Sun

Sun is the resource players need to build their plants (and zombies in I, Zombie). That makes it essential to create defenses against zombies. It falls from the sky (daytime areas only) and is produced by specific plants.

How to Use It
Each plant costs a certain amount of sun to plant, limiting the number of plants you can place on the lawn at a time. Your planting is also limited by the recharge or the "cool-down" rate of the seed packets. Also, upgrade plants cannot be planted unless the plant of which they are an upgrade of have already been planted, even if you have enough sun for them. The only plants that do not cost sun to plant are the Puff-shroom, its aquatic counterpart the Sea-shroom, and Imitater versions of them.

How to Obtain It
Sun is obtained from sun-producing plants, such as the Sunflower, Twin Sunflower and Sun-shroom, in addition to the player starting every ordinary level with 50 sun. That's important, as that's the sun cost of the primary sun-producing plant, the Sunflower. Sun also falls from the sky in the daytime areas day, pool, and roof. During the night and fog levels, sun-producers are the only way to obtain sun, so some of these plants must be planted to obtain more than the starting 50 sun. Sunflowers produce 25 sun, Twin Sunflowers produce two 25-value sun which is 50 sun together, and Sun-shrooms produce 15 sun until they grow, after which they produce 25 sun. Sun can also be obtained when it is daytime. Sun falls every ten seconds. Additionally, in Whack a Zombie, Zombiquarium, and Last Stand, zombies can drop sun.

Conveyor-belt Levels
The player cannot obtain any sun in these levels. Instead, they are supplied with plants that come up from a conveyor belt.

Normal Sun
Normal Sun gives the player 25 sun. It is the most common form of sun. It is given by all Sun-producing plants and it falls from the sky in most game modes. In Plants vs. Zombies 2: It's About Time, Normal Sun gives the player 50 sun.

Small Sun
Small Sun is given to you from Sun-shrooms until they grow more. They are worth 15 sun. It looks like a smaller version of regular sun. If you plant ten Sun-shrooms and collect all of the sun until they grow big, you will soon end up with a sun value that is a multiple of 25.

Big Sun
Big Sun falls from the sky in Sunny Day. It is worth 50 sun each. It looks like a larger version of normal sun.

Tiny Sun
Tiny Sun is worth 5 regular Sun, and is given when a plant is removed after a Shovel Refund bonus has been given, providing 25%-75% of the plant's original Sun cost. The maximum amount of sun refund possible without purchasing with real money is 50%, and an additional 25% is available for USD $3.99. Free plants will never drop tiny sun, as they do not cost any sun to plant, and plants before Last Stand waves are started will instead reward 100% of the original cost, regardless of already existent sun refunds.

Sun Bombs
Sun Bombs fall from the sky in Sun Bombs Brain Busters. They are purple and explode if you touch it. Once it lands on the ground, it will become normal sun and no longer explode when you touch it.

Zombies
In Survival: Endless, the sun cost of each upgrade plant will increase by 50 for every one of those planted. For example, the player's first Cattail will cost 225 Sun, but their second Cattail will cost 275 Sun.

Plants vs. Zombies Adventures
Note: VIPs do not cost any sun.

Trivia

 * The maximum amount of sun players can get is 9990, or if they cheat, they can have more sun. Any sun collected after this figure is achieved will not be added; however, new plants consume sun and decrease the available amount below 9990 (after which more sun can be gained again until the limit is eventually reached once more, etc.).
 * This, along with the 15 sun increments from small Sun-shroom, is the only way to get sun that is not in multiples of 25.
 * The mini-game Seeing Stars is another good one for getting this. If the player uses Garlic and Spikeweeds, along with a couple of Threepeaters, they can have all of the incoming zombies dying automatically with minimal effort, only needing to replenish a Garlic in the middle row. The rest of the empty squares can be filled with Sunflowers and they can collect sun without worrying about being overrun.
 * Survival: Endless levels and Co-op Endless are the only levels where the cost of plants rises (the cost of Upgrade Plants rising for every one of each type currently planted on the lawn).
 * In Versus Mode, the zombie's currency is brains, it works the same as sun.
 * But in I, Zombie, players can collect sun by eating Sunflowers. That's peculiar since zombies need brains, not Sun like plants. Maybe it has to do that if only one currency is used it will be sun.
 * If the player digs up a plant, they don't get back the sun you used to plant it.
 * That's why they have to be careful in Last Stand, due to how you only have limited sun and cannot gain more.
 * In Last Stand puzzles and Last Stand in Plants vs. Zombies 2: It's About Time, the player can get back the sun cost of a plant (unless it is damaged).
 * Sun will not fall from the sky during the Night or Fog levels. That is why it is risky and practically suicide (exception is on 2-1) to not use Sunflowers or Sun-shrooms.
 * There is Big Sun, which produce 50 Sun, this kind of sun will only appear falling from the sky only on Sunny Day, a Cancelled Mini-game.
 * The sun costs of non-free plants are multiples of 25.
 * When the Sunflower is producing or the sun is falling, the sun is like as the Big Sun in Plants vs. Zombies in Plants vs. Zombies 2: It's About Time in Version 1.7.