Guidelines for sizing solar-powered electronics?
Never is a long time ! :-)

Obtain sunshine hours per typical day on a typical month at your location from the superb gaisma insolation et al site here. As you are in NZ Ive chosen the Wellington page.

Th 4th graph down says the monthly mean insolation in kWh/m²/day  Jan to Dec is as below. This is the equivalent full sunshine hours. Lowest is 1.4 hours/day in 6th entry = June

5.83  
5.06  
3.97  
2.78  
1.85  
1.40  
1.63  
2.32  
3.32  
4.13  
5.26  
5.60  


1.4 hours per day means over a 24 hour period you will get a mean solar Wattage per square metre of


Watts = 1000 x 1.4/24 = 58 Watts per square meter of isolation 
= 5.8% of full sun panel power 


Solar panel efficiency = Zp          typically ~ 13%.
Bird poop degradation factor = Kbp = depends on cleaning etc.
 Say Kbp= 0.75.  

Average Wellinton June day = 1.4 sunshine hour/day but some days Watts ~= 0
 So D = days you want to run with NO solar input.

I'm going to stop naming K factors and lump them all into "Kother".
 Kother is a degradation factor comprised of all the factors you can think of  MULTIPLIED together.

Battery charge to discharge energy return - say 80% - depends on chemistry and several  other factors. 

Temperature - about 90%of rated at 25C as panel gets hot.

Panel matching to battery - addressed by eg MPPT controllers - Panel will be 18V oc for a 12V battery and energy loss without an MPPT controller will vary - say 80%.

SO

A Panel rated at say 100 Watts will power equipment run 24/7 in June in Wellington with  a Wattage of 


100 x 1.4 hrs/24 hrs x 0.75 bp x Kother (0.8 battery x 0.9 temperature x  ...)  / D days

= 100 x 0.058 x 0.75 x 0.8 x 0.9 x ... =~ 3 Watts / D


Below, Zp is allowed for by using the panel rated power. 

ie a 100 Watt panel will run 3 Watts of equipment in Wellington in June IF days are typical. If you get super black dark and stormy and want to last say 3 days with about no sun you get 3/3 days =1 Watt 24/7 per 100 W of panel with 3 days holdup.

Pretty stunning !

Battery sizing = 1 Watt Hour per Watt of load x 24 hrs/day x D days holdup / Kbattery to load.

Say a 1 Watt load and 3 days holdup = 1 x 24 x 3 / 0.75 say = ~= 100 Watt hours.

E&amp;OE !!!!!!!!!!! - the battery size and panel size don't seem quite correct. I may have dropped  a figure somewhere there BUT the principle should be obvious and straight forwards A major aspect is getting the degradation factors correct.

Ask questions if interested.