Penalised on Prepaid Meter

I would like to enquire why I am penalised when using Prepaid electricity I buy R100 and only get R82 electricity. They say at the office that we are only allowed 500 Watts a month and we go over that we are penalised. The electricity has gone up and we still have that 500 watt allowance. They say at the office to get a higher watt we will need to purchase a new box. <br> <br> No one can explain this allowance and they say it is just charged. I would like to know why am I penalised if I am paying and why do I have to buy another box which the office at City Power cannot answer and this information is not on their website. Is this suppose to be paid.