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