What’s with the SunPower panel amps?
People often get confused with the amps on SunPower panels as the amps is the same on all panels rated between 85W and 150W.
People are more used to seeing the panel amps increase as the watts increase while the panel voltage remains around 18V.
A typical (not SunPower) ‘12V panel’, will have 36 cells in series, each contributing 0.5V to provide a panel voltage of around 18V. This is sufficient voltage for charging a 12V battery using a conventional PWM or MPPT charge controller. The negative of a cell is the topside and the positive the underside – this can be seen by inspection of the conductors in a panel. The cell footprint can be cut and/or trimmed as needed to provide the required current rating, noting that the cell voltage remains a nominal 0.5V. The conductors are laid onto the cell prior to assembly into a panel. This is how ‘12V panels’ can be varied in power.
In SunPower cells both negative and positive conductors and the connector pad areas are on the underside of the cells in a SunPower cell – hence, they are frequently referred to as back-contact cells. They are 125mm square and, due to the connector pad areas it is only possible to cut the cells into halves, thirds or sixths. The voltage of each cell is around 0.6V, so only 32 cells are needed for a ‘12V panel’. Applying a bit of mathematical rounding, a 0.55A maximum power current * 0.6V * 32 gives 105.6W – this would be classified as a ‘100W panel’. Note that the actual Gen III cells provides a higher wattage than this – we have measured in excess of 7Amps short-circuit in Melbourne sun.
Therefore, a higher wattage from a SunPower panel requires more cells. For example, four more cells gives the same amperage but the voltage increases by 4 * 0.6V (i.e. 2.4V). For this reason, the RADpower panels between 85W and 150W all have the same current rating but the voltage increases as the number of cells increase.
To get the benefit of the increased voltage and power an MPPT charge controller must be used. With a PWM controller you would get a 5.5A maximum charge current regardless of which RADpower panel was attached.