NPower has a split price for gas consumption, now at 6.504 p per kWh for the first 4572Kwh/year and 2.098 for the following. In their own website is a table to be found how many of these units are to be charged per month. http://www.npower.com/web/At_home/customer_service/yourquestionsanswered/faq_understanding_my_prices

I received a bill in the beginning of November charging me only on the higher charge and after complaining and submitting a new meter reading I received an amended bill with exactly the same mistake. I had to complain again and only now as I am leaving for a better company received my final bill, which may or may not have corrected the original mistake, as they now charged me a lower amount of units on the higher charge than stated in their price table. Their billing is still everything but transparent and I can only warn every costumer to check their bills properly. I do not belive that this is a singular "mistake" and Npower will make thousands of ponds by overcharging every individual customer just a few pounds.