Balooned account bill

I pay my grandparents electricity bill on a monthly base and it has always been upto date but to my surprise we just received a bill amounting to R13 700+ and when I went to the municipal offices to enquire I was told that the amount is accurate as we were billed on assumptions for the past three(3) months. I mean who's fault is it and where do they expect pensioners to get such money??? I tried to explain to the people at the customer care that the house is occupied by to elderly people who are on pension