Petco Park is a nice field.

http://sandiego.padres.mlb.com/sd/ticketing/seating_pricing.jsp

As you can see from the map, which is oriented correctly, the visitor's team is located on the western side of the stadium, so depending on the time of day the game (Sunday suggests a afternoon game) then your opportunity for shade is on the west side. The 200 seats are above the field on the first elevated level, so you should be in the shade most of the game.