Nominal voltage is a "named" voltage, rather than its actual voltage right now. Many products and systems specify a nominal voltage to give you an idea of what it will be close to when in use. For example, the most common battery bank voltage for nomads is 12v, but nominally 12v banks are rarely actually 12.0v.
Nominal battery voltage
- lead-acid banks can range from ~12.1v (full discharged) to something like 14.8v (absorption voltage). When full/rested they are something like ~12.7v.
- LiFePO4 lithium banks (typically 4S) range from ~12v discharged to ~14.6v when full. (This is why LFP is so well suited for drop-in lead acid replacement.)
- Non-LiFePO4 li-po (actually Li-Ion, typically 3S) range from 9v discharged to 11.3v when rested. Just after charging you may see ~12.3v. (Not a good match for 12v systems, which is why this chemistry is relatively rare.)
Nominal solar panel voltage
Similarly, "12v" solar panels are called that because they are used to charge/power 12v systems. The voltages on these vary even more widely:
- As low as battery voltage in the morning, ~12.1v (pwm)
- 12v panels make maximum power (Pmax) at a particular voltage (Vmp) which is usually way above battery voltage, like ~18v.
- When unconnected (i.e. "open circuit"/Voc, or in late Absorption with MPPT controllers) voltages are very high, like 22v.
Nominal 20v (~30Vmp) panels are common on residences in the U.S., while nominal 24v panels (~36Vmp) panels are more common in commercial installations. Solar farms may use panels of much higher voltage.