I don't know for sure, but you probably need a battery maintainer that steps the power down to the 12 volt battery from the 220 volt current you have in Hong Kong.
As I am in the U.S., we have battery maintainers that plug into 110/120 volt outlets and step that power down to 12 volts so that the 12 volt battery in the car is maintained.
Do you have a Porsche Club in your area? If so, contact them or a instead contact a dealer near you to get a battery maintainer. Maybe you can use a transformer that converts 220 volts into 110/120 volts that American appliances use? However, if I were you, I'd get a proper battery maintainer that can run when plugged into the 220 volt outlets that are standard overseas. In any case, find out from them or email Porsche Club of America (PCA) if you are a member and they will be glad to send out an email answer to you and all members of PCA.
I understand you can let the car go without running it for up to three weeks before the battery will not start the car. Some of the intermediate side effects of letting your car sit for that long (and believe me, I know, as I have what is known as a "garage queen," only under 12,000 miles (or under 20,000 km) after two years years of ownership) are a clock that runs more slowly and probably an alarm system that doesn't run properly.
Get the proper battery maintainer as soon as possible. It's worth it.
By the way, using the battery maintainer will ultimately cut down on the power of your battery over time so have your battery tested once a year so you don't get a "dead" battery.
Jim