Solar panels only produce the output in the advertising material if they are aimed directly at the sun in bright cloudless conditions.
Unless you can be sure of maintaining these conditions it would be wise to assume the solar panel can only produce about 25% of its advertised output.
I would have a suitable lead-acid battery (perhaps 6v) in the circuit which would be charged by the solar panel and could provide power in low light. You should check whether the solar panel has a diode to prevent the battery discharging into the panel.
...R