There are a few sources available for you to finance your project.
If you are able to finance your project by cashing in some savings / investments you are likely to get a much better return on investment; and it is guaranteed by the Government. We would be able to calculate this for you but estimates are in the region of 8 to 10%.
If you need to borrow money the cheapest way is to increase your mortgage. We will not be able to help you with that but will be able to provide all the information they will need. If your mortgage has more than 10 years to run your monthly income is likely to be more than your monthly payments.
A personal loan is another option and you might want to look at Zopa. This is a social lending site that cuts out the banks. To learn more click here.
Rule of thumb borrowing money for Solar PV at up to 6% interest per year still makes the project viable – between 6% and 10% it is a dubious proposition and if you have to borrow at over 10% the project is not usually economical.