backchannel: panel misinformation

A spammer posted an article in /r/VanLifeUK and hooked several posters into commenting as if it were a good faith conversation. I will respond to this commenter here because I don’t want to add to the spammer’s visibility.

Oh the 12V/24V [panel] thing is basically meaningless.

24v nominal is meaningful to

  • PWM controllers on 12v banks, and to
  • MPPT with low PV input limits. e.g. the Ctek with a 22v limit or the Renogy DCC combo DC-DC/MPPT with 25v limit.

Economically it is meaningful because higher-voltage panels tend to be larger and cheaper by the watt. And that doesn’t even include the used market where the price differential between 12v vs 20v/24v nominal panels is massive.

The open circuit voltage of the panel varies wildly depending on the sunlight it’s catching.

Panel voltage is relatively stable above ~15% insolation, at least until MPPT starts dorking with resistance to select points on the power curve. Panel current varies “depending on the sunlight it’s catching”.

A decent MPPT controller is able to charge off even tiny voltages.

MPPT controllers require higher input voltages than PWM, particularly for startup. Victron, for example, is Vbatt+5v for startup. Renogy and EpEver are Vbatt+2v for startup and +1v to run.

Boosting MPPT do exist but they are relatively rare.

Flexible are easy to just stick on the roof, low profile but because of the flexibility a shorter lifespan. If you’ve glued them to a roof this now becomes a bit of an issue.

Cell temperature derating with crystalline panels is why airgap is a thing.

Always go mono crystalline if you can afford it. More efficient but a little more expensive.

Panel efficiency is a function of area vs. rated output. Higher efficiency does not mean that a 100w mono will outperform 100w poly.

With normal MPPT mono vs poly is usually a wash. With PWM controllers poly will generally outperform mono because of poly’s lower voltage / higher current.

Specific to the UK, poly panels tend to be more responsive than mono to the blue-shifted (shorter wavelength) light caused by cloudy or overcast conditions. If so, poly might be prefereable in the UK.

What you’re looking at is wattage and efficiency.

Rated wattage, yes.

Panel efficiency is a red herring unless the space available for panels is already maxxed out.. 500w of 17% efficient panels and 500w of 23% efficient panels will make the same power in the field.