You will earn 1.5% fixed interest per year on the automatic savings you accumulate with Plum over the first 6 months of being a user. Automatic savings are those initiated by Plum's system and they appear as 'Automatic deposit' in your transaction list. Interest will be paid for the first 18 months of being a Plum user.
Interest is calculated daily on the minimum of your total automatic savings and the balance in your Plum account at the end of the day, and paid quarterly (every 3 months). For example, let's assume that Plum has automatically saved £600 for you. If your balance is £1500 (because you've deposited £900 manually), interest will be calculated on £600. If your balance is £400 (because you've withdrawn £200), interest will be calculated on £400.
You sign up with Plum on the 1st of March. Plum automatically saves £100 by the end of March (£20 every 6th day) and continues doing so for April, May, June, July and August. By the end of August you have £600 in automated savings and you'll have earned approx. £1.99 in interest, £0.58 paid on the 1st of June and £1.41 paid on the 1st of September. From then on, if your savings do not drop below £600 (which is the total automated savings for the first 6 months), you'll earn interest of £2.25 every 3 months for another 12 months (in total 18 months). At the end of the 18 months, you will have earned a total of £10.99.