amount of R74.06 extra.
We call this a ‘fee’ as the reason for it is legion. Go find out at the municipality what the reason is.

The reason for this extra amount can be many things. Here is a few reasons.
1. You are on a business meter.
2.  Your landlord loaded your meter with higher tariffs. (Illegal)
3. Your account is in areas and they deducted an amount.