Legal Question in Wills and Trusts in California

No. But it may be wise.

There are two separate documents. A Will dictates what happens with your assets after your death. A power of attorney dictates who manages your assets during your lifetime when you are unable to do so yourself. A power of attorney has no control over your assets when you are deceased and a Will has no control over your assets when you are living. You do not need to have both. However, everyone over the age of 18 who as capacity should do both a power of attorney and an advance health care directive.

A power of attorney is automatically revoked on the grantor's death. So it is not for determining and running your estate after you die.