Once your Retirement Account is formed, evidently interest earned and credited to your Retirement Account (and to your claim to the CPF Lifelong Income Fund) is not relevant to the annual increase in the Enhanced Retirement Sum. So you can make additional top ups each January.
Let's suppose for example that you celebrated your 55th birthday in January, 2019. Also within January, you topped up your new Retirement Account to the Enhanced Retirement Sum, so your RA account balance is $264,000.
OK then, on December 31, 2019, your RA is credited with interest for the year (visible in early January). Here's the interest you get in the RA, if my math is right (I might be off slightly, but I'm very close):
1. All base bonus interest = 1% of $60,000 = $600
2. All age 55+ bonus interest = 1% of $30,000 for 11 months = $275
3. 4% interest for 11 months = $9,680
TOTAL interest credited to RA = $10,555
So your RA balance grows to $274,555.
On January 1 the new ERS takes effect: $271,500. But your RA is already bigger than that. So does that mean you cannot make another top up? No. Interest evidently isn't counted for these purposes, so you can make a top up in January of $7,500 (the difference between the 2020 ERS and the 2019 ERS). Do that and your RA balance grows to $282,055. Loop, repeat, if you wish, and your RA runs way, way ahead of the ERS.
All these top dollars earn 4% interest compounded annually, and then those dollars (with compounded interest) drive up your CPF LIFE monthly income stream no matter when you start payouts, and they also boost your residual at every age at death that there's still a residual. It's a great deal.
