I know this has probably been asked already...

Yes, hours of sun also matter. For example, a fixed array in Las Vegas will see "average noon time hours of sun per day" of:
http://solarelectricityhandbook.com/solar-irradiance.html

Las Vegas
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 54° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.63	5.11	6.32	6.70	6.74	6.63
Jul	Aug	Sep	Oct	Nov	Dec
6.27	6.06	6.15	5.85	4.99	4.58

In general, we use 5% as the minimum rate of charge for a flooded cell lead acid battery system (generally emergency/backup, weekend, seasonal usage). And 10% to 13% or so for a full time off grid system (more solar panels makes managing your battery bank state of charge easier). If you have a 10%+ rate of charge solar array, for many folks where there is lots of sun, you will get plenty of power (if you sized your battery bank for your loads).

So, some quick calculations. A 1,000 Watt*Hour per day system is a good emergency backup system (will run most small stuff around the house--Use 3,300 WH per day system if you want to run a refrigerator):

• 1,000 WH per day * 1/0.85 AC inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 maximum discharge = 392 AH @ 12 volt battery bank

Next, a 5% to 13% array would be:

• 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.05 rate of charge = 369 Watt array minimum
  
• 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.10 rate of charge = 738 Watt array nominal
  
• 392 AH * 14.5 volts charging * 1/0.77 panel+controller dratings * 0.13 rate of charge = 960 Watt array "cost effective" maximum

Then there is how much power your array can supply based on hours of sun... Sizing the battery bank (above) assumes (more or less) you charge during the day and discharge at night. You can, of course, use power during the day+charge and use battery at night. Tossing the bottom three months (assuming you may use a backup genset during bad weather), for February with (for example) a 10% array, you would get:

• 738 Watt array * 0.52 off grid AC system eff * 5.11 Hours of Sun = 1,961 Watt*Hours per day
  

So--In Las Vegas, you can get quite a bit of power for use during the daytime too. It may be large enough to run an efficient Energy Star refrigerator (with a 1,200 to 1,500 Watt minimum AC inverter). Otherwise, if you are powering smaller loads, a 300 Watt AC inverter would be very nice.

Lots of choices to make here... You can get a Kill-a-Watt type meter to measure your AC loads and figure out how large of backup solar system you would like.

A 1 kWH (1,000 WH) system per day is a great backup system (run a genset for larger loads). A 3.3 kWH per day will run a small home/cabin year round just from solar (and some backup genset during winter) with a Refrigerator, washing machine, well pump, lights, TV, computer, etc....

But solar power systems when strickly used as backup power (for less than 2 weeks per year) are a bit on the "expensive side". Using a Honda eu2000i genset + 10-20 gallons of gasoline (or propane conversion) can be more cost effective (use fuel stabilizer and change out the gasoline once or twice per year).

-Bill

I am not sure--You may have missed my post just above yours (I posted a little before you).

In general, running a 120 VAC energy star refrigerator/freezer will push you from a "small" to medium size solar power system (high surge current requires a larger battery bank and AC inverter).

If you have stable utility power--Off Grid Solar Power is not cheap. Just run utility power and, perhaps, a smaller backup genset + fuel.

If you have terribly unreliable power (lose power for many weeks/months at a time, or have afternoon brown outs--Then solar+battery bank+AC inverter can make sense.

And in Nevada, last I heard, they have changed the billing plans for home based GT Solar power (cheapest/most efficient of solar power, can turn your meter backwards), the state PUC + utilities have removed the subsidies for solar power and pretty much have shut down the GT Solar industry in the state.

You are doing the right thing--Researching first before you start buying hardware.

No easy answers here... Conservation (if you have an older home/appliances) can help save money (insulation, new Energy Star rated appliances, new A/C or Heat Pump system, etc.) and be a better investment than solar electric power.

-Bill