Your an idiot if you think you can get away with this scam. The Electricity companies know how much credit you have in your meter, they also know when you top up the card. If you add credit to the card yourself they will work out how much you have paid legitimately, and realise that there is more credit in the meter than you have paid for. They will monitor the amounts going into the meter and how much you have paid onto the card and will prosecute you for fraud. They use a closed system and unless you want a jail sentence I suggest you forget the idea.
Regards
Liam