Every week, your grocery list will show you exactly how much of each ingredient is needed to follow the meal plans. After you go grocery shopping, move the stuff you bought from the grocery list to the pantry.
The ingredient amounts we ask you to buy reflect the quantity used in the plan, so if you want the Pantry to have a totally accurate record, you can adjust the amount in the pantry to match the package size you picked out (we're working on getting accurate package size data so this happen automatically).
Foods will be removed from the pantry the day after you're supposed to have eaten them (either automatically when the weekly generator runs, or if you hit the "update pantry" button on the grocery list page). If things don’t go according to plan, add or remove foods from the plans and then update the pantry to send back unused foods for use in future meals.
The weekly planner will give priority to using up your pantry foods to reduce waste. During testing, we've found the planner to be 80%-90% efficient with using up groceries, and the average American is about 70% efficient with grocery usage. Our grocery data still needs improvement, but if you're anything like me, the planner will do a significantly better job of using everything up.
Can the generator create meals using only the foods in my pantry?
Sortof - the weekly planner will place some emphasis on using up your pantry foods, but you'll likely still need to go grocery shopping to follow the plans completely. In the new version of the planner (coming September 2016), you can tell the generator to focus on efficient grocery usage which will give even more priority to pantry foods.
If you want the generator to use *only* your pantry foods, you can generate one meal at a time by clicking the "More" -> "List alternative meals" button next to each meal (web version only for now). There, you can choose to show which meals you can make with your pantry. You can also browse recipes that can be made using your pantry, in the Food Bank -> Pantry tab. The just drag+drop these recipes into your plan.
If you want total control of what the planner recommends, there's always the Recurring Foods feature. Find the foods + recipes you want in the Food Bank, then add tjem as "recurring foods" that recur "often", and then select the checkbox for those meals that says "Only use recurring foods for this meal". This will force the generator to use only the small subset of foods you already own.