I thought light was light?

You were mistaken.

Sunlight is really, really, *very* bright. 1.3KWH per square metre or there
abouts. Creates more global warming than, well, anything else really ;-)

Artificial lights produce an, erm, 'artificial' light. Very low colour
temperature, zero energy up towards (and beyond) the violet end of the
spectrum.

You can buy pro light meters (I have one cos I worked on interior lighting
systems many eons ago), or buy a ten quid market-stall solar battery
charger and measure the voltage/current.

Sunlight is very, very strong

Al.