Re: formula for battery amp hours / watts used etc
Getting the numbers absolutely precise is tricky. But you can get darn close. Good enough for government work.
Watt hours are Watts * hours. Using a Kill-A-Watt meter can get much more accurate results than multiplying the rated Watts by the estimated hours of use. That's the load side. For now let's assume 720 Watt hours daily is right.
What can you get out of a pair of T105's?
The most you want to discharge them would be 50%, so you have 112.5 Amp hours to work with.
In actuality, the rate of depletion is not linear - but it's hard to calculate that way so you can use the nominal Voltage as an approximate mean. That's 112.5 * 12 = 1350 Watt hours. Since that's nearly double the expected use, it would probably work.
Now that you're in the ball park you have to start examining things more closely. For instance, including the power draw of the inverter. That could be 20 Watts over 24 hours: another 480 Watt hours. All of a sudden you're a lot closer to that maximum potential.
Looking at the panel capacity it is woefully insufficient. Using the famous "Icarus Formula" for an approximate AC yield we get:
270 Watts * 4 hours / 2 = 540 Watt hours
It looks like you'd need one more 135 Watt panel and an inverter that doesn't suck up too much power to be good to go.
But check the loads first. The more accurate you can be on that the happier the outcome.
1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.
Ohm's Law: Amps = Volts / Ohms
Power Formula: Watts = Volts * Amps