if i put ERS into RA, it can't earn me interest after 65. the actual yield will depend on how long I actually live..
I think you're misunderstanding how Retirement Account top ups work. Let's assume you choose the CPF LIFE Basic Plan. (I'm not necessarily recommending you do that, but let's assume.)
Here's how the CPF Board explains it:
CPF Board said:
Q. How does the CPF LIFE Basic Plan work?
A. CPF LIFE Basic Plan is a legacy plan carried over from the time CPF LIFE was first introduced in 2009. Unlike the CPF LIFE Standard Plan that gives higher and stable monthly payouts, the monthly payouts under the Basic Plan are lower and will get progressively lower when your combined CPF balances eventually fall below $60,000. This is because the extra interest is earned in your Retirement Account and paid as part of your monthy payouts, which will decline when balances fall as payouts are made.
Under the LIFE Basic Plan, about 10-20% of your Retirement Account (RA) savings will be deducted as CPF LIFE premium when you join CPF LIFE, which can be anytime from age 65 to age 70.
Your monthly payout will first be paid from your RA and is estimated to last until age 90. Thereafter, monthly payouts will be paid from your CPF LIFE premium. If your CPF LIFE premium is depleted, you will continue to receive monthly payouts for as long as you live from the interest that you and other members have accumulated.
OK, so let's assume the following:
1. You are celebrating your 55th birthday this year (2020).
2. Your Retirement Account is funded to the Full Retirement Sum.
3. You make a $90,500 cash top up within your birthday month, to boost your Retirement Account to the ERS. Thus the $90,500 is your principal in this analysis.
4. You then get 10 years of 4.0% interest on this top up. The accumulated interest boosts your top up to about $133,962.
5. You choose the CPF LIFE Basic Plan and start payouts at age 65. (You're allowed to start as late as age 70, but let's assume 65 here.)
6. The CPF Board subtracts a "worst case" maximum figure (20%) from your top up, or about $26,792 (probably too high, but let's assume 20%), and shifts it to the CPF Lifelong Income Fund, leaving $107,170 in your Retirement Account.
7. That $107,170's gradually dwindling residuals continue to earn 4.0% interest compounded annually. This amount supports a portion of your monthly CPF LIFE payout until about age 90.
8. You die in precisely the worst possible month from a net effective yield point of view, so you don't earn any interest on the 20% ($26,792). You/your nominee(s) only get principal return from that portion.
Got all that? OK, so if you make all those worst case assumptions, then it turns out you and your nominee(s) still earn about 3.2% net effective interest on your cash top up from the FRS to the ERS, not counting possible reinvestment. That's pretty attractive, of course.