how much power from 100w solar and an isolator?

this response was unwieldy in the reddit editor so I moved it here.  I am chiming in on a helpful response by another commenter.

In this thread, OP stipulates:

  • 100w of panel w/pwm controller

  • 100Ah AGM

  • an isolator

  • 150w of loads

and wanted to know how many hours he could run the loads each day.

Impossible to know how much solar you'll get.

Insolation averages to the rescue!

Using Phoenix as our example since OP is in AZ, on average in November he would see 3.52 hours of Full Sun Equivalent (FSE) per day.

The average daytime high in Phoenix in Nov is 73.3°F/22.94°C. This puts cell temps at 54.19°C, so we derate the output to by 14.01% to ~86w.

86w * 3.52 hours of FSE means his unshaded, flat-mounted 100w panel run at Vmp could produce 302.7w each day.

If he were running an MPPT controller we’d take away ~5% for DC-DC conversion losses, for about 288w. Since he is talking about PWM, I’m guessing the lower Vpanel would take about ~15% on average across the day, for about 257.3w. If he gets good isolator charging in the mornings the difference between our MPPT and PWM numbers would be decreased.

Say 40A for one hour, which would be roughly 500W

A 100Ah AGM of average consumer quality is likely to accept a max of C/3 charging. ~33A in this case. That would put us down closer to 400W if it accepted that amount for the hour. But current acceptance starts falling pretty quickly once we hit the alternator’s voltage, so we’d see a lower average current across the hour, maybe 15A? So my WAG on that would be 187.5w assuming the hour’s drive occurs when the battery bank is depleted.

So let’s say alt + solar output for any given day in November would be something like 445w, not including lead-acid charging inefficiencies. So maybe he gets 3 hours of power for his 150w load.

You could improve the efficiency of the alternator charging by getting a B2B charger

I really like Sterlings because their setpoints can be configured rather than just having predefined profiles like the others in that market segment.

I was going to push back on this since B2B chargers are typically limited to 20-30A in our price range. But then I remembered the C/3 figures from above and realized the 30A Sterling would be fine (even if $$$) for a 100Ah AGM.

The benefit of adding b2b to solar instead of adding a regular isolator would be found in two places:

  1. in the voltage range between the alternator’s highest voltage output (Valt?) and Vabs

  2. in early Absorption when acceptance is still greater than what solar alone could produce