Panel to battery ratio

[Swampwood]

I keep thinking I'm going to need 2 80a/hr batteries for my 70w panel..Overkill or having that much reserve good? Actual usage guess(not in use yet) is 26watts of lights/1hr and, 100watt fan/4 hrs a day...Equal 10.5a/hr? for 4 hrs.

[Sunking]

You question is hard to answer and depends mainly on location vs daily watt hour usage, and type of charge controller used. Forget Amp Hours and think Watt Hours.

Here is a classic example that demonstrates my point. Let's take two locations; Seattle WS and Tuscon AZ and both require 1 Kwh of energy each day.

Battery capacity is the same for both of 5 times daily capacity so 5 Kwh. Does not matter what battery voltage we operate at this point as it takes the exact same amount of battery capacity regardless of the voltage. To find the Amp Hours take Watt Hours / Voltage.

The real difference is the size of the solar panel wattage and charge controller amperage needed. In Tuscon AZ in winter receives 5.6 Sun Hours in December and January. So if using a MPPT charge controller with a system efficiency of 66% the solar panel wattage required = 1000 wh /.66 / 5.6 h = 270 watts. Assuming a 12 volt battery system requires a charge controller amperage rating of 270 watts / 12 volts = 22.5 amps.

Now for Doomy Gloomy Seattle only receives 1.2 Sun Hours in December and January. So using a MPPT charge controller requires 1000 wh / .66 / 1.2 = 1262 watts round up to 1300 watts. For this system you would have to use a 24 volt battery system because 1300 watts is more than most MPPT CC'c can handle at 12 volts so you would need 1300 / 24 volts = 54 amps.