Solar under control
QI’m planning a 400W solar power system, with two pairs of 100W panels each connected in series, then the two pairs connected in parallel, sending 24v to my controller which is to be connected to a 12v battery bank. I am looking at a 20amp MPPT controller – I understand that this will handle 400W at 24v, and covert this to 12v to charge the battery bank. Is this correct, or will I need either 2 x 20A controllers or one that takes more amps?
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I think you may be misunderstanding things. The controller will automatically set itself for either a 12v bank or a 24v one. Just ensure the maximum current and voltage the panels supply is within the limits for the controller you choose.
Most MPPT controllers could handle an input of 100 volts or more. If yours can, connect all the panels in series (to keep the current and thus the cable voltdrop to the minimum, allowing thinner cables) unless they are modern 30v+ ‘domestic’ type panels. Typical 21-volt panels will give 84 volts in series, well within the capability of a 100-volt controller.
However, 400 Watts at a charging voltage of 13 volts gives a theoretical current of just over 30 amps. That would only happen if the panels are ever able to supply their full output (unlikely in the UK, especially with panels horizontal on the roof) but personally, I would be looking for a higher current controller.