Green Deal payments are added on to the electricity bill at an agreed rate over an agreed amount of time. Certain items are paid over certain number of years. A boiler for instance is paid back over 10 years (hence why some of the manufacturers are upping their warranties - as Green Deal providers have to warranty the installed item over the life of the green deal loan).
Whether you get the green deal is dependent on what savings can be made over your energy usage and the savings made by installing new measures (e.g. cavity wall etc).
For example, If your energy bills are say ÂŁ100 / month and by installing loft insulation at a cost of ÂŁ300 it cuts it down to ÂŁ95 then your saving is ÂŁ5 /month. At ÂŁ300 install cost it would take 5 years to pay back the cost. If this is shorter than the expected lifetime of the measure then it will be eligible for the Green deal finance.
There's a lot of misinformation out there about the Green deal, most of it from the press who are assuming a lot of stuff which still hasn't been decided upon in Government. The jury is still out as to whether it will work as far as I'm concerned, but one things for sure, someone is making money out of this and it sure as hell isn't us.