We have POA. What are the requirements to enable it's use?

Springing or Standing in my state. Springing requires the opinion of two doc's to state the person is incapacitated. Standing is effective always does not require opinions of two doc's. Check your state's laws.

For a springing POA, you need a doctor's letter. My husband's doctor wrote one for him last year.

It depends on if it is durable or springing. A durable POA takes effect when it is signed. A springing POA takes effect only when the person becomes incapacitated. Many banks don't like to deal with a POA, so you need to ask them their requirements so you can conduct business. Government agencies, such as SS, do not recognize POAs.