How can these GPS/GPRS bike tracking locks be so power saving?
There are a couple of things you can do to cut down on the transmission rate.


Only transmit when you have moved more than a certain distance... eg, 50ft or so.
Cut down on the transmission size.. 1 second seems like a long time for an I am here packet. 40mS seems more in the ball park.
Use adaptive transmission based on available power. i.e reduce transit rate when dark and have not moved in a while.


From a sanity check, something also seems off with your math.

If it's 12V and max current is when it transmits at about 0.5A, that's only 6W if you are transmitting 24-7. At one second every five, that's just over a watt. From that alone, a 225W solar panel would be serious overkill even if you do want to charge a battery so it works for a few days in a garage.