The individual savings bonds (I and EE) offered by the US treasury can be a good (conservative) subset of an investment portfolio, but some basic calculation can help decide whether and how to invest.
The current (fixed) rate of the EE-bonds is 0.1 percent, which, if compounds for 20 years, will increase the original face value by only 2 percent!
The treasury will guarantee to double the face value at the 20 year anniversary. This translates to about 3.53 percent increase per annum, which is not too bad given the current low rate environment. But we have to wait for 20 years and trust the treasury not to mess with the rates or the policies. A lot of things can happen in just 3 years, not to mention 20 (e.g. inflation).
I-bonds offer higher rates (around 2.5 percent recently) which are adjusted for inflation (unlike the EE bond rates which remain fixed). Their values are not guaranteed to at least double after 20 years, but they have a much lower opportunity cost than EE bonds without betting on interest rates or treasury policies.
