Ive been with NPOWER for six years, and I can say they took a lot of money from us. They charged me 125pounds a month which we did not used much of our heater and electricity because we are working 4 days to 5 days a week from 8am to 8pm. When I moved in other house i switch to AVRO which is more reliable and cheap.
My advise is dont get npower they will rip you off. Read the reviews is very bad. Sad that I only know when i search them in google about their reviews. Shame company