There are benefits from using higher voltage panels that can be realized when a system is designed for an MPPT controller. $/watt ratios can become quite favorable with large, used panels and overpaneled controllers.
The downside is that MPPT requires some amount of excess panel voltage over charging voltage to do its work (see below). MPPT advantage may be minimal or nonexistent when nominal panel voltage matches nominal battery bank voltage (12v panels and 12v battery, for example). Consider this worst-case scenario:
- low Vmp 12v panel, like a poly panel with Vmp of 17v
- 90F ambient temps, which derates panel by 18% (Vmp is now 13.94)
- wiring losses of 2% (panel voltage entering controller is now 13.66v)
- a controller that requires Vbatt + 1v to maintain MPPT operation
In this situation the controller can't do MPPT charging above 12.66v; we have effectively wasted our money on that expensive MPPT. So make sure there is sufficient (voltage) headroom for the MPPT to operate. This can be done with higher-Vmp panels[1], higher-voltage panels[2], or panels in series. Example: 2x 12v panels in series is nominal 24v, a great fit for charging a 12v battery bank.
Input voltage
Input voltage is a balancing act between efficiency / heat reduction (requires lower input voltage), the ability to overcome self-consumption losses under all conditions (requires higher input voltage), wire size and length (higher input voltage), and shading mitigation (higher input voltage).
MPPT manufacturers always specify a maximum voltage coming into the controller; some also specify a minimum or optimium voltage for MPPT operation:
- Victron: Vpanel >= Vbatt + 5v to start MPPT, Vbatt + 1v thereafter.[3]
- Genasun (nominal 12 systems): 15-18v for constant power supplies[4], <= 27v for PV.[5]
- SNA (grid tied): Vpanel = desired output * 1.4142((https://www.researchgate.net/profile/Aalok_Bhatt/post/How_to_select_MPPT_Range_of_Solar_Inverter/attachment/59d624496cda7b8083a1f834/AS%3A377507284045824%401467016158673/download/SMA+Datasheet.pdf))
- EpEver: Vpanel >= Vbatt +2v to start MPPT.((https://www.epsolarpv.com/upload/cert/file/1811/Tracer-AN(50-100A)-SMS-EL-V1.1.pdf))
- Midnite requires Vbatt+30%[6]
- generic rule of thumb: Vpanel >= maximum Vbatt + 30%
- generic rule of thumb: Vpanel <= maximum Vbatt * 2.[7][8].
Other uses
An MPPT controller can also be the centerpiece of a DIY converter.