Hi Smanfa,
There is some guidance out there on making the most of Economy 7 and even some on NPower's own website I think.
As I understand it, to make the most, you need to ensure that you do not use expensive heating or washing machines during Peak hours (7am - Midnight), but use them during the 12 - 7am economy 7 time.
If you are out of the house for most of this time, it shouldn't be too bad, especially if you have a bath or shower before work, and put the washing machine on at midnight (if possible)
If you are heating then this is something that you need to ensure is off during the day if you are not there. There are some tips to cutting down your bill.
Regarding calculating your bill, I think where you have dual fuel (gas and electric) it is probably about 20-30 for each per month.
If it is a middle floor or upper floor flat then the heating from lower floors should help you!
If you run the electric oven, cooker, washing machine, dishwasher, heating and hot water all by electric, then these can put your bills up.
If you put the charges on a post, someone will be able to give you an idea if it is expensive rates or not.
Typically the Low rate is about 1/2 the Normal rate. This is where they make their money from you, in the normal rate!
Sparky
PS It doesn't hurt to have a look at switching, but before you can switch you will still have to pay what you owe NPower.