The amount of sunlight that is available to grasslands depends on the latitude in which the grassland is located. Tropical savanna lies at lower latitudes than temperate prairies, so light from the sun strikes the ground there at a more-direct angle, delivering more energy per unit area than elsewhere.
In the tropics, each square meter of ground can receive as much as 6 kilowatt-hours of energy every day. Northern latitudes and areas where cloud cover is common, receive less. Above 40 degrees, which is approximately the latitude of Boston, the energy available per square meter drops to only 3.6 kilowatt-hours per day.