Below 10%. The most efficient plants are food crops we've genetically modified. I believe corn absorbs around 10%.
However, plants also reflect a lot of energy, unlike black/asphalt roofs that most people in the US install.
When you say efficient it makes me suspicious. Just because a plant doesn't absorb the energy for conversion doesn't mean it doesn't absorb it as heat. Remember, they're quite connected to the ground so they could be good heat dissipation (I have no idea, just speculating.)
One thing trees/plant help with (instead of concrete/asphalt in cities) is with the added impact of evapotranspiration (besides providing shade).
Plants also evaporate water from the extra light energy, which further cools things.