In addition to Chris' answer I'm going to give you a (IMO) clearer and more generic way of doing this:
If you need to convert units, always reduce your units to more fundamental units of the International System of Units that your two terms have in common.
See International System of Units
A Watt is a Joule per second, so 1 W/m² = 1 J/(m²s)
(I'm using brackets for clarity, but they are not required)
Likewise 1 kWh/m²/day reduces step-by-step
... and if you insist on using Watts this is 41.666 W/m²
Strictly speaking I could have left out the step converting 1 W to 1 J/s, but we are so used to express energy in Joules* that I reduce to that.
And as Chris says, whether to use use min/max/avg depends on what you want to calculate, not on your units.
* If you are not, you should. Distinguishing the absolute amount of energy from the amount per second makes things much clearer.