to handle pressure changes in the system caused by temperature variations throughout the year.

Solar fluid isn't like standing water: it experiences an enormous range of temperatures under normal circumstances, so it's extremely important to get the right size expansion tank. If the pressure gets too high, you'll blow the relief valve or perhaps even a joint; if it gets too low, you'll risk sucking air into the system and ruining its smooth operation.

Expansion tank manufacturers have their own guidelines for tank sizing, depending on the amount of fluid in the system and the expected temperature range for your location. Make sure you consult their recommendations and follow them carefully.