backchannel - Is there such thing as too much solar?
the original post was intended as a joke (AI-generated image of a van obscured by panel) but the question comes up regularly.
Some of this is obvious and some is more subtle.
overcurrent
vs the controller
While MPPT solar charge controllers are typically overpaneled to some degree1 there are limits:
- Exceeding the controller’s output rating on the input side can damage or deactivate certain protections. An example is Victron’s PV reverse polarity protection.
- at some point there is no more benefit to adding more panel to a given controller; 5000w of panel on a 10A controller won’t make more Watt-hours than 4000w would.
- PWM controllers should not be overpaneled at all; the rated current of the controller should be greater than the Isc of the array.
vs the battery bank
The main threat here is production of more current than the bank can safely accept. To be fair, it takes a lot of solar to do this, and safe can mean different things. I’m using it here to mean something like jeopardizing the bank’s ability to make it to the rated number of cycles.
My personal rules of thumb follow:
- Gel - ≤0.2C
- AGM - between 0.2C and 0.33C
- LiFePO4
- at human-comfortable temps, Vabs <14.0v, ≤80% SoC - 0.4C
- at human-comfortable temps, Vabs ≥14.0v, >80% SoC - 0.2C
- colder or hotter than human-comfortable - 0.1C
voltage
- a panel or string with with Voc greater than the controller’s PV input rating will cause damage.
- a panel or string on PWM with Vmp much higher than Vabs wastes the panels’ abilities
weight
Excessive panel, racking, and/or tilt mechanisms can upset the vehicle’s center of gravity and make the array more susceptible to damage.
opportunity cost
Spending lavishly on solar components or mounting could derail capital investments in other charging sources. A 1000w solar setup in Canada might produce less and cost more than alternator or generator charging with a smaller amount of solar
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the Victron MPPT calculator typically advises 10-30% overpaneling. They call it 110-130% overpaneling, but I think that is sloppy language. ↩