While both policies provide an additional layer of insurance above the underlying policies, and Excess policy will only pay if the underlying insurance covers the loss and exhausts policy limits. An Umbrella policy will also pay under the same circumstances; however, it also could “drop down” and cover claims that are not covered under the primary policy.
These are the basic differences but since most carriers do not use a standardized umbrella or excess liability coverage form, you or your agent will need to closely review the coverage form to determine the coverage intended for a specific circumstance.