hours of usable sunlight per year

Google Project Sunroof can give a general idea of how many hours of good sunlight a given roof-mounted solar setup will receive.

maximum and minimum day length

check timeanddate.com's Sunrise and Sunset Calculator for day length on the following days:

summer solstice (June 21st) (longest day)
winter solstice (December 21st) (shortest day)

calculation

solar panel's rated power output	W	P
hours of usable sunlight per year	hr	S_yr
average hours of usable sunlight per day	5.475851494907458 hr	S_d = S_yr / 365.25
longest day length	hr	D_l
shortest day length	hr	D_s
average day length	12.25 hr	D_a = avg(D_l, D_s)
average solar effeciency guess-timate^[0]	45 %	E = S_d / D_a
longest day solar power output^[1]	2593 Whr	= P * E * D_l
shortest day solar power output^[1]	1788 Whr	= P * E * D_s

^[0] the average solar effeciency is over the course of the whole year. therefore, it's probably optimistic in the worst case and pessimistic in the best case
^[1] temperature should also be taken into consideration