question

Help with upsizing charging capacity

I have two solar panels, one 140 watt, one 116 watt, mounted on Bimini, each with its own mppt 75/15 controller wired to battery. No connection to load output so for charging only. Daily usage around 110 ah. Panels will keep up with usage during day but will nor recover over night usage the next day. Considering adding two 120 watt panels to Davitts. No room for more. Will this be adequate in general? Series or parallel? What size controller? Any other recommendations?

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Take a look online for the average daily solar irradiance per month for the location that you are in.

Then use the effective 'peak irradiance hours' figure to determine the solar array peak power required to cover your estimated daily usage + losses during the worst months of the year.

Obviously more solar is always better/safer if it's feasible.

As a rough worked example, assuming you have a 12v system and use 110Ah/day = 1320Wh/day.

Now assuming efficiency and losses of say:

-90% for the inverter efficiency

-95% for the charge controller efficiency

-95% for wiring losses

-90% for battery charge efficiency

-90% for the solar panel related losses

=~66% total efficiency

So, 1320/0.66 = 2000Wh/day.

So if there is only 2 hours of effective peak irradiance per day during the worst case month of the year, than you would need a minimum solar array size of 2000Wh/2h = 1000W.

Anything less and you will probably be short during the worst case months of the year... but during the best winter months you will probably have ~3x the energy available, so you could get away with a far smaller solar array.

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