Assuming it's a 34-50 compact and you're sticking with the same size tires and cranks on your single speed as your current bike: using your current 50-20 gear will be 67.5 g-", which is between a couple common single speed setups on the lower/easier side: 44-18 (66 g-") and 44-17 (69.9 g-"). Sticking in that gear for awhile should give you a good idea whether one of those gearings will work well for you. Try going up or down one cog if 50-20 seems too easy or hard after a full commute of sticking with it.