Use the Annuity equalling a Future Amount Calculator to work out the value of savings that you need to build up in order to guarantee a certain lump sum annuity payment in the future. Firstly enter the future value that you wish to have, then enter the interest rate that you have been offered and enter the length of time for which you plan to have the savings account.
If the lump sum that you have entered is not sufficient, given the current interest rate and the length of the savings period, try playing around with different interest rates to see if this gives you the end result that you want. It might also be worth seeing if you are able to budget to save a larger amount - perhaps you can put away a little more each month with a relatively small number of cutbacks or lifestyle changes. Using the Annuity Equalling a future amount calculator is a good way to start thinking about your retirement savings.