Excessive panels???

Re: Excessive panels???

Using 29 volts (or 28.8, etc.) is just reflecting some of the losses in the system...

Lead Acid Batteries are nearly 100% efficient when looking at Amp*Hours... Use 100 Amp*Hours for your loads, it will take ~100 Amp*Hour to recharge your battery bank.

However, when looking at Power or Watt*Hours, batteries are less efficient. Remember the equation for power is:

• Power = Volts * Amps
• Energy (in Watt*Hours) = Power * Hours = Volts * Amps * Hours

So, running your loads, your battery bank will be around 12.5 volts to 11.5 volts. Recharging will be around 13.2 to 14.5 (or even higher) volts. So, using the ~29 volts is just one more place to account for power losses in the system design.

• Efficiency = Volt out / Volt in = 12.0 volts / 14.5 volts = 0.83 = 83% cycling efficiency

Batteries tend to be around 80-98% efficient (old lead acid to new AGM). Also, it depends on how you use your bank... If you cycle deeper (say 50% to 90% state of charge), battery banks tend to be more efficient. If you cycle between 85% and 100%, they tend to be less efficient. And for sake of being conservative, I use 80% efficiency for flooded cell and 90% efficient for AGM.

-Bill

Re: Excessive panels???

The "real" charging model is more complex... But drawing 13% rate of charge at 14.5 is real and very doable if you have enough solar panels or large enough battery charger (this is the transition point between Bulk and Absorb charging stages--going from current limit to voltage limit on the charge controller).

Normally, one would expect to hit 14.5 / 29 volts at 80-90% state of charge (depending on lots of variables, size of battery bank, rate of charge, temperature of bank, etc.).

So, if you discharge to 50% or less--you will be charging in the 13.X volt range for quite a while. If you discharge to 80% state of charge, then you will be much closer to the bulk/absorb transition voltage of 14.5/29 volts.

I try to avoid "double counting" losses... So, sometimes you will see "all in one" deratings (like 0.52 for solar panel + charge controller + flooded cell battery + inverter losses) for solar panel to AC outlet available power calculations.

Others, like sizing loads to battery bank, use individual deratings to make sure we don't double count (recharing battery bank does not need inverter losses).

If you want "more accurate" calculations--we can certainly attempt to make them... However there is limited return. For example, clean solar panels, brand new flooded cell batteries (less self discharge), cycling more deeply (typically in winter, not so much in summer), warmer batteries (more true in summer, less true in winter), and the 10-20% variation in solar radiation due to monthly/yearly weather variations, specific weahter (3+ days of bad weather exceeds battery storage capacity), etc.

So, trying to get better than 10% accuracy is not really worth the effort as there are so many (sometimes competing) variables that it is not worth it.

Also, I try to design the system around "end of life" (old/dirty panels, old about to fail batteries, etc.) so that they work well for the first year and then you have "power shortages" for years 2-8.

In the end, it is not a good idea to plan on using 100% of your available power every day unless you will be taking the time and energy to manage the loads (i.e., moving wash day and vacuuming day around) based on daily weather.

Some loads do tend to manage automatically (people probably pump more water during hot/sunny weather for irrigation and pump less during poor weather).

-Bill