backchannel - solar rules of thumb

[this is a response to a thoughtful post on reddit.]

two ratios that I followed in my build

Valid starting points. I want to bring up a couple things that may warrant consideration.

Solar should be twice the Watts as batteries have usable Amp-hours.

The usable part could result in undersizing the array, especially problematic with lead chemistries.

  • The traditional ratio rule of thumb is 1:1 panel-to-Ah. So 200w for 200Ah.
  • basing panel wattage on usable Ah would mean 100w for 200Ah of lead (ie, 100Ah usable), or 0.5-to-1.
  • 100w of panel would require ~14 hours of FSE (full sun equivalent) to fully recharge 100Ah usable (lead), about 2x what is available anywhere in CONUS. (blog post on this topic)

Undercharging isn’t a problem for Li chemistries. But since usable % is greater (~80%) the usable criteria trips us up in a different way. Even assuming perfect charging efficiency a 0.8:1 ratio won’t cut it in CONUS. Let’s use 200Ah again for consistency.

  • we need to replace 2,048Wh (160Ah x 12.8v) with 160w of solar to get back to 100%
  • assuming 85% solar harvest efficiency this will require over 15 hours of FSE (2,409Wh÷160W), again ~2x what is available in CONUS even under optimal conditions.

Batteries should have triple the Ah of daily usage to account for cloudy days.

Reasonable, assuming lithium. There is little downside to running larger Li banks.

With lead banks having more bank than can be charge fully/regularly is a recipe for battery murder.