How many Watts of Solar is enough?

[Iceni John]

I went through this calculation when deciding on the house batteries for our 4108. The Trojan dealer told me that they recommend a charging rate of approximately 10% of battery capacity. We're going to have about 780 Ah between our four L16 batteries (set up as a 12v system), and he said that a 70-amp plug-in charger would be the proper size. In setting up solar, ideally we would be able to charge the batteries at about the same rate.

Going with a formula of Amps=Watts/Volts, a 1000-watt solar setup would provide about 83 amps output under optimal conditions. Since there is no way to always have optimal conditions, bigger would be better. Seems like there are a lot of 'kits' available that include six 160-watt panels (960 watts total), which would be about the smallest we'd consider given our battery setup.

The solar gurus on the NAWS forum recommend charging FLA batteries at between 5 and 13% of their 20-hour rate.   Anything less than 5% runs the risk of deficit charging that will eventually sulfate the plates, and anything over 13% will overheat the electrolyte and cause too much water loss.   If you will be charging at the upper end of the range it's prudent to have battery voltage and temperature sensors so the charge controller can precisely charge without risk of battery damage.   My setup will charge at about a 13% rate in theory, but with typical real-world conditions it will probably be less than that.   I still want to push as much power into the batteries as quickly and safely as possible  -  ideally I want my banks fully recharged by noon or early afternoon, then any PV power produced after that time can be used for discretionary loads such as water heating or running big power tools.   My intention is that even in winter I will still be able to fully charge batteries most days from solar, and only if it's raining or heavily overcast will I need my emergencies-only generator.   Well, that's the plan!

John