How PoS works

There is probably a lot of errors, oversight... Thank you to clarify, I will update accordingly.

I will use Mintcoin as an example because it is the coin I know the most.

How PoS works
First: yearly-vs-NVCS
There are two kinds of proof-of-stake.
- Static rate (like Peercoin and Mintcoin). A diminishing return may be hardcoded (like in Mintcoin) or not (like in Philosopher's Stone)
- Dynamically-adjusting rate, base on the amount of staked coins - the more stakers, the less interest. A lot of Novacoin descendants use this method of diminishing returns. Most high-stake coins use this method (Philosopher's Stone and Mintcoin being notable exceptions). This is called NVCS (NoVaCoin Stake)

Second: Weight
The actual time required to get reward (seems) to be a function not only of the hardcoded value (from one month for TEKcoin, to eight hours for Blackcoin) but also of the number of coins - yeah, rich just get richer, at least for HoboNickel, I did not check for other coins). I don't know why this uselessly complex calculation.

Third: Compounded interest
Leaving a wallet open 24/7 costs more electricity (and CPU power, more on this later) but also make you benefit more from compounded interest (interests adding to capital so that next time capital to base interest upon will be bigger - in a bank, you compound your interest once an year, in crypto, at the end of each period, so every twenty days for Mintcoin). If you received all of your coins in just one big transaction, you could just stick to open wallet at every stake period and this is it. You receive your coins, and they count for the next stake. But if you received them in several parts (99% of users) there are end-of-period occuring a lot and so opportunity for compounding interests occuring a lot (in other words, "it's every day twenty days"). By just opening you wallet every 20 days, you are actually losing compounded interest. Notice that with compounded interest, you should achieve more than 20% interest on Mintcoin (at the cost of electricity, though and less CPU-mining power, though). It is not worth it if you don't have a lot of coin (except if you don't pay for your electricity).
Beware though: the multiplication of the end-of-period generates "dust", something similar to fragmentation on a disk. This won't increase the size of your wallet, but it will require more CPU power. You may end up with 100% CPU usage. Raspberry or VPS are ways to mitigate this. Another way is to resort to "spring cleaning": every once in a while, you move all your coin to another wallet; you sacrifice every coin age to get back to a normal level of CPU usage. Heartbreaking.

Hopes this helps, thank your for correcting.

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