How can daily brightness be quantified or measured? It occurred to me that there might very will be a metric out there that measures and quantifies "daily brightness". Meaning, how bright any given day is, taking weather conditions and cloud coverage into account.

A general brightness metric is lumens, but I can't find anything on this subject with respect to meteorology.

And, if such a metric does exist, what's its range, 0 - 100? What weather/ambient conditions are represented at this range's extrema?
It can be measured in units of energy per surface area, for example, kWh/m2.  The official SI name would be radiant exposure but I haven't seen that phrase used in a climate or weather context.

For example, here is a map by NREL of average radiation per day in the US for July:


(Source: National Renewable Energy Laboratory)

And here is a map for Sweden for the same quantity per year (hence the much larger numbers):


(Source: SMHI)