How to Assess Light Brightness

Most solar lighting uses Led lights as the choice of lighting technology because:

Most operate in a range of voltage from 10 volts to 30 volts and are perfect for 12V or 24V solar applications.
Their power consumption relative to useful light output is better than halogen or CFL lights

To select the appropriate light for your application first determine:

The total area to be illuminated
How bright the light needs to be

Once you have calculated the total floor or surface area to be illuminated, then calculate how much light power is needed in terms of lux levels. General purpose lighting requires a minimum of 40 Lux and office work requires a minimum of 80 Lux.

Lumen and lux explained
Lumens are a measurement of the total light output from a light source, while lux is the light brightness measured at a certain point and at a certain distance from the light source.
Most LED lights will specify the total light output in lumen But when choosing a light we need to measure the Lux levels to determine the brightness of a work area illuminated by the light. The lux level depends on how focused the light is and the distance from the light source to the work area.

For example the two lights below both have total light outputs of 1000 lumen.
The left one has a wide beam spread of 120°, the other has has a narrow beam spread of 60° so its total light output is concentrated in a smaller area.

The light with the 120° beam spread, when shone onto a surface 1 meter from the light will illuminate an area of 9 square meters. A Lux meter will show a reading of 111 lux at the illuminated surface one meter from the the light.
The other light with the 60° beam spread will light up a smaller surface area of only 1 square meter when shone onto a surface 1 meter from the light, but it will be 9 times brighter, so the light meter will read about 1000 lux at a distance of one meter from the light source.

Therefore the illumination provided by a light depends on:

The total output of the light in lumen
The spread of the light beam
The distance from the light to the area that needs to be illuminated