If you need to refund orders and you do not have enough money in your Stripe account, you need to add funds from your bank account. This used to be complicated, using wire transfers, by recently Stripe has added the functionality to make a simple bank transfer.
Click on Balances in the top menu:
Then, select 'Top-ups' from the left hand-side menu:
On the top right-hand corner of the Top-ups page, click on the [+New] button:
Enter the amount you want to transfer and click [Next}:
You will be presented with the information you need to use to make your bank transfer using the FPS or BACS option. (FPS stands for 'Faster Payment Service'; it is, in fact, the standard bank transfer process which takes up to 2 hours; BACS takes up to 5 days).
Make a note of all the information provided (account number and sort code): these are your own Stripe account details and will ensure that eth money you send from your bank account will arrive directly in your Stripe account (the details shown above are for illustration only). When you have made a note of these details click [Done].
IMPORTANT: Next, go to your bank account and make a bank transfer using the details you have been given (account number and sort code). The money should be available for you to process refunds within the specified time (FPS within 2 hours or BACS within 5 days).