OK, it's complicated, but basically we are getting our electricity free from now on.
The 15k was the cost to install solar panels (photovoltaic) on the roof. These are expected to generate much less electricity than we will use, but the electricity company pays us four times as much for what we generate than they charge us for what we take from the grid, even when we use it ourselves (and of course we are taking less from the grid), and they will continue to pay for the next 25 years (we have a contract). We pay our normal electricity bill as usual, but email or phone in a meter reading from the generation meter once a quarter and get paid for what we generated directly into our bank account. The estimated yearly output of the PV panels (we haven't had them a year yet) will pay us back for all the money spent on electricity bills.
Read that again- it's hard to get ones head around it. It seems to good to be true, but this is a government devised and regulated scheme to promote the use of domestic scale renewable energy. They have distorted the market in your favour, if you have the resources to take advantage.
You need a big enough roof, facing mostly South, and the money for the installation. We got ours from Tesco Direct (double clubcard points!). If you have £15,000 languishing in a bank account earning almost no interest, then it makes sense to put it to work. If you would have to borrow the money, then probably not. Some people spend this on a new car, which will never pay you back anything.http://www.tescohomeefficiency.com/renewable-energy/