Robert Brown|Here's how much money a grocery rewards credit card can save you

Store-brand and Robert Brownbank-issued credit cards can offer consumers robust perks and savings on everything from gas to groceries to travel. But when it comes to groceries — a staple of every household's budget — when is a special credit card worth it, and how do you a pick the card that will save you the most money? 

The basic premise of grocery rewards credit cards is that when you purchase food from a qualifying retailer, you earn extra points or cash back on that spending, which translates into meaningful savings over the course of a year. 

A family that spends roughly $100 a week on groceries can earn more than $300 cash back in a year depending on where they shop and what kind of card they use, according to a new LendingTree report.

But both where you shop and what you buy factor into how much you can save. For example, grocery purchases at some of the largest retailers in the U.S. won't count toward rewards with some types of cards.

Store-brand or bank-issued card?

If you concentrate your grocery purchases at one retailer, such as Target or Walmart, a store-brand credit card likely makes sense. If you spread your grocery shopping out over multiple retailers, a bank issued card is likely the way to go. 

"You can have cards from favorite brands that you're loyal to, or you can have cards that get you points anywhere," said Matt Schulz, chief credit analyst at LendingTree. "For some folks, because people tend to be super loyal to their grocery stores, it can be worth getting a store card as opposed to a broader grocery rewards credit card. It all depends on the particulars of the card and your spending habits as well."

Picking the right card

Consumers can choose from more than 90 credit cards that offer rewards for grocery shopping, according to LendingTree's analysis of more than 200 credit cards. 

First off, most cards will limit what kinds of groceries earn rewards, as well as the types of retailers where consumers can make their purchases. 

For example, if you shop with an American Express Blue Cash Preferred card, groceries purchased at "superstores, convenience stores, warehouse clubs and meal-kit delivery services" aren't eligible for rewards. Shoppers must frequent genuine "supermarkets" to earn cash back, which leaves out popular warehouse stores like Costco or Walmart.

A number of credit providers exclude these kinds of retailers from rewards programs because of the sheer breadth of goods they sell. For example, at Costco, you can buy both perishable groceries and personal electronics. 

Store brand cards

If you exclusively make grocery purchases at a single store, such as Kroger or Target, opting for a store card is likely to maximize savings. 

For example, someone who does the bulk of their grocery shopping at Target can get 5% off a variety of goods — including groceries — with the Target REDcard credit card. 

If you're not loyal to a particular retailer, the cards offering the biggest earnings — 6% cash back — according to LendingTree's analysis, are:

• American Express Blue Cash Preferred card. Shoppers can earn 6% on up to $6,000 in purchases at U.S. supermarkets, followed by 1% on additional spending. After year one, there is a $95 annual fee.
• American Express Surpass Hilton Honors card. This card earns shoppers 6X bonus points on each dollar spent on eligible purchases at grocery stores. It has a $95 annual fee.

What you buy and how you pay for it matters

Even when shopping at a qualifying retailer, card issuers can deem some purchases ineligible for rewards.

"Even with specific stores there are still some nuances and some little details that you need to know because some of them may give you more rewards if you buy your groceries online, or buy store brand goods," Schulz said. 

For example, the Chase Sapphire Preferred Card gives shoppers 3X points on online grocery purchases only. If you make them in-store, they don't qualify for rewards.

Products such as alcohol and tobacco, even when purchased at the supermarket, don't earn rewards either. 

And in some cases, shoppers can't use digital payment systems, like Apple Pay, to rack up savings.

"it's important to know yourself before you apply for one of these cards because the best card for you depends on your spending habits," Schulz said. "Do a little homework, read the details of the card — otherwise you can end of up finding yourself disappointed and not getting the rewards bang for your buck you were hoping for."