Originally Posted by GallenH
So for those of us who get confused continually..........
What does 400 w/h/day mean in terms of charging a battery or bank of batteries? Say, for example, you had a single 100AH battery and it was depleted to 70%. What does 400 w/h/d mean in terms of charging that battery back up to 100% Or if you had 200AH of battery? It seems these are vital bits of knowledge for deciding solar needs vs battery capacity.
There is a bit of a disconnect between the watt hours per day from the solar and the actual charge you replace in your battery because of the charge efficiency, which varies between battery types and brands plus it varies over the range of a single charge cycle.
Nearly all the energy from the charging gets stored in most batteries when the SOC is low (under 70%, maybe) so charge efficiency is nearly 100%. By the time you get to the top end of charging with over 95% full batteries, that charge efficiency drops a lot, probably under 50% of the energy gets stored. This is for AGM. Lithium is nearly 100% all the time.
Because of this, you can't really predict a consistent and really accurate amount of power you will get per day, especially if you are going to full or near full on the SOC.
The old standard 30 amp hours of recovery per day that has been around is probably a good place to start for planning, as it is on the conservative side for high sun conditions and maybe a bit low for low sun times of the year. We have seen as low as 5 amp hours per day in clouds, rain, shady times.
All of these are for a typical install 100 watt panel.