Compatibility summary
You will need a travel adapter in United States; plug types used there differ from Spain. Voltage differs by more than 20% — a voltage converter may be needed for some appliances.
Plug comparison
Spain plug types: C F
United States plug types: A B
There is no common plug type between Spain (C F) and United States (A B). You need a travel adapter.
Voltage comparison
Spain: 230V, 50Hz
United States: 120V, 60Hz
Voltage converter recommended (Spain 230V vs United States 120V).