Sun

Sun is a type of resource in the Plants vs. Zombies series that the player needs to build up his or her defenses (and zombies in I, Zombie). It is essential to create defenses against zombies. It falls from the sky (daytime areas only) and is produced by sun-producing plants. It is the Plants' equivalent to brains.

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; and in Plants vs. Zombies 2: It's About Time also includes Iceberg Lettuce and Grave Buster.

How to obtain it
Sun is obtained from sun-producing plants, such as the Sunflower, Twin Sunflower and Sun-shroom and falls from the sky every ten seconds when it is day-time. The player starts every ordinary level with 50 sun, which is enough for one Sunflower. During the Night, Fog and Dark Ages 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 a normal sun, Twin Sunflowers produce two normal sun, and Sun-shrooms produce small sun until they grow, after which they produce normal sun, growing once more to produce big sun in Plants vs. Zombies 2: It's About Time. 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, which do not cost any sun.

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

Small sun
Small sun is given to you from Sun-shrooms until they grow and are worth 15 sun each (25 in Plants vs. Zombies 2: It's About Time).

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

Tiny sun
Tiny sun is worth five 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.

Large sun
Large sun is obtained from full grown Sun-shrooms in Plants vs. Zombies 2: It's About Time. It is worth 75 sun.

Sun bombs
Sun bombs fall from the sky in Sun Bombs. They are purple until they touch the ground, after then it reverts to normal sun. If the sun bomb is tapped while it is still purple, any plants or zombies nearby will take damage and the player will not be given any sun.

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 the player can get is 9990 in Plants vs. Zombies and 9900 in Plants vs. Zombies 2: It's About Time.
 * The 15 sun increments from a small Sun-shroom, the Sun Bean, and shovel refunds are the only ways to get sun that is not in multiples of 25.
 * Seeing Stars is another good way to max out the player's sun. 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.
 * However, in the Far Future trailer of Plants vs. Zombies 2: It's About Time, Tile Turnip's cost was 64000 sun. It is impossible for the player to do so. The maximum sun the player can collect is 9900, but Tile Turnip needs to be planted with 16000 sun before the sun cost doubles twice to 64000.
 * In Plants vs. Zombies, 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 which works the same as sun. But in I, Zombie, players can collect sun by eating Sunflowers. That is peculiar since zombies need brains, not sun like plants. If the player digs up a plant, he or she does not get back the sun used to plant it.
 * 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 Night, Fog or Dark Ages levels.
 * The sun costs of non-free plants are multiples of 25.