are constant current devices so the basics of ohms law does not apply. Good solar panels have a specs label on the back similar to this one from Evil Bay
- Max Power: 120W
- Max Power Voltage: 18V
- Max Power Current: 6.67A
- Open-Circuit Voltage: 21.6V
- Short-Circuit Current: 7.34A
- Cell Efficiency: 16.25%
- FF: >74%
- Maximum System Voltage: 1000V
- Temperature Range: -40¡ãC - 85¡ãC
- Power Tolerance: ¡À 3%
- Standard Test Conditions: 1000W/m2, AM1.5, 25¡ãC
- Solar Cell: Mono
- Front Glass: 3.2mm Tempered Glass
- Maximum Hail Load: steel ball fall down from 1m high
- Frame: Anodized Aluminium Alloy
- Dimension: 120 x 81 x 3.1cm
- Weight: 15kg
the important parts
are, the max power current, in this case 6.67A, the max power voltage, in this case 18v and the open circuit voltage, in this case 21.6v.
If a Pulse Width Modulation only (PWM) type controller is used then the best you will get is 6.67amps, the rest is lost
through inefficiency due to voltage mismatch.
A Multiple Power Point Tracking (MPPT) type controller is designed to that the max power voltage (Vmp) at the Max Power Current (Amp) and convert it to match the voltage of the battery it's connected to.
Sounds great in theory and if you believe the B/S in the manufacturers ads an MPPT controller will perform miracles, fact is, they don't. If 12v nom. panels (Vmp upt to 18v) are used to charge a 12v battery about the best you will get is an extra 10% more than the PWM controller will give you, that was confirmed on another forum by a technician's email
If you use panels that have a Vmp higher than 18v then an MPPT controller is a must or a lot of charge current will be lost. MPPt cotrollers aren't cheap
, well good ones aren't cheap
, cheap ones aren't worth even bothering with. The best of the best by far is the Outback FM80, if you are going to get serious with solar then spend the $$ and get one of these.
If you just want a small system using 12v nom. panels then I'd recommend a Plasmatronic PL20 from Aust, very well built, includes system monitoring and can have a shunt kit plugged in to measure big loads in and out like generator
and mains charging and inverter
Hope that helped