The Finch inline MC4 Fuse connector is a one-way diode, also known as a bypass diode, prevents current from flowing in the wrong direction, which can damage solar panels and other electronic equipment: 

Note: To work out what size fuse is required please follow the following example:  

200-Watt Solar Panel – is calculated as per example: 200W ÷ 12V = 16.7A therefore a 20amp fuse would be required to safeguard against potential spikes.