What are higher power 12v panels for, obsidian, 1500 days off grid
higher-power nominal 12v panels; what are they good for?
I was in a discussion about the Grape Solar 200w 12v panel1, and it got me to thinking about what need this class of panel is intended to address.
Here are the relevant specs
- Price ~$310 at Home Depot, which usually prices Grape gear fairly well. Available off pallets from CL around $200.
- Vmp = 20.31V
- Voc = 24.35V
- Imp = 9.85A
- Isc = 10.33A
PWM controllers?
It’s a nominal 12v panel, so it will work with PWM controllers and 12v battery banks.
But for a given rating, more voltage (20.31V) means less current (9.85A). And since PWM cannot2 use panel voltages > bank voltage, higher Vmp will decrease yield from PWM. Consider the following:3
- this panel: 14.4v bank voltage * 9.85A = 141.84W.
- panel with 18Vmp: 14.4v * 11.11A = 160w.
So, not particularly well-suited for PWM.
MPPT controllers?
MPPT controllers prefer a decent amount of panel voltage headroom to work their DC-DC magic. It is common for MPPT controller specs to suggest something like 1.5x-2x panel voltage compared to battery voltage. So it’d be happiest with at least 21.6Vmp (actual, not lab-rated) and preferably 28.8Vmp.
So, not particularly well-suited for MPPT.
compared to 2x 100w panels
for PWM
2x 100w in parallel would cost less, often ship for free, and lower Vmp panels could be found to optimize power output.
for MPPT
2x 100w in series would cost less, often ship for free, and the doubled voltage from arranging in series would give MPPT room to work.
my WAG: DC-DC controllers with unusually low limits.
I’m guessing these 200w panels are made to accomodate MPPT controllers with unusually low panel voltage input limits. Most freestanding MPPT solar charge controllers have input limits ≥100V, although some very small MPPT are limited to 60V. But built-in controllers can have very different limits:
- The wildly-popular Renogy DC-DC chargers with MPPT have an input limit of 25V.
- The Redarc and Wagan combo units have an input limit of 32V.
- The once-popular Ctek 250-series has an input limit of 23v. This one is dicey; the 20.41Vmp is fine but the 24.35Voc is not. Admittedly, with the usual ~10% cell temp derating might fall to 21.92Voc, but I wouldn’t bet my controller on it.
- low-to-midrange “solar generators” typically have restrictive panel input limits. The one I have access to has an input limit of 22V (pdf).
obsidian
I’ve been walking by a piece of what I assumed was black glass.
Then I realized there were not other pieces of black glass around.
Then I remembered this area is part of the San Francisco Volcanic Field:
an extensive region of nearly 2,000 square miles that contains some 600 identified volcanoes
and Sunset (volcanic) Crater national monument is just a few miles away. Sometimes it takes me a while to catch on.
another milepost off-grid
Today is day 1,518 off-grid, so I passed the 1,500-day mark. At my average rate of production/consumption the main solar setup has made about 2,505kWh of energy.
It’s also 1 year on the LiFePO4 battery bank and about 2 years since I set my phone to Do Not Disturb; incoming calls go to voicemail and I answer when I have the desire, the cell connectivity, and the ability to remember that voicemail exists. :-P
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