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An MPPT controller must match both the electrical data of the panels and the battery. I therefore use the panel data sheets before I select a controller based on the total wattage.
What does 100/20 mean, for example?
For the Victron SmartSolar MPPT 100/20, 100 indicates the maximum open-circuit voltage from the panels in volts, and 20 the maximum charging current to the battery in amperes. These are two different sides of the controller. The panel current must be evaluated against the controller's separate limit for PV short-circuit current.
The meaning of model names should always be checked with the manufacturer. A name alone is not a complete data sheet.
Find this information on the panel
- Voc: Open-circuit voltage, used when checking the controller's voltage limit.
- Vmp: Voltage at the panel's maximum power point.
- Isc: Short-circuit current, which must be included in the evaluation of the PV input.
- Imp: Current at maximum power point.
- Temperature coefficient for Voc: Used to check voltage in cold weather.
The values change for the overall system when panels are connected in series or parallel. The final configuration must be checked, not just each panel individually.
Calculation example: 200 W to a battery while charging
If 200 W reaches the controller and the battery is being charged at 14.2 V, this theoretically corresponds to 200 / 14.2 = 14.1 A before controller losses. This is no guarantee that a 200 W panel will provide this current in practice.
The example shows why the panel's amperes and the charging current to the battery do not have to be the same.
Cold weather must be included in the voltage check
The panels' open-circuit voltage rises in cold weather. A calculation that only just stays below the controller's maximum under the data sheet's standard conditions may therefore be insufficient. Use the panel's temperature coefficient and the relevant lowest temperature.
The controller must also have sufficient voltage to start charging. For the mentioned SmartSolar series, the manual describes a start-up requirement of battery voltage plus 5 V. The requirement during continuous operation is different. Always check the manual for the selected model.
A larger controller provides room – but does not create more sun
Extra capacity may be relevant if you are planning more panels. However, it does not make a small panel more productive. When intentionally oversizing the panels, voltage, short-circuit current, and other manufacturer limits must still be observed.
Send the panel model, quantity, desired connection, and battery type. Then I will help with choosing a controller and clarifying cables, fuses, monitoring, and the possibility of later expansion.
Sources and manufacturer documentation
The guide is written by SejlStrøm. Technical information was verified on September 22, 2026. Always use the documentation for the specific model and version.