redux: why overpaneled systems don’t make braggable power

I’ve talked about this before but will revisit because it is so danged counterintuitive: overpaneled solar configurations rarely make impressive levels of power.

For example, this morning the system achieved Vabs (14.7v) at 0950 local time.   By definition, charging acceptance will drop from this point until float (or sundown in my case).  So barring the imposition of big loads the peak power of 278.26w observed moments before Absorption will be the most it makes all day.   To be fair, solar conditions were excellent:  cold, clear sky, no obstructions.

This is only . ~36% of rated power (278.26/770w since the portables are set out), unlikely to impress casual onlookers or participants in solar d*cksize contests.

why does this happen?

Solar noon (when insolation is strongest) in Parker, AZ will occur at 1242,  almost 3 hours later than we hit Absorption.

A lower-wattage system that hits Absorption a moment after solar noon will see that config’s maximal output.  So  a smaller array that hits Abs at solar noon  might actually make a higher peak wattage than the overpaneled system.

so why overpanel?

Because overpaneling is intended to counteract adverse solar conditions.  Early morning sun is such an adverse condition.

Because lead-chemistry banks need meaningful charging ASAP so they spend as little time as possible at deep levels of discharge.

Because conditions are not always excellent.  In overcast or partly cloudy conditions the overpaneled system will make more power.  This can make the  difference between “fully charged” and “partial state of charge”.

Because hitting Vabs at solar noon might not allow sufficient absorption duration.  Let’s say it’s winter and the system can no longer hold Vabs by 1630 (the case here and now).  That would allow only 4hrs of absorption.  That’s great if it’s all your bank needs, but if it needs more time then the bank does not get fully charged.   The problem is worse if the config doesn’t hit Vabs until well after solar noon.  Tick-tok, the sun is setting.     BN:  the controller dropping to Float does not mean Absorption was completed and the batt is fully charged.   On most controllers it just means Absorption duration timed out at 2-3 hours.

Because overpaneled systems can sustain bigger loads if needed.  I might stress the system to see what it will actually make in winter here at solar noon.