Determining battery size

[mav15]

Just a quick question..
How do i determine what kind of batteries to get?

Let's say i want to power up a light bulb (20watts) and a radio (15watts).
so the total wattage for both would be 20+15 = 35watts
and lets say i'm going to run them both for 3hours each day.
35watts x 3hours = 105watt-hrs/day

Now, i heard that you need the Amp-hrs/day
so i divide the 105watt-hrs/120Volts = 0.875Amp-Hrs/day

in the area i live i have hypothetically 5 days of sunshine-less days and i want my batteries to only go as low as 70%.

so i use the following formula:

AmpHours/Day X # Cloudy Days / Discharge level = Amp Hour rating of Batteries

0.875 x 5 / 0.70 = 6.25Amp Hours of battery

So this would be the rating i would need to look for in a battery...
Is this correct?

[Mike90250]

Is this correct?

Almost.

You have arrived at AH/day for 120V. Now to convert that to battery voltage (12V) just multiply. or go back to watts, and redo
> 105watt-hrs/120Volts = 0.875Amp-Hrs/day

105Wh / 12 Volts = 8.75 Ah day

8.75 x 7 = 61.25Ah consumed in 7 days
times inverter losses of 20% = 73.5 Ah consumed

You never want to let any deep cycle battery get below 50% State of Charge, so a 147Ah battery would be the smallest.

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Problem: You don't want a battery below 70% State of Charge for more than a day. Below 70%, sulphation begins, the longer it sits , and the lower it gets, the more acid is converted to hard sulfur crystals that both block the plate area, and deplete the acid. No good way (or hard way) to recover from this, its a permanent loss of capacity.

There are "De-Sulphater gadgets" on the market, but little evidence they do much at all, but make sales folks rich.

[mav15]

@Mike90250 - Thanks! got it.